16th International Reflectometry Workshop (IRW)

13 May 2024, 08:30 → 16 May 2024, 17:00 Europe/Berlin

Günter-Grieger-Hörsaal (Max Planck Institute for Plasma Physics (IPP Greifswald))

The 16^th International Reflectometry Workshop (IRW16) will take place at the Max-Planck-Institute for Plasma Physics, Greifswald, Germany between the 13^th May 2024 and the 16^th May 2024. 
For the attendance of the workshop no fee will be charged.

Scope

This workshop is an in-person meeting for discussions among experts of microwave reflectometry and related topics. It brings together experts, master- and PhD-students with the aim to discuss common interests in reflectometry and plasma physics. The scientific objective is to exchange the latest results in plasma physics, theory and simulation of microwave propagation in plasmas, as well as the latest achievements in hard- and software development for reflectometry and radar applications. This workshop also invites scientists working on the design and construction of active microwave components to participate.
Stimulating discussions build the heart of this workshop. This workshop publishes proceedings for later citation.

 

Videoconference (ZOOM) Link

Monday, 13 May

08:30 → 09:00 Check-in: Conference check-in and information desk

Conveners: Claudia Schoenian (IPP Bereich Greifswald), Gavin Weir (Max Planck Institute for Plasma Physics), Matthias Hirsch (IPP Bereich Greifswald), Thomas Windisch (IPP)

09:00 → 09:15 Welcome: Welcome to IPP-Greifswald

Welcome

Convener: Robert Wolf (Max Planck Institute for Plasma Physics)

09:00 Welcome to the IPP-Greifswald and Wendelstein 7-X

Speakers: Robert Wolf (Max Planck Institute for Plasma Physics), Matthias Hirsch (IPP Bereich Greifswald), Claudia Schoenian (IPP Bereich Greifswald), Gavin Weir (Max Planck Institute for Plasma Physics), Thomas Windisch (IPP)

09:15 → 10:45 Talks: Reflectometry

Conveners: Roland Sabot (CEA, IRFM, CEA Cadarache, F-13108 Saint Paul Lez Durance, France), Filipe da Silva (Instituto Superior Técnico-Instituto de Plasmas e Fusão Nuclear)

09:15 Introduction - Density measurements in fusion plasmas: reflectometry and other measurements

A tutorial introducing the place of microwave diagnostics, and reflecometry especially, in plasma physics and controlled magnetic confinement fusion research.

Speaker: Roland Sabot (The French Alternative Energies and Atomic Energy Commission (CEA))

09:55 New density profile reconstruction method from the time delay in O and X mode reflectometry

Profile reflectometers are ubiquitous on magnetic fusion devices. For X-mode reflectometry, the standard method for density reconstruction has been the Bottollier-Curtet method [1] or its derivatives [2]. All these methods reconstruct recursively the density profile from the phase. However, the phase is an ill-defined parameter: the phase extraction is highly non-linear becoming very noisy at low signal to noise ratio or with multiple components; the total phase shift $\varphi$ is not directly accessible but wrapped into $\varphi\in[-\pi,\pi]$ requiring unwrapping from the plasma edge $a$ to the cutoff layer $R_c$. On the other hand, the time delay $\tau=\frac{1}{2\pi}\frac{\partial\varphi}{\partial F}$ is an instantaneous physical quantity: directly measured in pulse radar reflectometry or proportional to the beat frequency $f_b$ with $f_b=\frac{1}{2\pi}\frac{\partial\varphi}{\partial t}=\frac{\partial F}{\partial t}\tau$ in swept reflectometry system.

We have developed a new method to reconstruct the density profile from the time delay. It overcomes the divergence of the WKB approximation integral at the cut-off layer $R_c$, $\tau(F)=\int_{a}^{R_c(F)} \frac{dR}{v_{g}(R,F)}$. The method has been developed for X and O mode polarizations. It can be applied to compute the group delay for synthetic diagnostic applications with known radial profiles of density, magnetic field and temperature.

For the direct profile reconstruction, the best approximation to evaluate the last part of the time delay integral $I=\int_{R_c(F_i)}^{R_c(F_{i+1})}\frac{dR}{v_g(R,F_{i+1})}$ leads to a second order equation in $\Delta R=R_c(F_{i+1})-R_c(F_{i})$. The method has been used to fasten the profile inversion in preparation for real-time reflectometry measurements [3].

[1] H. Bottllier-Curtet and G. Ichtchenko, Review of Scientific Instruments 58 (1987) 539.

[2] R.B.Morales et al., Plasma Sci. Technol. 22 (2020) 064005.

[3] M.Carrard et al., this conference.

Speaker: Roland Sabot (CEA, IRFM, CEA Cadarache, F-13108 Saint Paul Lez Durance, France)

10:15 Status of the EUROfusion Enabling Research Project Advances in real-time reflectometry plasma tracking, for next generation machines

Reflectometry is anticipated to play a significant role in plasma positioning, shaping, and tracking for DEMO, potentially replacing magnetic diagnostics. Initial progress has been made through experiments, theory, and simulations, but there is still much work to be done. Our EUROfusion Enabling Research (EnR) Project, involving experts and developers of reflectometry systems in Europe, aims to address lingering questions and create a unified approach for implementing a reflectometry system.
We are presenting the objectives and associated outcomes, which are divided in two main branches with its own specificities and requiring different approaches: (i) The ability to track and monitor the position and shape of the plasma in the initial stage of the discharge, in the start-up phase and also at the ramp-down phase; (ii) To improve the capabilities of operation in the stationary phase (flattop) in order to provide an accurate and precise substitute to the positioning magnetic diagnostics in real time. Contained in the EnR tasks are also important advances on Finite-Difference Time-Domain (FDTD) simulation codes allowing access to 3D simulations that can accurately depict scenarios such as the antenna prototype of the Divertor Test Tokamak (DTT) plasma position reflectometer tested at IPFN. An important issue that must be addressed is the synchronization between all reflectometers with an experimental validation on the tokamak WEST. The talk will also report on the advances on hardware with a prototype of a compact coherent fast frequency sweeping radio frequency (RF) back-end being developed using commercial Monolithic Microwave Integrated Circuits (MMIC) with Direct Digital Synthesis (DDS).

Speaker: Dr Filipe da Silva (Instituto Superior Técnico-Instituto de Plasmas e Fusão Nuclear)

Talk_16thIRW_FdSilva_EnR.pptx

10:45 → 11:00 Coffee break

15 min+

(Coffee should be ready by 10:30 in case of short discussion in previous session)

11:00 → 12:30 Talks: Real time systems and profile inversion

Conveners: Maylis Carrard (CEA), Jia Huang, Dirk Stieglitz (IPP Garching)

11:00 Real time reflectometry measurement of density profiles in WEST tokamak

The electronic density ne is a major parameter, both for real-time tokamak control and plasma physics. On the WEST tokamak, X-mode reflectometry provides post-treatment radial density profiles ne(r) with a centimeter precision[1]. The current reconstruction method initializes the profile using the signal amplitude jump at the edge plasma position and reconstructs the profile using a step-by-step algorithm[2] within a computational time of 10 to 20 ms per profile. A real-time measurement of the density profile would enable the control of density at the plasma edge, a key parameter for plasma stability, such as for maintaining the equilibrium of the Radiative X-Point configuration to optimize confinement time.
Real-time reflectometry measurement on WEST is being developed using the innovative Nectarine digitizer[3]. The data processing has been adapted to be performed in real-time. For the profile initialization, a new method was tested relying on the jump in the beating frequency signal, due to the wave reflected faster on the inner wall than on the plasma. In addition, a new method is proposed for the profile reconstruction, based on the time of flight instead of the phase is used[4]. Then, the data extraction is easier and less sensitive to noise and biases, enabling a faster reconstruction with a reduced number of radial positions.
The single profile reconstruction code is programmed in C language and runs in 2 ms, a duration comparable to the WEST control cycle time.
References
[1] Clairet, F., et al. "Fast sweeping reflectometry upgrade on Tore Supra." Rev. Scien. Inst. 81.10 (2010).
[2] Bottollier‐Curtet, H. "Microwave reflectometry with the extraordinary mode on tokamaks: Determination of the electron density profile of Petula‐B." Review of scientific instruments (1987)
[3] C. Bouchand, et al. “The data acquisition system of WEST’s New Thomson Scattering diagnostics” 24th IEEE Real Time Conference, 22–26 avr. 2024
[4] Sabot, R., This conference

Speaker: Ms Maylis Carrard (CEA)

11:30 Real-time density profile inversion with deep neutral network model for X-mode polarization density profile reflectometer on EAST

The real-time distribution of density profiles serves as valuable data for monitoring plasma density and position. Presently, devices rely on magnetic measurement diagnostics to obtain reference data for these purposes. The microwave reflectometer serves as a widely employed density profile diagnostic system on magnetic confinement fusion devices across various nations. It is also planned to be utilized on the future ITER facility for measuring plasma density distribution, aiming to determine the plasma boundary position and density.
Traditional physics-based profile inversion algorithms require manual extraction of time-delayed data and suffer from slow computation speeds, rendering them incapable of providing real-time density profiles during experimental discharges for plasma position and density feedback control. To achieve real-time density profile data acquisition and enable online data processing, there is an urgent need to reduce the computation time for profile inversion. This work presents a rapid density profile inversion algorithm based on a deep neural network model. Leveraging a large volume of density profile data for training and inference, this approach circumvents the intricate physical processes involved in microwave propagation within magnetized plasmas, as encountered in traditional physical algorithms. The described method has been successfully applied on the EAST device, achieving rapid and accurate density profile inversion.
The next step involves integrating this data-driven real-time density profile algorithm directly into the deployed profile reflectometer data acquisition system on EAST to enable parallel computation of data acquisition and processing. This integration aims to provide real-time density profile distribution, particularly during density feedback control experiments, such as gas or pellet injection experiments.

Speaker: Dr Jia Huang (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, Peopleʼs Republic of China)

IRW16-JiaHuang-5.13.pptx

12:00 Bayesian O-mode profile inversion with dynamic initialisation

The ASDEX Upgrade tokamak has a swept O-mode reflectometer for measuring density profiles, which is used as a stand-alone diagnostic and as part of the Integrated Data Analysis framework. This contribution presents developments for the stand-alone analysis using Bayesian methods for initialisation and inversion, with uncertainty quantification for both.
Stand-alone analyses using techniques such as Abel inversion rely on an initialisation for densities below the first probed cutoff density. A common choice is a linear interpolation of two group delays, representing a parabolic density profile. Often, the starting point of the profile along the line of sight is set a priori, so that the first measured group delay can be used to fully define the initialisation profile. However, discrepancies with the actual profile in that region can affect significant parts of the remaining profile and its gradients.
The proposed alternative is compatible with more general initialisation profiles with more degrees of freedom, allows their uncertainties to be quantified, incorporates prior knowledge of profile shape, and propagates all of this to the profile error bars.
Part of the approach is to extend the information for the initialisation by exploiting the trend of the measured group delays of the first measurement frequencies, as they contain information about the density gradient. Additionally, this reduces the susceptibility for outliers. This information is used to infer the free parameters of the initialisation model and their uncertainties including correlations. Prior knowledge about the initialisation profile, e.g. about the gradients or the decay length, from theory or other measurements is easily incorporated.
The subsequent inversion of the observed densities is based on posterior samples of the initialisation model. The result can be used to determine profiles with asymmetric error bars as, in general, the initialisation does not have a linear effect on the inversion.

Speaker: Dirk Stieglitz (IPP Garching)

12:30 → 13:30 Lunch

13:30 → 15:00 Talks: Profile systems

Conveners: Janmejay Buch (Institute for Plasma Research), Frederic Clairet (CEA), Umesh Kumar (EPFL)

13:30 Density profile measurements from the developed FMCW Reflectometry system for Aditya-U tokamak.

Results from FMCW reflectometry measuring density profiles for the first time on Aditya-U tokamak are reported. Two channels in 18-28 GHz and 26-40 GHz range are established at IPR. The reflectometer was operated in O-Mode with its horn antennae placed outside the vacuum vessel. The diagnostic was calibrated in laboratory and also verified in-situ. Characterisation of the instrument for system dispersion and phase offset correction are presented. A signal processing algorithm developed to filter the noise and frequencies other than those due to plasma. An algorithm to remove amplitude variations is used on the raw baseband signal after which the group delay is estimated using the spectrogram technique. Bottolier-Curtet method is used for profile inversion after correcting the measured group delay for contributions to it other than plasma. Edge density profiles for different plasma conditions were measured and analysed.

Speaker: Mr Janmejay Buch (Institute for Plasma Research)

14:00 Embedded reflectometry into additional heating launchers of WEST

The performances of the WEST tokamak plasmas relies on additional heating system such as ICRH (Ion Cyclotron Resonance Heating) and LHCD (Lower Hybrid Current Drive). However, the coupling efficiency of these additional power systems strongly depends on the electron density in front of their launchers. Thus, in addition to the reflectometer with waveguide routes already installed in a LH launcher [1] we have now the possibility of performing density measurements in front of an ICRH antenna at three different poloidal and toroidal positions. The objective being to study eventual density asymmetries due to expected cell structures and sheath effects, in front of the antenna. The reflectometer is a W-band fast frequency swept system operating in X-mode which provides density profiles at the very edge. We will present careful waveguide routes design for the attempt to eliminate, or at least minimize, possible higher mode conversion generation generated by waveguide bends. In front of the heating launcher a new design of the bistatic emitting and receiving antennas has been done to find the best compromise, within the limited place available, between the maximization of the reflected signal detection and the direct coupling signal between antennas which provides the phase reference signal. An improvement of 20 dB compare to simple open waveguides, used for the LH launcher, has been achieved. We will discuss the signal processing challenges to perform density profile reconstruction in such metallic environment subject to substantial multi-reflections due the plasma proximity.

[1] F. Clairet, B. Ricaud, F. Briolle, S. Heuraux, and C. Bottereau Rev. Sci. Instrum. 82, 083502 (2011)

Speaker: Dr Frederic Clairet (CEA)

14:30 Recent advancements in Short-Pulse Reflectometry in the TCV tokamak

The accurate and precise measurement of the density profile and characterization of turbulence in a fusion plasma is of paramount importance and achieving this through non-invasive diagnostics is a key area of interest. The microwave reflectometer stands out as a potential candidate for such measurements, with short-pulse reflectometry presenting a uniquely appealing approach.
In this method, 1 ns pulses with a defined carrier frequency are launched into the plasma. Their round-trip group delay is measured using an accurate timing system [1]. In the TCV tokamak, these pulses are generated using a 65 GS/s AWG with a carrier frequency ranging from 48 to 75 GHz in 0.1 GHz steps. The returned pulses are recorded using a 32 GS/s ADC and post-processed to derive the TOFs for 1.76 s of the TCV shot [2]. Data processing improvements have reduced computation time to 13-15 minutes, less than the typical interval between TCV shots, with potential for further reduction.
These TOFs can be directly utilized to estimate the density profile and characterize the fluctuations and turbulence within the plasma [3, 4]. The typical pulse repetition rate thus far has been 8.33 MHz, sending each pulse only after the previous one has returned. By relaxing this constraint, the repetition rate has been increased to 41 MHz, providing a 20-point plasma density profile every 0.5 microseconds. This offers sub-microsecond temporal and millimeter spatial resolution, significant for exploring rapid transitions such as the fast variation in the pedestal density profiles during edge localized modes [5].
The presentation will discuss the details, challenges, and advantages of this diagnostic method.
[1] P Molina Cabrera et al, 2019 Rev. of Sci. Instrum. 90, 123501
[2] U Kumar et al, to be submitted, 2024
[3] V Shevchenko et al, 1993, Int. Journal of Infra and mm Waves, 14(9) 1755
[4] O Krutkin et al, 2023, Nucl Fusion, 63, 076012
[5] P Molina Cabrera et al, 2021 Plasma Phys. Control Fusion 63, 085019

Speaker: Dr Umesh Kumar (EPFL)

UKumar_IRW16_presentation.pptx

15:00 → 15:30 Extended Coffee break

15:30 → 17:00 Talks: Reflectometry development on HSX

Conveners: Xiang Han (University of Wisconsin-Madison), Michael Richardson (University of Wisconsin-Madison), Henrique Oliveira Miller (University of Wisconsin Madison)

15:30 Microwave reflectometry diagnostic for density profile and fluctuation measurements on Helically Symmetric eXperiment

A dual band frequency-modulated continuous wave (FMCW) reflectometry is developed to measure the density profile and its associated fluctuation on Helically Symmetric eXperiment (HSX). This reflectometry equips a switch to select the frequency source between two voltage controlled oscillators (VCOs) to the transmitter, realizing an extended operational frequency range from Ku to K bands (14.5 - 25 GHz). The polarization of the system can be tuned between O- and X-modes by rotating the horn aiming to accommodate with the magnetic field and the density coverage on HSX. More specifically, the reflectometer is in X-mode operation at 0.5 T of the magnetic field and in O-mode when the magnetic field is above 1.0 T, corresponding to a cutoff density range of about $\rm 0.2\sim2\times10^{18}\ m^{-3}$ in X-mode at 0.5 T and $\rm 2.6\sim7.8\times10^{18}\ m^{-3}$ in O-mode regime.

This reflectometry was originally installed in 2007[1], recently an upgrade of the system is ongoing for the bi-static antenna structure and the Ka-band extension. Here we introduce the current reflectometry system and the plan for the upgrade. To validate the system performance on HSX, the experimental measurements of the density profile and an estimation of the density fluctuation level under different magnetic configurations and plasma parameters are presented.

This work was supported by the U.S. D.O.E. contract DE-FG02-93ER54222.

[1] K. Likin, et al, poster, 16th International Stellarator / Heliotron Workshop, Oct. 15-19, 2007, Toki, Japan

Speaker: Xiang Han (University of Wisconsin-Madison)

IRW16_XiangHan_v2.pdf

16:00 Ongoing design of Doppler Reflectometry for use on the Helically Symmetric eXperiment

Reflectometry is one of the workhorse diagnostics at the Helically Symmetric Experiment (HSX) to study turbulence-induced density fluctuations in the frequency range from 10 – 100 kHz. To further optimize the system for turbulence and flow studies, it is planned to upgrade the HSX reflectometer system into a Doppler Reflectometer. With the new setup, the diagnostic will be capable of providing perpendicular (to the magnetic field) flow measurements and perpendicular turbulent k-spectrum measurements. The perpendicular flows in HSX are in the range of 5 km/s, as measured previously by Charge Exchange Recombination Spectroscopy (CHERS). The expected observable perpendicular wavenumber range of 0.5 to 4.5 cm$^{-1}$ is compared to TRAVIS [1] simulations. Design elements for a motor-controlled rotatable mirror apparatus are shown alongside ray-tracing results. A discussion of the design considerations for a magnetic field strength of 1.25 T (in addition to 1 T standard operation) is presented including an adapted frequency source in the Ka band (28-40 GHz). Once installed, the new system will be used to identify turbulence induced density fluctuations and flows, which can be further used to study radial electric field, and will provide insight into the physics of turbulent transport in HSX.

Speaker: Michael Richardson (University of Wisconsin-Madison)

Richardson_IRW2024.pptx

16:30 Modelling of the HSX Reflectometer and Planned Doppler Reflectometer with the Ray-Tracing Code TRAVIS

A density fluctuation and profile reflectometer diagnostic is in place on the Helically Symmetric Experiment (HSX). It is currently installed as a perpendicular incidence reflectometer with a shared launching and receiving antenna. Probing microwaves range from 14.5-25 GHz, and either O or X-mode propagation can be selected. Preliminary modelling of this reflectometer with the ray-tracing code TRAVIS is discussed here. Specifically, analysis of beam width and wavenumber resolution of the existing perpendicularly-oriented reflectometer for both O and X-mode propagation and an assortment of cutoff layers and magnetic configurations is presented. Plans for modifying the existing reflectometer to allow for Doppler reflectometry are underway. Modelling of this planned Doppler reflectometer for HSX is shown, including predictions for perpendicular wavenumber sensitivity and resolution for a range of launching/receiving mirror angles and cutoff layers.

Work supported by US DOE Grant No. DE-FG02-93ER54222.

Speaker: Henrique Oliveira Miller (University of Wisconsin Madison)

Tuesday, 14 May

09:00 → 10:45 Talks: Theory and synthetic diagnostic development

Conveners: Garrard Conway (IPP Garching), Kaixuan Ye (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, Peopleʼs Republic of China), Mr Antoine Jamann (CEA, IRFM, F-13108 Saint Paul-lez-Durance, France)

09:00 2D full-wave and 3D beam-trace modelling of Doppler reflectometry response in realistic geometry

This contribution reports the conclusions of a lengthy modelling study (cf. previous reports [1]) using 2D full-wave (IPF-FD3D) and 3D beam-tracing (Torbeam) codes, on the behaviour, and in particular the physical and spectral resolutions, of microwave Doppler reflectometry / backscatter diagnosis of fusion plasmas. Using a large database (including simulation spatial weighting-functions & instrument response to test density perturbations) of ASDEX Upgrade tokamak configurations covering a broad range of magnetic equilibria shape, plasma density and probing beam geometry in O & X-mode wave polarization with upper and lower cutoffs, a set of generic empirical models for the principle diagnostic sensitivities & associated error-bars have been distilled. These models require only easily obtainable experimental plasma & wave parameters, such as the permittivity scale length, wavelength, refractive index and beam spot size etc. There have been significant refinements in the models over previous database fits [1], which, are now supported by new theory considerations. Comparisons have also made between the 2D full-wave parameter models and similarly distilled diagnostic models from 3D beam-tracing. Beam tracing provides an alternative, and more experimentally accessible, approach to modelling the diagnostic behaviour. Finally, new results and models have also been obtained on the diagnostic back-scattered power sensitivity. This opens the discussion on the need for fully 3D full-wave simulations and their potential benefits.

[1] G.D.Conway et al. Proc. 12th Intl. Reflectometer Workshop, Julich (2015) and Proc. 14th Intl. Reflectometer Workshop, Lausanne (2019)

Speaker: Dr Garrard Conway (IPP Garching)

conway-irw16_talk.pdf

09:45 Development of full-wave simulation codes for synthetic profile reflectometry in EAST tokamak

Microwave reflectometry is a powerful diagnostic that can measure the density profile and will be used in the future fusion devices such as ITER, so understanding of how the reflected signals are affected by plasma perturbations is of prime importance. In EAST, profile reflectometry has been developed as routine diagnostic since 2013, [1]–[3]. In this work, a set of full-wave simulation codes via the finite-difference time-domain (FDTD) method has been developed to implement synthetic profile reflectometry including 1D/2D ordinary mode (O-mode), and 2D extraordinary mode (X-mode) codes. The characteristics of the reflect signal, also known as the time-of-fight (TOF) signal associated with the local density perturbations are studied in both EAST experiments and numerical simulations.
Using the 1D O-mode code, it is well validated that the local density flattening could induce the discontinuity of the simulated TOF signal and an obvious change of reflect amplitude. Experimental TOF signals under different types of MHD instabilities are studied in detail and show agreement with the simulation [4]. Furthermore, a ‘dual-reflection’ phenomenon prior to an ELM crush is observed in the TOF signals and comparatively studied in the experiment and simulation. Using the 2D X-mode code, it is suggested that this phenomenon is due to the local density peak at the pedestal top leading to the tunneling and partial reflection. The effects of local density peak parameters (amplitude, width and position) on the TOF signals are investigated. From these results, it is concluded that the TOF signal analysis from profile reflectometry can provide a straightforward and localized measurement of the plasma perturbation from the edge to the core simultaneously and, is beneficial for the study of the ELM physics.
[1]S. Zhang, Plasma Sci. Technol. 16(2014).
[2]Y. M. Wang, Fusion Eng. Des. 88(2013).
[3]H. Qu, Plasma Sci. Technol. 17(2015).
[4]K. X. Ye, Plasma Sci. Technol. 26(2024).

Speaker: Kaixuan Ye (ASIPP)

10:15 Full-wave simulations for synthetic reflectometry diagnostic development

In magnetized plasmas, turbulent mechanisms play a crucial role in governing the anomalous transport of energy and particles. These mechanisms are challenging to measure and require high-resolution diagnostics. Microwave reflectometry emerges as a versatile and cost-effective tool capable of measuring electron density fluctuation resulting from MHD or micro-turbulence. On the other hand, reflectometry can benefit from the use of simulations, (i.e. synthetic diagnostic) which aim to support the interpretation of the measurements.

Our synthetic diagnostic FeDoT [1] is based on 2D Finite Difference Time Domain (FDTD) full-wave numerical scheme and uses absorbing boundary conditions. The code is specifically designed to emulate a fixed-frequency reflectometer with a monostatic antenna launching X-mode polarized waves at normal incidence. The code is adapted to run with different turbulence maps, including maps resulting from GYSELA gyrokinetic simulations.

This study focuses on enhancing the reliability of reflectometry data interpretation by investigating the Mazzucato transfer [2], which establishes a linear relationship between acquired phase-shifts and fluctuation level. The investigation includes a thorough assessment of the transfer function's validity domain, employing both 1D (R) and 2D (R, Z) full-wave codes coupled with simplified turbulence maps. This study lays the groundwork for 2D full-wave code verification, which is essential for paving the way for further studies. More precisely, this should enable meaningful confrontation between experimental data and computed structures of turbulence, ultimately advancing our understanding of turbulent mechanisms in tokamak plasmas.

[1] A. Medvedeva, et al. “Development of the synthetic diagnostic for the ultra-fast swept reflectometer“. 14th International reflectometry workshop, IAEA (2019)

[2] E. Mazzucato. “Microwave reflectometry for magnetically confined plasmas”. Review of Scientific
Instruments: 69.6 (1998)

Speaker: Mr Antoine Jamann (CEA, IRFM, F-13108 Saint Paul-lez-Durance, France)

IRW16_Jamann.pptx

10:45 → 11:00 Coffee break

15 min+

Coffee should be ready by 10:30 in case of short discussion in previous session

11:00 → 12:30 Talks: Multi-channel microwave system development on EAST

Conveners: Liutian Gao (USTC), Wenxiang Shi (University of Science and Technology of China), Gongshun Li (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China)

11:00 Upgrades of the five-channel tunable W-band doppler backscattering system on EAST

The W-band doppler backscattering system on EAST, which was installed in 2018 [1], has been upgraded. The new design uses a beam splitter and two antennas to separate signal launching and receiving, thus avoiding signal noise caused by the reflection of the W-band filter. The frequency intervals between adjacent channels have been increased from 400 MHz to 1 GHz to broaden the diagnostic area. After carefully optimizing the power distribution between different channels, the maximum bandwidth is increased slightly from 97.8 GHz to 100.4 GHz and the performance in high frequency is significantly enhanced, which are more favorable for tokamak core turbulence measurements. Additionally, the corresponding ray tracing script [2] has been optimized by solving its numerical abrupt density change at the Last Closed Flux Surface, providing more reliable calculation results for the scattering position and wavenumber.

[1] Feng, X. et al. Five-channel tunable W-band Doppler backscattering system in the experimental advanced superconducting tokamak. Review of Scientific Instruments 90, 024704 (2019).

[2] Zhou, C. et al. Ray Tracing for Doppler Backscattering System in the Experimental Advanced Superconducting Tokamak. Plasma Sci. Technol. 17, 728–732 (2015).

Speaker: Liutian Gao (USTC)

11:30 A ten-channel E-band Doppler backscattering system on EAST

A muilti-channel E-band (60–90 GHz) Doppler backscattering (DBS) system with X-mode polarization has been installed on the Experimental Advanced Super-conducting Tokamak (EAST)1. In September 2023, this system was upgraded to the ten-channel E-band DBS in order to cover a larger radial range in one shot. The upgraded system can launch 16 fixed microwave frequencies in the range of 60–90 GHz with a 2 GHz interval into the plasma, and ten probing signals are selected by employing a reference signal and multiple filters. During discharge experiments, the frequency of the reference signal is tunable in the E-band, and the selected probing signals can be changed as needed without any other adjustments. Furthermore, the incident angle can be adjusted from −10°to 20°, and the wavenumber range is 4–25 cm−1 with a wavenumber resolution of Δk/k<=0.35. Ray tracing simulations are employed to calculate the scattering locations and the perpendicular wavenumber2.
[1] Liu, S., et al. (2023). "An E-band multi-channel Doppler backscattering system on EAST." Review of Scientific Instruments 94(12).
[2] Zhou, C., et al. (2015). "Ray Tracing for Doppler Backscattering System in the Experimental Advanced Superconducting Tokamak." Plasma Science & Technology 17(9): 728-732.

Speaker: Wenxiang Shi (University of Science and Technology of China)

12:00 Observation of Doppler shift f_D modulated by internal kink mode using conventional reflectometry on EAST tokamak

As a conventional reflectometry, W-band Poloidal Correlation Reflectometry (PCR) has been operated successfully on EAST tokamak since 2018 and has become a key diagnostic tool for measuring electron density fluctuation in plasma core. In this report, a new experimental observation that the turbulence spectrum detected by this PCR system exhibits an asymmetry and induced Doppler shift f_D during internal kink mode (IKM) rotation phase, will be presented. This Doppler shift f_D is the target measurement of Doppler reflectometry, but captured by this conventional reflectometry. Results show that the Doppler shift f_D is modulated by the periodic changes in the effective angle between the probing wave and cutoff layer normal, but not by plasma turbulence during IKM rotation. Fishbone mode and saturated long-live mode (LLM) are typical IKMs on EAST NBI-heating plasmas, and this modulation phenomenon is observed in both cases. In addition, the value of Doppler shift f_D is positively correlated with the amplitude of IKM, even when the latter is small. However, the positive and negative frequency components of the Doppler shift f_D can be asymmetric, which is related to the plasma configuration. A simulated analysis is performed by ray-tracing to verify these observations. These results establish a clear link between the Doppler shift f_D and IKM rotation, and are beneficial to study the characteristics of IKM and related physical phenomena.

Speaker: Dr Gongshun Li (Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China)

12:30 → 13:30 Lunch

13:30 → 15:00 Talks: Microwave system development on HL2A/3

Conveners: Yu Zhou (Southwestern Institute of Physics (SWIP), China), Weichu Deng (Southwestern Institute of Physics, China), Ruihai Tong (SWIP)

13:30 A flexible Doppler backscattering system on the HL-3 tokamak

Recently, a flexible Doppler backscattering system (DBS) has been developed for the HL-3 tokamak to obtain radial profiles of the perpendicular propagation velocity during L-mode and H-mode. [1–4] It operates within the frequency range of 24.4-50.4 GHz, encompassing both the Ka-band and Q-band. In frequency hopping mode, its minimum step frequency is 0.2 GHz. For heterodyne detection, the single side band modulation (SSBM) technique is employed to achieve an intermediate frequency (IF) of 400 MHz. Ray-tracing code BORAY [5] is utilized to estimate the scattering location and wavenumbers of the density fluctuations. [6] The new DBS has been installed and tested in HL-3 2023 experimental campaign. Preliminary results demonstrate its ability to measure radial velocity profiles and turbulence behaviors. In future, this system will be utilized for turbulence analysis and L-H transition investigation.

[1] X. L. Zou et al., 4 (1999).
[2] G. D. Conway et al., Plasma Phys. Control. Fusion 46, 951 (2004).
[3] P. Hennequin et al., Nucl. Fusion 46, S771 (2006).
[4] W. L. Zhong et al., Physics of Plasmas 23, 060702 (2016).
[5] H. Xie et al., Computer Physics Communications 276, 108363 (2022).
[6] Y. Zhou et al., Review of Scientific Instruments 94, 013508 (2023).

Speaker: Mr Yu Zhou (Southwestern Institute of Physics (SWIP), China)

14:00 Development of 105 GHz Collective Thomson Scattering System on HL-2A

A 105 GHz collective Thomson scattering (CTS) diagnostic for measurement of velocity distribution of fast ions has been developed on HL-2A tokamak. A high gain Cassegrain antenna installed below the gyrotron launcher is used to receive the scattering beam from the central chord inside the vacuum vessel. The transmission system and notch filters provide a suppression level >60 dB at 105 GHz, to protect the electronics in receiver system. The measured position is determined by the steerable gyrotron launcher, and the spatial resolution range varies from 70 mm at LFS to 260 mm at HFS1.
Positive linear relationships are found between the power of the CTS signal and Neutral Beam Injection (NBI) power or neutron count, indicating that the scattering signal contains a contribution from fast ions. Via signals with NBI dividing signals without NBI, a measured scattering spectrum consistent with simulation is obtained.
The high-frequency range of signals enhanced by NBI is slightly wider than the calculation, which may not come from accelerated fast ions. There could be a small heating effect of the modulated gyrotron since the gyrotron frequency is in the range of the third harmonic electron cyclotron frequency. The absorption of probe radiation in the plasma broadens measured spectra2.
A 140 GHz CTS system is under development on HL-3. Notch filters with larger attenuation can be adopted for better signal-to-noise ratio and thus clear scattering spectra. Furthermore, the full electromagnetic model for the simulation of scattering spectra could be utilized to extract projection velocity distribution from results.

Speaker: Mr Weichu Deng (Southwestern Institute of Physics, China)

14:30 A novel comb-based multi-channel microwave Doppler backward scattering diagnostic on the HL-3 tokamak

A novel multichannel Doppler backscattering (DBS) system based on a comb generator has been designed and tested for application on the HL-3 tokamak. DBS diagnostic is widely used to measure the localized density fluctuations and the propagation velocity of turbulent structures[1-4]. Microwave is launched at a frequency that approaches a cutoff layer in the plasma and at an angle that is oblique to the cutoff layer. Bragg backscattering occurs near the cutoff layer for fluctuations with k⊥=-2ki, where ki is the incident probe wave vector at the scattering location. The turbulence propagation velocity can also be determined from the Doppler shift in the return signal together with the knowledge of the scattering wavenumber. With the comb generator and heterodyne scheme, the stability and flexibility of the DBS system are greatly improved. The turbulence information can be obtained with high temporal and spatial resolution. This article introduces the system design, laboratory test results, and initial experimental results from HL-3.

1. Conway, G.D., J. Schirmer, S. Klenge, W. Suttrop, et al., Plasma rotation profile measurements using Doppler reflectometry. Plasma Physics and Controlled Fusion, 2004. 46(6): p. 951-970.
2. Shi, Z., W. Zhong, M. Jiang, Z. Yang, et al., A novel multi-channel quadrature Doppler backward scattering reflectometer on the HL-2A tokamak. Review of Scientific Instruments, 2016. 87(11): p. 113501.
3. Rhodes, T.L., C.A. Michael, P. Shi, R. Scannell, et al., Design elements and first data from a new Doppler backscattering system on the MAST-U spherical tokamak. Review of Scientific Instruments, 2022. 93(11): p. 113549.
4. Pinzón, J.R., T. Estrada, T. Happel, P. Hennequin, et al., Measurement of the tilt angle of turbulent structures in magnetically confined plasmas using Doppler reflectometry. Plasma Physics and Controlled Fusion, 2019. 61(10): p. 105009.

Speaker: Dr Ruihai Tong (SWIP)

15:00 → 15:30 Extended Coffee break

15:30 → 17:00 Talks: Technology development

Conveners: Antoine Sirinelli (ITER Organization), Valentina Nikolaeva (Commonwealth Fusion Systems), Yilun Zhu (UC-Davis mmWave Research Center)

15:30 Calibration options for ITER reflectometry systems

Calibration is a key step in the development and operation of any measurement instrument. It allows to quantify the accuracy and precision of the measurement. A careful calibration enables the user to understand the performance of the different parts of the instrument and therefore permits the compensation of systematic errors. Regular calibrations also give some insights on the long-term stability of the measurement. While being an important contributor of the performance of a diagnostic, calibration is not always integrated into the early design of fusion diagnostics. This is why ITER has decided to organise a series of workshops dedicated to diagnostic calibration to make sure calibration is well defined and implemented into the diagnostics design and operation.

The first ITER microwave diagnostic workshop was held in November 2023. Calibration of reflectometry systems was discussed. The calibration experience from JET and KSTAR reflectometers was presented. The calibration plans for future ITER reflectometers were detailed as well.

We will present the outcome of the ITER calibration workshop: the lessons learnt from other tokamaks, the actual provision for ITER reflectometry calibration and the open questions on some of the calibration options which are not yet implemented. After sharing this first feedback with the IRW community we would like to gather contributions on best-practices for reflectometry calibration before organising a second workshop to review in detail the final implementation of calibration within the ITER microwave diagnostics.

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization

Speaker: Antoine Sirinelli (ITER Organization)

16:00 SPARC Edge Scanning Reflectometry Overview

SPARC is a compact, high-field, D-T tokamak that is presently under construction and will be used to de-risk the high-field path to commercial fusion energy. Diagnostic systems are entering the final design stage and will be used for real-time control and to close science gaps needed to design ARC. An Edge Scanning Reflectometry (ESRL) system for SPARC is included in the Campaign #1 diagnostic set, integrated into one of SPARC’s midplane port plugs. The engineering challenges for this reflectometry system include space constraints and high thermal and electromagnetic loads, mainly in the port area, long transmission lines from the port towards the laboratory (approximately 20 m), and the necessity for radiation protection of signal processing equipment. Given limited in-vessel access due to activation, developing in-situ diagnostic calibration techniques is crucial for success and to plan for reflectometry on ARC.

The ESRL system aims to determine the edge density profile through measurements in both O-mode and X-mode, utilizing probing frequencies in the K, Ka, U and E bands, corresponding to the 18-90 GHz range. ESRL is being designed to measure the electron density profile from 3x10^18/m^3 to 4x10^20/m^3, corresponding to 0.004 - 0.5 of the Greenwald density limit for Ip = 8.7 MA, the highest plasma current at SPARC.

The ESRL system is being designed to decrease the signal loss while optimizing for cost and manufacturing time. This design overview includes electromagnetic simulations to identify the optimal distance from the plasma, optimization of the horn and waveguide geometries, tolerance analysis for inner surface roughness, development of manufacturing options for the antenna box, simulations of vacuum windows and design of waveguide bends for the wall penetration across multiple frequency bands.

This work was supported by Commonwealth Fusion Systems.

Speaker: Dr Valentina Nikolaeva (Commonwealth Fusion Systems)

16:30 System-on-Chip Technology Application on Millimeter Wave Reflectometers

The microwave reflectometer serves as a crucial diagnostic tool for plasma density measurements. In modern large-scale, high power fusion plasma facilities, reflectometers require lengthy millimeter-wave transmission lines to shield the microwave transmitting/receiving systems from harsh radiation environments, thereby ensuring the diagnostic system's integrity. Enhancing the output power of the transmitting module and the conversion gain of the receiving module is essential to compensate for transmission line losses. Leveraging advancements in semiconductor technology, System-on-Chip (SoC) microwave transmitter and receiver modules have been developed and implemented in magnetic confinement fusion diagnostic equipment. The inclusion of millimeter-wave low-noise power amplifiers in both transmitting and receiving components directly improves the signal-to-noise ratio of reflectometer measurements while minimizing back-end electronics noise. These microwave chips, scaled down to square millimeter dimensions, are enclosed individually in shielding enclosures, occupying only about 150 cubic centimeters. This compact design, coupled with high power output and conversion gain, facilitates easy installation, and broadens the application potential of microwave reflectometers. In 2023, the successful development of the GaN-based W-band receiver chip marked a significant milestone. The utilization of wide bandgap materials like GaN promises enhanced radiation tolerance for millimeter-wave electronics, a pivotal advancement in the realm of fusion reactor microwave diagnostics.

This work is supported by the US Department of Energy under US DoE grants DE-SC0021353, DE-FG02-99ER54518, DE-FG02-99ER54531, DE-SC0023500.

Speaker: Dr Yilun Zhu (UC-Davis mmWave Research Center)

Wednesday, 15 May

09:00 → 10:30 Talks: Intensity refractometry, Plasma density fluctuations and Turbulence

Conveners: Maria Usoltseva (Max Planck Institute for Plasma Physics), Oleg Krutkin (EPFL), Jason Smoniewski (IPP Bereich Garching)

09:00 Intensity refractometry: choosing optimal diagnostic parameters and understanding distortion in experimental data

Intensity refractometry is a recently developed technique for plasma density measurements in laboratory plasmas with large density gradient, such as in tokamaks or stellarators. It relies on measuring the change of both phase and amplitude of a microwave beam with the O-mode polarization passing through the plasma. The direction of the microwave beam propagation is perpendicular to the density gradient and perpendicular to the magnetic field. This ensures that different density profiles cause a large variation of the beam refraction. A broad phase-power parameter space is mapped to the density values. Dedicated numerical analysis based on 3D full-wave modelling allows interpreting the measured data [M.Usoltceva et al, Rev. Sci. Instrum. 93 (2022) 013502].
An optimization routine has been developed, which allows choosing optimal parameters of an intensity refractometer (including the number of the receivers) for a certain plasma device and for a target range of plasma density. This work has deepened the understanding of the influence of a density distribution on values measured by intensity refractometry and allowed improving the existing algorithm of density reconstruction. For any diagnostic, which relies on forward modelling for data interpretation, an optimization of the model selection approach is crucial. The developed routine is applied to optimize the configuration of Microwave Intensity refractometer in the Limiter Shadow (MILS), installed in ASDEX Upgrade, planned to have three receivers.
Experimental factors can influence the accuracy of measurements of the intensity refractometry and, therefore, the accuracy of density reconstruction. We have shown that the signal distortion is low enough to allow density reconstruction with accuracy better or comparable to other density diagnostics [M.Usoltseva et al, Fus. Eng. Des. 192 (2023) 113783]. This study has been expanded with a more detailed analysis of the experimental conditions and of ways to reduce errors.

Speaker: Maria Usoltseva (Max Planck Institute for Plasma Physics)

2024.05 Usoltseva - IRW16.pptx

09:30 Density fluctuation measurements with Short Pulse Reflectometry

A Short Pulse Reflectometer (SPR) diagnostic has been developed for the TCV tokamak [1]. It utilizes short (~1 ns) microwave pulses to probe plasma in the presence of a cut-off. Pulse delays corresponding to different probing frequencies are then used to reconstruct the electron density profile, similar to conventional fast-sweeping reflectometry.
Within this work, the capabilities of SPR to provide information on plasma turbulence are studied. A fluctuation SPR (FSPR) method for obtaining the amplitude and the radial correlation length of the turbulence from statistical properties of pulse delays is developed [2]. The method is based on a simplified 1D WKB model. Theoretical results are validated utilizing 2D full-wave modeling with the CUWA code [3]. While some limitations are observed due to 2D geometry and nonlinear scattering effects, the method is shown to provide relevant information about the turbulence amplitude, radial correlation length and frequency spectrum.
The FSPR approach is then applied for interpreting results from the SPR system on TCV. Measurements are carried out in a set of discharges for which the plasma shape is modified from positive to negative triangularity, reproducing the decrease of the turbulence amplitude previously observed with other turbulence diagnostics [4]. The synthetic diagnostic combining local gyrokinetic GENE simulations and CUWA computations is used to validate these experimental measurements.
Finally, recent efforts include development of a more advanced synthetic diagnostic by utilizing global GENE simulations. At the same time, the FSPR method is further developed to account for the possible impact of the nonlinear scattering regime.
[1] P. A. Molina Cabrera et al., Rev. of Sci. Instrum. 90.12 123501 (2019)
[2] O. Krutkin et al., Nucl. Fus. 63.7 076012 (2023)
[3] P. Aleynikov and N. B. Marushchenko, Computer Physics Communications 241 40–47 (2019)
[4] Z. Huang et al., Plasma Phys. and Control. Fusion 61 014021 (2019)

Speaker: Dr Oleg Krutkin (EPFL)

10:00 TEM turbulence in simulation and experiment with degraded quasisymmetry in the HSX stellarator

The Helically Symmetric eXperiment (HSX) has previously demonstrated reduced neoclassical transport and reduced flow damping with quasisymmetry (QHS) as compared to a degraded-symmetry (Mirror) configuration, but the effect of quasisymmetry on the anomalous transport is less clear. Here, experimental heat flux and density fluctuation measurements are compared to gyrokinetic simulations of Trapped Electron Mode (TEM) turbulence in the QHS and Mirror configurations. Density fluctuation amplitudes are measured by a conventional heterodyne O-mode stepped-frequency reflectometer ($15-26\,\mathrm{GHz}$), capable of scanning across the minor radius for densities available in HSX. The reflectometer views the outboard midplane of an up-down symmetric cross-section, matching the center of the flux tube used in simulation and enabling a relatively direct first-order comparison without a complicated synthetic diagnostic.

Instead of trying to reproduce an individual discharge in simulation, a large number of plasma discharges are collected in a database to extract scans across the density and temperature gradient. The gradient dependence of the heat flux and density fluctuation amplitude are reproduced in simulation, and the simulated heat flux matches measurements within experimental uncertainties, confirming that $\nabla n$-driven TEM turbulence is the dominant source of anomalous transport in HSX at the mid-radius. The experimental heat flux also shows that anomalous transport is larger in the Mirror configuration when temperature and density profiles are matched. A shift of turbulence propagation from the electron to the ion direction suggests a target for future investigations of the TEM in HSX.

This work was supported by US DOE grant DE-FG02-93ER54222.

Speaker: Jason Smoniewski (Max-Planck Institute for Plasma Physics)

10:30 → 11:00 Coffee break

11:00 → 12:30 Talks: nT-crossphase estimation: Reflectometry and CECE

Conveners: Carsten Lechte (Institute of Interfacial Process Engineering and Plasma Technology IGVP, Stuttgart, Germany), Branka Vanovac, Gavin Weir (Max Planck Institute for Plasma Physics)

11:00 Fullwave Simulations of Radial Correlation Doppler Reflectometry and Cross Correlation with Te Fluctuations in ASDEX Upgrade

Connecting experimental results with turbulence simulations can require fullwave simulations of regular and Doppler reflectometry when high fluctuation levels are probed with high frequencies (X mode). For this, the experimental kinetic profiles and power fluxes are matched in the gyrokinetic turbulence code GENE, whose output density fluctuations are used in the fullwave simulations with IPF-FD3D to determine the non-linear properties of the microwave plasma interaction.
Previous work [1,2] has concentrated on the perpendicular velocity and the wavenumber spectrum of the turbulent density fluctuations, which will briefly be reviewed.
In this contribution, fullwave simulations are done to take a closer look at how misalignment of the probing beams and diffraction effects affect the interpretation of (Doppler) correlation measurements. Diffraction effects turn out especially to play a role in the cross correlation between temperature fluctuations and reflectometry, because of the different probing frequencies, which lead to different beam paths. The electron temperature fluctuations for this analysis are taken directly from the GENE simulations. First results hint at systematic shifts in the cross phase for misalignments of the reflectometer beam as low as 0.5°. This might in part explain the observed frequency-dependent cross phase in correlation ECE measurements at ASDEX Upgrade [3,4].

[1] C. Lechte, G. Conway, T. Görler, T. Happel, and the ASDEX Upgrade Team. Plasma Sci. Technol., 22 064006, 2020. doi:10.1088/2058-6272/ab7ce8
[2] C. Lechte, T. Happel, K. Höfler, U. Stroth, T. Görler, A. Frank, and the ASDEX Upgrade Team. Poster at the 49th European Conference on Plasma Physics, 2023, Bordeaux
[3] K. Höfler. “Turbulence measurements at the ASDEX Upgrade tokamak for a comprehensive validation of the gyrokinetic turbulence code GENE”. Ph.D. thesis, TU München, 2022
[4] P. A. Molina et al. Phys. Plasmas, 30, 082304. 2023. doi:10.1063/5.0143416

Speaker: Dr Carsten Lechte (Institute of Interfacial Process Engineering and Plasma Technology IGVP, Stuttgart, Germany)

11:30 CECE and nT-phase measurements in ASDEX Upgrade tokamak

Turbulent heat and particle transport are related to the amplitude of density, temperature,
potential fluctuations, and the cross-phase angles between these fluctuating quantities. While cross-phase between the density and temperature might not be directly linked to the transport, it can provide information on the relative changes of turbulent properties with changing plasma parameters and provide insight into the type of turbulence.
The density temperature cross phase (nT-phase) can be measured experimentally by cross-correlating co-located density fluctuation signals from a reflectometer and temperature fluctuation signals from a Correlation Electron Cyclotron Emission (CECE) radiometer [1,2]. A dedicated nT-phase system has been constructed for the first time at ASDEX Upgrade, using a W band X-mode reflectometer and a 24-channel CECE system. Initial measurements with this new system capture the nT-phase of broadband fluctuations in the outer core and pedestal region. Previous nT-phase measurements in AUG were performed using the standard CECE system coupled with Doppler reflectometers operating in normal incidence mode [3]. These measurements include those of the Weakly Coherent Mode (WCM), which is found to have out-of-phase nT-phase in both L-mode and I-mode [4].
In addition, CECE radiometer, toroidally separated from the nT-phase system, is operated routinely for the outer core and edge measurements at standard 2.5 T field configuration. Further upgrades that involve the extension of that system to cover a major portion of the plasma radius and the operation at a low magnetic field are discussed.

[1] Häse et al, Rev. Sci. Instrum. 70, 1014 (1999)
[2] White et al, Phys. Plasmas 17, 056103 (2010)
[3] Freethy et al, Phys. Plasmas 25, 055903 (2018)
[4] Bielajew et al, Phys. Plasmas 29, 052504 (2022)

Speaker: Dr Branka Vanovac (MIT PSFC)

12:00 Correlation reflectometry on Wendelstien 7-X: correlation properties, sensitivity, and prospects for nT-crossphase measurements

The poloidal correlation reflectometer (PCR) diagnostic on Wendelstein 7-X (W7-X)[1,2] is two-frequency system covering the Ka- and U-bands comprised of four receiving antennas and a single transmitting antenna. This diagnostic measures plasma density fluctuations in close proximity to two correlation radiometry antennas (CECE)[3] and two Doppler reflectometer systems (DBS)[4]. The PCR on W7-X is typically used to determine the poloidal flow velocity and correlation length from the signal correlation between adjacent receivers. During operational phase 1 of W7-X (OP1), the PCR antennas were uncooled and the system operated in the Ka-band. During OP2.1, the antennas were integrated into a monolithic water-cooled antenna head and the U-band reflectometer was added. Frequency hopping steps were separated into (1) a fullband scan in 0.5 GHz increments, and (2) a localized scan at 40 GHz with the two reflectometers separated in 0.2 GHz increments. In this work, the correlation properties measured by the two systems will be presented. Additionally, the signals from the receivers are combined to form a coarse phased array antenna system as in [5], with a poloidal steering range limited to approximately ±10 deg. and the wavenumber sensitivity of the diagnostic is investigated. For OP2.2, a water-cooled PCR antenna head has been installed that is optimized to match wave polarization to the magnetic field-line pitch at the last-closed flux surface. When combined with improvements to the CECE, the correlation between plasma density and electron temperature fluctuations may be measured to aid in the characterization of turbulence and transport.
[1] A. Krämer-Flecken et al., Nucl. Fusion 57 066023 (2017)
[2] T. Windisch et al., Plasma Phys. Control. Fusion 59 105002 (2017)
[3] G.M. Weir et al., Rev. Sci. Instrum. In-proc. HTPD 2024 (in-prep.)
[4] T. Estrada et al., Nucl. Fusion 61 046008 (2021)
[5] D. Prisiazhniuk et al., In-proc. 45th EPS (P1.1008)

Speaker: Dr Gavin Weir (Max Planck Institute for Plasma Physics)

12:30 → 13:30 Lunch

13:30 → 15:00 Talks: Reflectometry on W7-X

Conveners: Andreas Krämer-Flecken (Forschungszentrum Jülich), Thomas Windisch (IPP), Emmanouil Maragkoudakis (CIEMAT)

13:30 The Rotation of Quasi coherent Modes in W7-X

For the first time quasi coherent modes (QC-modes) are detected at the isodynamic stellarator W7-X. QC-modes are known from many tokamak experiments. They appear mostly in the plasma core at low collisionallity and some times in the plasma edge. They show up as density fluctuations in the power spectra, as structures with centre frequency ranging from $40\,$kHz$ \le f_c\le 120\,$kHz. Whereas in the plasma core trapped electron modes (TEM) are the cause of the QC-modes, density gradient driven TEMs are supposed to be the drive for the QC-modes in the edge.
At W7-X QC-modes are observed in plasma core of programs with $T_e>T_i$ and low collisionallity. The centre frequency ranges from $100\,$kHz-$250\,$kHz, depending on applied ECRH heating and the magnetic configuration. For certain conditions in the plasma edge such structures at higher frequency ($f_c\approx 800\,$kHz) are observed, too. The modes are detected by Poloidal Correlation Reflectometry (PCR), a set of 5 antenna looking from below the plasma mid-plane at the bean shape plasma cross section into the plasma and measuring density fluctuations allowing for the estimation of the mode-rotation and its properties, simultaneously. The measurement of the coherence spectra allows to discriminate all kind of uncorrelated fluctuations and allows to decompose the coherence spectra in two components (i) the central low frequency turbulence e.g. MHD at the $E\times B$-speed and (ii) the QC-mode rotation. A difference in the rotation speed is observed for the time intervals where the QC-modes are observed. To validate the PCR measurements they are compared with the rotation observed from Doppler Reflectometry and with neoclassical calculations.
The presentation will discuss the results from this analysis.

Speaker: Dr Andreas Krämer-Flecken (IEK Forschungszentrum Jülich)

14:00 Doppler-reflectometry at W7-X: Initial results from the campaign OP2.1

For the Wendelstein 7-X operation phase OP2.1 the Doppler-reflectometry (DR) systems have been upgraded significantly. Steerable mirrors allow for beam steering perpendicular to the local magnetic field to enhance the measured $k_\perp$-range and to investigate flow asymmetries and the poloidal localization of microinstabilities. The operation of three synced DR systems with ordinary mode polarization at two different toroidal positions is used to investigate the radial correlation of fluctuations and to characterize zonal flow components.
In this contribution, inital results from the OP2.1 campaign are presented. For various magnetic configurations and different heating scenarios the measured radial electric field is compared against neoclassical predictions and the measured backscattered power is analyzed and compared to linear growth rate calculations. The observation of edge localized MHD modes is investigated in detail during a rotational transform scan. The diagnostic resolution, sensitivity and limitation is investigated by means of full-wave simulations using CUWA [1]. The input data for CUWA is taken from nonlinear Gene-3D simulations [2] with realistic experimental profiles with kinetic electrons and imposed neoclassical radial electric field. The simulaton data is analyzed to disentangle the low-k contributions of ion temperature gradient-trapped electron mode
turbulence.

[1] P Aleynikov, NB Marushchenko, Comp. Phys. Com., 241, 40 (2019).
[2] F. Wilms, et al. J. Plasma Phys. 87 , 905870604 (2021).

Speaker: Thomas Windisch (IPP)

20240502_FBES.pdf

14:30 Characterization of DR profiles for various Wendelstein 7-X scenarios

This work concerns a newly created database of plasmas from the most recent operational phase of Wendelstein 7-X (October 2022 – April 2023). This database includes a comprehensive assortment of various plasma configurations, densities and heating schemes, for which profiles of the radial electric field ($E_{\rm r}$) and the back-scattered power ($S$) were measured using the V-band DR system located at the AEA-21 port, overlooking the bean shaped plasma cross-section. The database includes gas-puffed ECH plasmas from four island-divertor configurations and one limiter configuration. Additionally, we have examined a set of high-performance NBI-heated plasmas in three island-divertor and one limiter configuration: First, these plasmas are created with ECH heating, followed by a phase sustained solely by NBI and then a final phase with a combination of ECH and NBI. In this last phase, a significant increment of the stored energy is observed indicating a change in the transport properties. We analyze and compare the $E_{\rm r}$ and $S$ profiles for these plasmas alongside those of the gas-puffed pure ECH plasmas. Finally, we present our findings regarding the values of the Er measured in the SOL for all these scenarios and how the formed edge $E_{\rm r}$ shear influences the local density fluctuation amplitude.

Speaker: Emmanouil Maragkoudakis (CIEMAT)

15:00 → 15:30 Coffee break

15:30 → 16:30 Talks: Wendelstein 7-X: ECRH and radiometry

Conveners: Torsten Stange (Max Planck Institute for Plasma Physics), Dr Neha Chaudhary (IPP Bereich Greifswald)

15:30 The quasi-optical ECRH system of Wendelstein 7-X

The standard heating method at W7-X is a steady state capable electron cyclotron resonance heating system with ten 140 GHz gyrotrons and an overall installed power of currently 8.3 MW. The fully quasi-optical transmission line to the plasma vessel is the first of its kind. It consists of a section with single beam waveguide (SBWG) mirrors matching the non-perfect Gaussian beam of the gyrotrons to the subsequent section with broadband multibeam waveguide (MBWG) mirrors allowing low-loss transmission of up to 7 beams with a theoretical overall spurious mode generation of less than 0.2 %. All mirrors were aligned with the high power ECRH beam itself, and only two beams were necessary for the adjustment of the MBWG mirrors demonstrating the nearly perfect imaging properties of the imaging MBWG section that is followed by an additional SBWG section to the individual torus window. The overall theoretical power loss to the window is 6 % including diffraction, beam truncation, misalignment, absorption of the mirrors and the atmosphere. It was proven by reflecting a high power ECRH beam with the aid of a corner cube reflector in front of the torus window. Therefore, the beam passes twice the MBWG section to measure its remaining power in the same calorimeter as the input beam. High power operation in air could be successfully demonstrated by several 1-minute-pulses over a beam path length of 120 m and 31 quasi-optical mirrors.
In the plasma vessel a triple beam pass O2 heating scenario makes use of polarization correcting and holographic reflector tiles to allow high density operation beyond the X2-cutoff density of 1.2 ∙ 10$^{20}$ m$^{-3}$ and achieve an overall absorption of 95 %. To finally achieve average beta values of 4 - 5 % and demonstrate the optimization criteria of W7-X, the installed ECRH power will be doubled in the coming years. For this purpose, the number of gyrotrons will be increased from 10 to 12 and the power per unit from 0.9 MW to about 1.5 MW.

Speaker: Dr Torsten Stange (Max Planck Institute for Plasma Physics)

16:00 ECE diagnostics at W7-X stellarator

W7-X is currently equipped with two ECE diagnostics: a 32 channel heterodyne radiometer measuring X2 mode from 120-160 GHz corresponding to a central magnetic field of 2.5 T with a spatial resolution of 1-2 cm behind cold resonance position and a temporal resolution in the order of $ \mu $s, and a Michelson interferometer measuring the higher ECE harmonics in the spectral range of 50-500 GHz with a temporal resolution of 22 ms and a spectral resolution of 5.66 GHz. Both diagnostics are absolutely calibrated using a time integrated hot-cold calibration technique using a black-body ceramic hot source with a maximum radiation temperature of 600 °C and liquid nitrogen.
Stellarators inherently don't have Greenwald density limit and aim at achieving high confinement using higher $n_e$, hence, the X3 emission was explored for the high density, $n_e$, application of ECE. An X3 radiometer covering 190-220 GHz will be commissioned in the next operational campaign to track $T_e$ for high $n_e$ O2 ECR and NBI heated plasma beyond X2 mode cutoff of $1.2 \times 10^{20} \ m^{-3}$. W7-X is planned to operate at reduced magnetic field of 1.7 T to achieve high $\beta$ and a W-band radiometer is planned to be used for X2-mode measurement for these reduced field operations.
For $T_{e}$ fluctuation measurements and turbulence studies, a 32 channel correlation ECE system is used. Furthermore, a zoom system is available which can be frequency tuned to measure different parts of ECE spectrum with a higher spatial resolution for the purpose of ECRH power deposition measurement using heatwave experiments for understanding and controlling plasma profile shaping.
In addition, the recent observations of strong $T_e$ edge barriers also planned to be probed by the zoom system.
A collective Thomson scattering diagnostic is operated at W7-X in frequency range of 172-176 GHz to measure scattering spectrum of incident radiation from the collective plasma fluctuations.

Speaker: Dr Neha Chaudhary (IPP Bereich Greifswald)

16:30 → 17:00 The Wendelstein 7-X Project

Convener: Prof. Olaf Grulke

17:00 → 18:00 Wendelstein 7-X tour: Torus hall and Periphery

Conveners: Adrian von Stechow (IPP Greifswald, E5), Humberto Trimino Mora (IPP HGW)

19:00 → 21:00 Störtebecker Braugasthaus: Workshop Dinner

Thursday, 16 May

09:00 → 10:30 Talks: Phased array antennas and wavenumber measurements

Conveners: Seong-Heon Seo (Korea Institute of Fusion Energy), Dr Tokihiko Tokuzawa (National Institute for Fusion Science), Laure Vermare (LPP)

09:00 Design of a Doppler reflectometer based on frequency steering phased array antenna

KSTAR Doppler reflectometer is designed based on 32-channel frequency steering phased array antenna (FSPAA) operating in X-mode over the entire V-band (50-75 GHz). In the FSPAA system, the microwave radiation angle can be changed quickly without mechanical movement. This is a very attractive advantage in a fusion reactor environments where the plasma fluctuates continuously and the concept of maintenance-free is important. The microwave radiation angle varies as a function of frequency in the FSPAA. Currently, two 8-channel FSPAA prototypes have been manufactured. The performance of the manufactured FSPAAs are experimentally measured. The radiation angle scans over ±45˚ for frequency changes of several GHz. The angular scan is repeated 9 times across the entire V-band. A prototype of KSTAR Doppler reflectometer is assembled based on a bistatic antenna configuration with the two FSPAAs. The performance of the prototype Doppler reflectometer is tested using a corrugated reflecting wheel.

Speaker: Dr Seong-Heon Seo (Korea Institute of Fusion Energy)

09:30 Wavenumber spectrum S(k, t, r) measurements using a frequency comb Doppler reflectometer in combination with a phased array antenna

The simultaneous oscillation of multiple frequencies and the use of a phased array antenna make it possible to measure the instantaneous radial distribution of the wavenumber spectrum S(k, t, r). To realize this, we are developing the phased array antenna which is additive manufactured by a 3D metal printer. In particular, with regard to surface treatment, we have succeeded in applying copper plating to the antenna made of aluminum material,which was sandblasted and chemically polished in the previous workshop report. In addition, we will also show an example of antenna using stainless steel material.
We plan to combine this phased array antenna with a frequency comb oscillator that can generate multiple frequencies simultaneously, and this development is currently underway. Performance evaluation using an initial test bench will also be presented.

This work was partially supported in part by KAKENHI (Nos. 19H01880, 21H04973, 23H01161, and 23K25858), by a budgetary Grant-in-Aid from the NIFS LHD project under the auspices of the NIFS Collaboration Research Program (ULPP027, LHD115), by the Collaborative Research Programs of Research Institute for Applied Mechanics, Kyushu University and by Collaborative Research Programs of the QST. Additional support was provided by Japan/U.S. Cooperation in Fusion Research and Development, and by "PLADyS", JSPS Core-to-Core Program, A. Advanced Research Networks.

Speaker: Dr T Tokuzawa (NIFS)

10:00 Flow and phase velocity of turbulence in magnetized fusion plasmas

Turbulence and flows play a key role in tokamak plasmas performance.
The aim of the present contribution is to gain insight into the nature of the instability at the origin of the turbulence and its impact on the flow by studying the dynamics of the density fluctuations and by identifying the contribution coming from the “phase velocity” of the density fluctuations. Doppler backscattering (DBS) gives access to the intensity and velocity of density fluctuations at a selected spatial scale. The velocity detected is the sum of the mean flow plasma velocity plus the phase velocity of the density fluctuations. In the most common case, density fluctuations are considered as tracers that are advected by the plasma that give access to the velocity of the plasma flow. In this sense, the intrinsic velocity of the fluctuations, which corresponds to the phase velocity in a linear stability analysis, is considered negligible. This approximation seems appropriate at the edge of the plasma where the mean flow velocity can be large. However, depending on the plasma conditions, this phase velocity can contribute significantly to the measured velocity. An evaluation of its amplitude can be obtained by measuring the evolution of the density fluctuations velocity as a function of the wavenumber, since the mean flow velocity does not depend on the wavenumber. New experiments have been carried out in the WEST tokamak to evaluate the wavenumber dependence of the density fluctuations velocity by changing the probing angle during the stationary phase of a single discharge.
Furthermore, in order to go beyond this amplitude assessment, combined studies using full-wave simulations and results from first-principles numerical simulations in different plasma conditions (close or far above the instability threshold) are performed to characterise the density fluctuation velocity as a function of the wavenumber.

Speaker: Dr Laure Vermare (LPP)

10:30 → 11:00 Coffee break

11:00 → 12:30 Talks: Edge flows and L-H Transitions

Conveners: Sascha Rienäcker (Laboratoire de Physique des Plasmas (LPP), CNRS, Sorbonne Université, École polytechnique, Institut Polytechnique de Paris, Palaiseau, France), José Vicente (Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa), Ondrej Grover (Max Planck Institute of Plasma Physics)

11:00 Edge Flows Studies via Doppler Backscattering on the TCV Tokamak

A Doppler backscattering (DBS) diagnostic from LPP has been installed recently on TCV, enabling an extended characterization of the detailed edge $E_r$ structure on this tokamak. The DBS system consists of two continuous wave, independent V-band channels. It uses a quasi-optical launcher antenna[1], shared with TCV’s short-pulse reflectometry system. The polarization can be changed flexibly between O- and X-mode. The tiltable line of sight, which views the plasma diagonally from the upper low-field side, allows the top to outer mid-plane regions of the plasma cross-section to be probed. The unusual probing geometry offers some flexibility, in particular regarding access to the X-point region in upper single-null plasmas, or the study of poloidal variation of the $E_r \times B$ velocity, but also comes with technical challenges. In this contribution, we report on recent and ongoing experiments utilizing this new DBS system to address the physics of edge $E_r$ “well” formation in L-mode and approaching the L-H transition.
A special focus of these experiments is the role of magnetic topology (favorable vs. unfavorable $B \times\nabla B$ drift) in setting the $E_r \times B$ shear[2,3]. Guided by similar experiments on TORE SUPRA[4], WEST[5,6] and ASDEX Upgrade[7,8], $E_r$ profiles have now been measured on TCV for different topologies and plasma conditions, in Ohmic as well as auxiliary heated discharges. Further studies intended with the new DBS diagnostic include radial correlation measurements and the impact of shaping (especially negative triangularity) on $E_r$.

[1] P. Molina Cabrera Rev. Sc. Instruments (2018)
[2] T. N. Carlstrom et al. PPCF (2002)
[3] B. LaBombard et al. PoP (2005)
[4] P. Hennequin et al. 37th EPS Conference on Plasma Physics (2010)
[5] L. Vermare et al. Nucl. Fusion (2021)
[6] S Rienäcker et al. 27th Joint EU-US TTF Meeting, poster (2023)
[7] J. Schirmer et al. Nucl. Fusion (2006)
[8] U. Plank et al. PoP (2023)

Speaker: Sascha Rienäcker (Laboratoire de Physique des Plasmas (LPP), CNRS, Sorbonne Université, École polytechnique, Institut Polytechnique de Paris, Palaiseau, France)

11:30 Probing ELM Triggers in ASDEX Upgrade with Conventional Reflectometry

Edge Localized Modes (ELMs) have been associated with precursor oscillations that grow in amplitude leading to the ELM crash. However, the presence and characteristics of these precursors vary significantly across different tokamaks, ELM types, and plasma conditions. In ASDEX Upgrade, for example, the conditions for detecting magnetic precursors remain unclear. Nevertheless, signatures of ELM precursors have been reported in the edge electron temperature and electron density.

Recent research employed a synthetic framework to analyze the response of conventional O-mode reflectometry (simulated using the REFMUL code) during a Type-I ELM cycle reproduced by non-linear magnetohydrodynamic simulations (JOREK code). These simulations identified growing non-axisymmetric perturbations as ELM precursors, which were also linked to density fluctuations detectable by the synthetic reflectometer operating in a fixed-frequency mode. Building on this work, we investigate further the characteristics of the ELM triggering mechanisms and their link to density fluctuations. Additionally, we report on experimental efforts to use the conventional O-mode diagnostics at ASDEX Upgrade in probing ELM triggers.

Speaker: Dr José Vicente (Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa)

12:00 Innovative Doppler backscattering spectra analysis methods applied to comb reflectometer measurements in ASDEX Upgrade

The W-band tunable 7-channel Comb reflectometer (P. A. Molina Cabrera et al., Rev. Sci. Instrum. 94, 083504 (2023)) is used for simultaneous measurements of Doppler backscattering (DBS) spectra at several radial locations in ASDEX Upgrade, enabling the study of dynamical events such as the L-H transition. To facilitate the analysis of the resulting large volume of spectra from short time windows with a limited signal/noise ratio, the common DBS spectra fitting approaches are examined in terms of how reliably they can be automated and improvements are proposed.
Specifically, the common fit of the asymmetric decomposition of DBS spectra which avoids the m=0 reflection component is improved by additionally enforcing the positivity of the latter. This avoids common issues of convergence to unrealistic solutions when the width of the Doppler peak becomes comparable to its shift. Additionally, the formulation of the problem as a forward model enables the use of a non-gaussian likelihood and an associated chi-squared-like statistic more appropriate for measured spectra (S. Baker and R.D. Cousins, Nuc. Inst. And Methods in Phys. Research 221 (2), 437-442 (1984)), which in turn is shown to improve the fit accuracy.
Finally, the dynamics and temporal ordering of DBS spectra parameter changes across the L-H transitions are presented.

Speaker: Ondrej Grover (Max Planck Institute of Plasma Physics)

12:30 → 13:30 Lunch

13:30 → 15:00 Talks: Plasma position reflectometry (PPR)

Conveners: Jorge Santos (Instituto de Plasmas e Fusão Nuclear - Instituto Superior Técnico), Stéphane Heuraux (University of Lorraine, Institut Jean Lamour), Filipe da Silva (Instituto Superior Técnico-Instituto de Plasmas e Fusão Nuclear)

13:30 Integrated Microwave Diagnostic Design Workflow - a PPR antenna design use case

This work presents the development and application of an advanced engineering design workflow for the planning, design, and assessment of future fusion plasma diagnostics. The workflow leverages advanced simulation techniques and synthetic diagnostics for system performance evaluation and prediction, crucial in the decision-making process of the design cycle. A key component of this workflow is the integration of realistic representations of system components, including wave launcher structures and vessel wall structures. The procedure involves the use of CAD models of these structures, combined with parameterizable CAD models of the launcher, to produce a description suited for Finite Difference Time Domain (FDTD) 3D simulation. They can include electron density data from plasma scenarios obtained from proven state-of-the-art integrated modeling codes such as JINTRAC for the core plasma, and SOLEDGE2D-EIRENE or SOLPS-ITER for the scrape-of-layer and pedestal region. All used CAD models are shared between the wave propagation code and thermal, structural and neutronic simulators, being iterated according to the analysis of the various simulation results. The validation of this workflow is demonstrated through the design of a set of antennas for the DTT’s High-field-side (HFS) Plasma Position Reflectometry (PPR) system. Given the severe space and access constraints, optimized small-footprint bistatic and monostatic hog-horn antenna designs were proposed. The viability of the PPR implementation was confirmed through 3D full-wave simulations, laboratory measurements of a 3D-printed bistatic antenna prototype, and a preliminary thermal analysis of the antenna embedded in the plasma-facing wall structures under standard plasma operation conditions. This use case illustrates the power of the proposed workflow in addressing complex design challenges in the field of fusion plasma diagnostics.

Speaker: Dr Jorge Santos (Instituto de Plasmas e Fusão Nuclear - Instituto Superior Técnico)

14:00 Synthetic wave tomograph based on DEMO Plasma Positioning Reflectometer

Under the framework of an enabling research EUROfusion project, it was possible to explore the concept of µwave tomograph based on the Plasma Positioning Reflectometers (PPR) system planned to measure the plasma shape in a DEMO poloidal cross section from the early start-up phase to flat top and then the ramp-down phase. The possibility to do it in real-time is also considered based on the use of AI tools, but it requires a database of learning cases. Here the focus is put on how to build the needed database. The only possibility to build it at this moment is by using a raytracing code, considering the DEMO size. However, the synthetic tomograph elaboration requires the knowledge of the radiation patterns of each reflectometer antenna. According to the typical antenna structure, a full-wave simulation is needed to determine it and use it as an input parameter describing the launching beam in the ray tracing code at the emitter antenna mouth. After running the ray tracing code, the lightening reflectometers are identified for computing the electric field map at the receiver antenna mouth. This reconstruction of the electric map at the receiver is mandatory for determining the electric field and phase reaching the detector placed in a waveguide, assuming that only fundamental mode propagates. All the used procedure will be presented in details. Then a comparison in academic cases between full-wave code results and our synthetic model based on raytracing code was performed showing that the amplitude and phase have enough accordance to be used to describe the behavior of the DEMO microwave tomograph. Other information will be provided on the feasibility of such DEMO µwave tomograph such as the required source power, among others. Few words on a foreseen experimental on SPEKTRE device dedicated to the validation of the methodology in the case of the density profile reconstruction using oblique incidence will be also given.

Speaker: Prof. Stéphane Heuraux (University of Lorraine, Institut Jean Lamour)

14:30 Confronting simulations of the performance of an equatorial HFS PPR for DTT obtained using 2D and 3D synthetic diagnostics

Reflectometry, compatible with a full grade reactor implementation, has been proposed as a source of real-time (RT) plasma position and shape measurements for control purposes, in replacement or complement of standard magnetic measurements, to ensure reliability and safety on the machine. This new control technique, based on multiple, poloidally distributed, non-magnetic measurements, must be tested, in all of its aspects, before it can be fully implemented in future fusion reactors. The Divertor Test Tokamak (DTT) facility will enable the testing and validation of key non-magnetic control diagnostics, such as reflectometry, to assist with DEMO design implementation. To predict the behavior and capabilities of these new reflectometry systems, we propose a comprehensive simulation approach using finite-difference time-domain (FDTD) time-dependent code, including aspects such as propagation in realistic plasmas, the system location based on a CAD model of the vacuum vessel, and accesses to the plasma. Notwithstanding the comprehensive description of reflectometer diagnostics, the use of FDTD codes is computationally intensive, in particular if three-dimensional (3D) codes are used to evaluate the performance of the foreseen diagnostic, which requires access to HPC facilities. This fact makes the use of two-dimensional (2D) codes much more common. It is therefore important to have a good evaluation of the compromises made when using a 2D model in order to decide whether it is applicable to the problem under study, or if the problem rather requires a 3D approach. The present work makes a benchmark assessment of 2D against 3D of the behaviour and capabilities of a design for an Ordinary mode Plasma Position Reflectometer (PPR) system. In particular, this is done for an equatorial line of sight at the High Field Side (HFS) on DTT for a foreseen Single Null plasma scenario. The corresponding, synthetic diagnostics were set up using the 2D REFMULF and the 3D REFMUL3 codes.

Speaker: Dr Filipe da Silva (Instituto Superior Técnico-Instituto de Plasmas e Fusão Nuclear)

Talk_16thIRW_FdSilva_3Dvs2D.pptx

15:00 → 15:20 Coffee break

15:20 → 17:00 Talks: Closing session, Coffee and Cakes