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Description
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)
