Speaker
Description
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
