7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Validation of short pulse reflectometry diagnostic turbulence measurements inthe TCV tokamak with synthetic diagnostic based on gyrokinetic modeling
O. Krutkin1, U. Kumar1, S. Mazzi2, S. Brunner1, S. Coda1, M. van Rossem1 and the TCV team*1
Swiss Plasma Center, École Polytechnique Fédérale de Lausanne
2Institut de recherche sur la fusion magnétique, CEA Cadarache*See Reimerdes et al 2022 (https://doi.org/10.1088/1741-4326/ac369b).
e-mail (speaker): oleg.krutkin@epfl.ch

A Short Pulse Reflectometer (SPR) diagnostic has beendeveloped for the TCV tokamak [1]. It utilizes short (~1ns) microwave pulses to probe plasma in the presence ofa cut-off. Pulse delays corresponding to different probing
frequencies are then used to reconstruct the electron
density profile similar to conventional fast-sweeping
reflectometry. Recently, a fluctuation SPR (FSPR)
method for obtaining the amplitude and radial correlation
length of the turbulence from statistical properties ofpulse delays was developed [2]. The method is expected
to possess advantages over traditional fluctuation andradial correlation reflectometry. It was validated withfull-wave numerical modeling utilizing ad-hoc
turbulence with a simplified geometry.
Within this work, the first experimental application of
the FSPR is presented for shots with L-mode Ohmic
plasma. In particular, measurements are carried out inshaped plasma with positive and negative triangularity(PT and NT respectively), reproducing the decrease of
the turbulence amplitude previously observed with otherturbulence diagnostics [3, 4]. The difference in measured
turbulence amplitude is presented in Fig. 1, where thenormalized fluctuation amplitude in a wide radial range
is shown for the two selected pulses.

modified to have a pulse microwave source and a
Gaussian antenna pattern close to the experimental one.
For each timestep of the GENE output a synthetic pulse
delay was computed resulting in over 1000 samples foreach probing frequency. The FSPR method was thenapplied and geometrical effects were shown to not be
responsible for the measured difference between PT andNT.
Finally, the GENE output was rescaled to include a radial
variation of turbulence amplitude as a way to overcome
the limitations of the flux-tube model. This allowed us toobtain a direct match between synthetic andexperimental FSPR results, confirming experimental
measurements and further validating the FSPR method.
References[1] P. A. Molina Cabrera et al., Review of Scientific
Instruments 90.12 123501 (2019)[2] O. Krutkin et al., Nuclear Fusion 63.7 076012 (2023)[3] Z. Huang et al., Plasma Physics and Controlled
Fusion 61 014021 (2019)[4] M. Fontana et al., Nuclear Fusion 60 016006 (2020)[5] P. Aleynikov and N. B. Marushchenko, Computer
Physics Communications 241 40–47 (2019)

During the previous validation efforts [2], the FSPR
method was shown to be particularly sensitive to thecurvature of the cut-off surface. This curvature wasdifferent between PT and NT discharges in the probing
area, casting doubt on the obtained measurement. Thusto further validate the method under conditions asrealistic as possible and confirm the interpretation ofexperimental results, a synthetic TCV SPR diagnostic
was developed.
First, realistic density fluctuations related tomicroturbulence were computed with the flux-tube
version of the gyrokinetic GENE code. Simulations wererun with the experimental magnetic equilibrium anddensity and temperature profile and included collisions
and impurities. Profiles were varied within experimental
error bars to match the experimental heat fluxes.
Next, density fluctuations produced by GENE together
with the experimental density profile were used as an
input for the full-wave code CUWA [5]. The code was

Figure 1. Radial profiles of turbulence amplitude
measured in NT (blue line) and PT (red line) discharges
using FSPR.