Plasma Dynamical Processes and Turbulence Studies using Advanced Microwave Diagnostics
Small scale (electron) turbulence structure (AUG, TCV)
Background: Particle and energy losses in magnetically confined fusion plasmas are of particular importance for future devices, since they determine the particle and energy input to the first wall and also the overall plasma performance. Although significant progress in understanding turbulent transport in the ion-channel has been achieved experimentally, experimental understanding of electron thermal transport is lagging behind. This lack of knowledge is both due to the fact that it is particularly challenging to measure turbulence in the electron channel, and that historically only limited interest was dedicated to electron transport research, since it was believed to cause only a fraction of the losses caused by ion-scale transport. However, in the last decade the importance of electron transport has received new attention, in fact it is believed that a significant amount of thermal transport can be caused by so-called streamers, which are observed in simulations.
Approach: During the last 15 years Doppler reflectometry has emerged as a promising scale-selective turbulence diagnostic, which measures the perpendicular velocity of density fluctuations u_perp and their turbulence level S(k_perp), where k_perp is the perpendicular wavenumber of the turbulent density fluctuations. Doppler reflectometry is based on the backscattering of a microwave beam off density fluctuations at a reflection layer in the plasma. In contrast to conventional reflectometry, where the microwave beam is launched perpendicular to the reflection layer into the plasma, in Doppler reflectometry the probing beam is launched obliquely with respect to the reflection layer normal. This so-called tilt angle is of particular importance for the technique, since it determines the turbulence scale from which the microwave is back-scattered. If a system allows tilt angle scans, different turbulence scales k_perp can be investigated. By scanning the probing beam frequency, the radial position of the reflection layer is changed, thus radial profiles can be acquired.
Goal: Doppler reflectometers with steerable antennas and steppable frequencies have been developed at both ASDEX Upgrade and Tore Supra (now installed on TCV) to explore a broad wave-number spectrum of turbulence up to small scales, associated with electron driven turbulence modes. Turbulence responses in different scenarios, such as different heating powers or deposition locations, will be monitored at different scales, such that responses in ion- and electron-channels can be investigated independently.
Project lead by IPP & LPP:
Dr. Tim Happel
Dr. Laure Vermare