Effect of ECRH regime on characteristics of short-wave turbulence in plasma of the L-2M stellarator

Abstract

The present paper reports on studies of shortwave turbulence in plasma of L-2M stellarator under markedly different conditions: with doubling the ECR heating power (100 and 200 kW) and with restricting the plasma radius by a sector limiter. The role of such shortwave turbulence in anomalous transport can appear important for conditions of a thermonuclear reactor. Experiments were carried out in a basic magnetic configuration of the L-2M stellarator during ECRH at the second harmonic of the electron gyrofrequency (75.3 GHz) at average electron densities of 1.5-1.7⋅10 13 cm-3. The energy lifetime was ~3.5 ms at Р 0 = 100 kW and was reduced to ~2 ms at Р 0 = 200 kW. When the limiter was introduced inside the plasma to a depth of 2 cm from the last closed flux surface, τ Е decreased by a factor of 1.3-1.4. Plasma density fluctuations were measured from the scattering spectra of gyrotron radiation at the second harmonic of operating frequency (~150 GHz). A quasioptical receiving system allowed measurements of scattered radiation from plasma regions r/a≤0.6 at scattering angles π/4≤0≤π/2 (24 cm-1 ≤k ⊥ ≤44 cm-1). The shortwave turbulence was studied for two radial positions of the scattering region: r/a=0.3-0.4 and r/a= 0.5-0.6. Shortwave turbulence observed in the core plasma of the L-2M stellarator is assigned to the ETG mode instability. This type of turbulence exhibits features of strong plasma turbulence. It is experimentally established, that a change in the energy lifetime in L-2M stellarator correlates with the level of shortwave turbulence.

Quasi-optical approach to reconstruction of plasma fluctuations using amplitude distribution of transmitted microwave beam

“…The first one is based on the microwave reflectometry which basically allows the reconstruction of the turbulence spectrum by the spectrum of the scattered signal in the far zone [1]. The second one is phase contrast imaging (PCI), basing on reconstruction of the turbulence distribution by phase distortion of passing laser beams in geometrical optics zone [2].…”

The evolution of probability characteristics of low-frequency plasma turbulence

“…The processes of plasma microturbulence play a decisive role in the processes of heat and particle trans port in toroidal magnetic confinement systems (tokamaks and stellators) used for investigating the plasma for controlled thermonuclear synthesis [1,2]. During the past few years, investigations of the microturbu lence of the edge (near wall) plasma revealed deviations of the distribution functions of fluctuation ampli tudes and their increments from the normal (Gaussian) law, see, e.g., [3,4].…”

On the investigation of plasma turbulence by the analysis of the spectra

“…Исследования низкочастотной структурной турбулентности проводятся на стеллараторе Л-2М методами доплеровкой рефлектометрии [Pshenichnikov, Kolik, Malykh, Petrov et al, 2005] и рассеянием излучения второй гармоники гиротрона на флуктуациях [Skvortsova, Akulina, Batanov et al, 2010]. Комплексные фурье-спектры, измеренные рефлектометром на краю плазмы, изменяются разительно в интервале стационарных макропараметров плазмы.…”