M. S. Berezhetskii

Effect of Vacuum Chamber Boronization on the Plasma Parameters in the L-2M Stellarator

After boronization of the vacuum chamber of the L-2M stellarator, radiative losses from ohmically and ECR heated plasmas were reduced by a factor of 3–4. Under these conditions, radiative losses in the ECRH regime comprise only 10–15% of the input microwave power. Some effects have been detected that were not observed previously: a substantial increase in the gradient of the electron temperature near the separatrix, a preferentially outward-directed radial turbulent particle flux (both throughout the discharge phase and from shot to shot), and a longer (by a factor of 2–3) duration of the plasma cooling phase.

Testing of the method for water microleakage detection from OH hydroxyl spectral lines at the L-2M stellarator

Results are presented from L-2M stellarator experiments on testing a possible method for detection of water microleakages in the cooling system of the first wall and vacuum chamber of ITER. The method consists in the spectroscopic detection of spectral lines of the OH hydroxyl, which forms via the dissociation of water molecules in plasma. Emission in the spectral band of 305–310 nm can be detected even at water leakage rates less than 10−4 Pa m3/s. Chemical reactions between water and boron compounds on the vacuum chamber wall delay the detection of leakages up to ∼2000 s. A similar phenomenon can be expected when a leakage will occur in ITER, where the materials suggested for the first wall (Be, Li) can also chemically react with water.

Heat wave modulation experiments in the L-2M stellarator

The heat wave modulation was used in ECRH experiments in the L-2M stellarator to determine the absorption region and the thermal diffusion coefficient. The temperature response of the plasma was followed by ECE and SXR diagnostics. It is shown that the absorption region has a nonlocal character, and its size is dependent on the magnetic field. The estimated value of the electron thermal diffusivity coefficient turns out to be higher than expected.

Effect of the transverse magnetic field on turbulence and parameters of a plasma column in the L-2M stellarator

The influence of magnetic configurations with magnetic hills or wells on the parameters of a plasma column and turbulence characteristics were studied in experiments in which the plasma was created and heated by a microwave beam at the second harmonic of the electron cyclotron frequency. Calculations show that, for 〈β〉=(1.5−2)×10−, a configuration with a magnetic well takes place and the Mercier criterion for stability of the ideal MHD modes is satisfied. It is shown that the compensation of the Shafranov shift of the plasma column by a transverse (vertical) field (Bv/B0=5×10−3) leads to a configuration with a magnetic hill in which the Mercier stability criterion is violated in the central region of the plasma column. It is experimentally shown that the stored plasma energy in the magnetic-hill configuration is reduced by one-half in comparison with the magnetic-well configuration. In the case of a magnetic hill, the energy of fluctuations increases both in the plasma core and near the separatrix, and the quasi-regular components of the wavelet spectra grow. When the Shafranov shift is compensated only partially (Bv/B0∼3×10−3) and the system is near the instability threshold, the stored plasma energy and the central electron temperature are somewhat higher, and the radiation power of fast electrons from non-Maxwellian tails at the second harmonic of the electron gyrofrequency decreases. It is found that the wavelet spectra of fluctuations change, the coherence coefficient for spectral components increases, and the radial electric field near the separatrix decreases.

Effect of unstable MHD modes on the confinement of a stellarator plasma

The results of an experimental study of the influence of unstable MHD modes on plasma confinement in an L-2M stellarator are presented. The spectral and statistical characteristics of turbulent plasma simultaneously at both the edge and center of the plasma are investigated. It is shown that at constant power of electron-cyclotron heating of the plasma the energy content of the plasma depends strongly on the strength of the externally applied vertical magnetic field used to adjust the position of the plasma column. Appreciable degradation of plasma confinement is observed for values of the vertical field such that ideal MHD modes become unstable in the greater part of the plasma column. This in turn is due to the formation of a magnetic configuration with a magnetic “hump.” At the same time, in the edge plasma the instability of resistive-balloon modes grows, and turbulent particle transport increases.

Structure of the plasma and energy balance near the limiter in the L-2 stellarator

Abstract The results of a study of the properties of the edge plasma in the L-2 stellator are reported. The edge plasma can be divided into three radial regions: a low density region between the wall and the separatrix (ne ≤ 1 × 109 cm−3), a high gradient region near the separatrix and a region inside the separatrix zone with density ~ne ≤ 1 × 1012 cm−3. Edge turbulence is found to be sensitive to the mode of plasma heating. Edge plasma data from the stellarator is compared with that from the TV-1 tokamak. An energy balance in the near limiter zone of the stellarator is analysed from experimental measurements of the radial heat flux profiles and also from numerical simulations based on a theoretical model. It is shown that the experimentally measured limiter efficiency is well described theoretically.

Measurements and monitoring of the hydrogen and deuterium contents in the plasma of the L-2M stellarator

The program of experiments on ITER includes a sequential change of the plasma isotopic composition from pure hydrogen plasma in the initial stage of research to deuterium and, then, deuterium-tritium plasma with a gradual increase in the tritium content. In this context, the influence of the plasma isotopic composition on the processes of plasma heating and confinement are being actively studied on the existing tokamaks and stellarators. The plasma isotopic composition also depends on the composition of the gas desorbed from the vacuum chamber wall in the course of recycling. Therefore, the rate of change of the plasma isotopic composition after altering the injected gas also depends on the rate of change of the isotopic composition of the gas absorbed in the wall. These effects were studied in the experiments carried out on the L-2M stellarator in which the working gas was changed from hydrogen to deuterium. Spectral measurements of the intensity ratio between the Hα and Dα lines made it possible to monitor the isotopic composition of the plasma in the course of cleaning of the chamber wall from earlier absorbed hydrogen and its replacement with deuterium. After returning to hydrogen, the rate of cleaning of the wall from deuterium was also determined. The results of these experiments show that the plasma isotopic composition varies exponentially with the number N of shots after transition to another isotope, ∼exp(−N/47). Hence, the isotopic composition can be changed almost completely over 2 to 3 working days. This allows one to study the influence of the plasma isotopic composition on plasma confinement during the same experimental session.

Study of plasma confinement in the L-2M stellarator during the formation of an edge transport barrier

A plasma confinement mode characterized by the formation of an edge transport barrier (ETB) was discovered in the L-2M stellarator after boronization of the vacuum vessel wall. The transition into this mode is accompanied by a jump in the electron temperature by 100–200 eV at the plasma edge and a sharp increase in the gradient of the electron temperature Te in this region. The threshold power for the transition into the ETB confinement mode with an increased electron temperature gradient is Pthr▿Te = (60 ± 15)ne [1019 m−3] kW. The formation of the ETB manifests itself also in a substantial change in the electron density profile. A density peak with a steep gradient at the outer side forms at the plasma edge. The threshold power for the transition into the ETB confinement mode corresponding to a substantial increase in the plasma density gradient near r = a is Pthr▿Te = (67 ± 9)ne [1019 m−3] kW, which agrees to within experimental error with the threshold power for the transition into the ETB confinement mode determined from the sharp increase in the gradient of the electron temperature Te. The value of Pthr for the L-2M stellarator agrees to within 25% with that obtained from the tokamak scaling. In the ETB confinement mode, the plasma energy W and the energy confinement time τE determined from diamagnetic measurements increase by 20–30% as compared to those obtained from the stellarator scaling for the confinement mode without an ETB. When the heating power increases by a factor of 2–3 above the threshold value, the effects related to improved energy confinement disappear.

CHARACTERISTIC FEATURES OF THE BEHAVIOR OF ECRH-PRODUCED MODERATE- AND LOW-DENSITY PLASMAS IN THE L-2M STELLARATOR

D. K. Akulina, G. M. Batanov, M. S. Berezhetskii, G. S. Voronov, G. A. Gladkov, S. E. Grebenshchikov, I. S. Danilkin, N. P. Donskaya, L. V. Kolik, N. F. Larionova, A. I. Meshcheryakov, K. A. Sarksyan, 0. I. Fedyanin, N. K. Kharchev, Yu. V. Khol’nov, and S. V. Shchepetov Institute of General Physics, Russian Academy of Sciences, ul. Vavilova 38, Moscow, I19991 Russia, e-mail: akulina@pl.gpi.ru Abstract Results are presented from experimental studies of the behavior of plasmas with moderate ( n , l 3 x m”) down to low (n, = 0.3 x I O l 9 m”) densities produced and heated by microwaves If= 75 GHz) with power PS 400 kW It is shown that, as the plasma density is reduced, the plasma emission spectrum is modified. In spite of the fact that the second harmonic emission from the plasma increases in this case, the plasma energy measured by the diamagnetic diagnostics does not increase. This casts doubts on the correctness of ECE measurements of the plasma temperature under these conditions. The formation of an internal transport barrier was not observed in the available magnetic-field structure of L-2M.

Influence of the Plasma Density and Heating Power on the Intensity of Electron Cyclotron Emission in the L-2M Stellarator

Results are presented from experiments on studying the plasma behavior in the L-2M stellarator in regimes with a high power deposition in electrons during electron cyclotron heating at the second harmonic of the electron gyrofrequency (X mode) at heating powers of Pin=120–400 kW and average plasma densities from ne≤3×1019 to 0.3×1019 m−3. It is shown that, as the plasma density decreases and the heating power increases, the electron cyclotron emission spectrum is modified; this may be attributed to a deviation of the electron energy distribution from a Maxwellian and the generation of suprathermal electrons. At low plasma densities, the emission intensity at the second harmonic of the electron gyrofrequency increases, whereas the plasma energy measured by diamagnetic diagnostics does not increase. This poses the question of the correctness of determining the plasma electron temperature by electron cyclotron emission diagnostics under these conditions.