Plasma was heated at the second harmonic of electron cyclotron resonance (ECR) in the L-2M stellarator and the T-10 tokamak. The concept of equivalent tokamak and stellarator discharges was extended to the case of both full and partial absorption of EC power. Comparison of experimental electron temperature profiles with profiles calculated using the canonical profiles transport model allows us to estimate the efficiency of ECR heating in the L-2M discharges without suprathermal electrons, which distort the distribution function, preventing reliable measurements of temperature. The dependence of the ECR heating efficiency on the plasma density was obtained, describing experiments on the L-2M and TJ-II stellarators, and on the T-10 tokamak. The energy characteristics (the stored energy and the confinement time) for L-2M discharges were calculated. Predictions for ECR heating in the T-15MD tokamak are considered. The features of solving the ill-posed transport problem for the L-2M are discussed.

2D and 3D simulations of the penetration of neutral particles into the plasma with parameters corresponding to the ohmic heating regime in the L-2M stellarator were performed, and the simulation results were compared. Radial distributions of neutrals density in plasma and model energy spectra of fluxes of charge-exchange atoms escaping from the plasma were obtained. For the conditions of the ohmic heating regime in the L-2M stellarator, the limiting plasma densities were determined, above which it is necessary to take into account the recombination processes when performing simulations. Comparison of the model energy spectra of fluxes of charge-exchange atoms escaping from the plasma with experimental data made it possible to construct the radial distributions of the neutrals density in absolute units. In this case, the calculated density of neutrals at the plasma axis turned out to be ~10 15 m –3 , which is four orders of magnitude less than the density of charged particles 10 19 m –3 .

Выполнено 2D- и 3D-моделирование проникновения нейтральных частиц в плазму с параметрами, соответствующими режиму омического нагрева в стеллараторе Л-2М, и проведено сравнение результатов моделирования. Получены радиальные распределения плотности нейтралов в плазменном шнуре и модельные энергетические спектры потока атомов перезарядки, вылетающих из плазмы. Для условий омического нагрева в стеллараторе Л-2М определены значения плотностей плазмы, выше которых необходимо учитывать при моделировании процессы рекомбинации. Сравнение модельных энергетических спектров потока атомов перезарядки, вылетающих из плазмы, с экспериментальными данными позволило построить радиальные распределения плотности нейтралов в абсолютных единицах. При этом расчетное значение плотности нейтралов в центре плазменного шнура составило около 10 15 м –3 , что на четыре порядка меньше плотности заряженных частиц 10 19 м –3 .

Results of measuring longitudinal electric current excited in the toroidal plasma of the L-2M stellarator as a result of high-power pulsed microwave heating (power of up to 600 kW, pulse duration of up to 20 ms) are presented. Microwave radiation generated by gyrotrons and at frequency of 75 GHz, the second harmonic of the electron cyclotron resonance for magnetic field with induction B = 1.34 T in the plasma-column center, was used in the experiments for creation and heating of plasma in the stellarator. Diagnostic systems of the stellarator designed for detection of changes in transverse and poloidal magnetic fields with time were used for measuring currents in plasma. It is demonstrated that the presence of ohmic-heating iron transformer in stellarator construction substantially affects the temporal dynamics of equilibrium currents due to considerable inductance of the toroidal plasma column. The magnitude of the current excited in plasma can be as high as 7 kA upon compensation of inductance of these devices.

The experimental results are presented from the experiments on the high-power electron cyclotron resonance heating (ECRH) of the current-free plasma with the power 200–500 kW (the specific power was 0.8–2 MW/m3). The spontaneous transport transitions were observed that resulted in an increase in the plasma density and energy. At the ECRH powers exceeding 150 kW, the processes are observed accompanied by the abrupt changes in the plasma edge parameters, while the core plasma parameters change only slightly. At the ECRH powers exceeding 400 kW, a jump-like increase in the plasma density and energy is observed. At a power of approximately 500 kW, the regime was obtained, in which at time of transition, the plasma energy and lifetime increase by 20%, despite a decrease in the electron temperature. At the same time, at heating powers up to 700 kW, the energy lifetime corresponds on average to the L-2M single-machine stellarator scaling.

ИМПУЛЬСНО-ПЕРИОДИЧЕСКИЙ РЕЖИМ ЭЦР-НАГРЕВА ПЛАЗМЫ В СТЕЛЛАРАТОРЕ Л-2М

In experiments on multipulse on-axis electron cyclotron resonance heating (ECRH) of plasma by a series of microwave pulses at the L-2M stellarator, several phases of plasma energy loss were observed: the short stage of low-energy loss, the stage of rapid increase in energy loss, the quasi-steady stage, and the relaxation stage between the heating pulses. In the stage of rapid increase in energy loss, the energy loss power is two or more times higher than that in the relaxation stage at the same energy of the plasma column. Short-wavelength plasma density fluctuations were measured using both the ordinary and extraordinary microwave collective scattering technique. It is found that, in the quasi-steady stage, the amplitude of density fluctuations is much lower than that in the preceding heating stages. The fluctuation amplitude lowers just after the restructuring of the density profile and establishment of a steady-state hollow density profile due to the density pump-out effect. The amplitude of large-scale density fluctuations at the plasma periphery recorded by a Doppler reflectometer remains unchanged during the ECRH pulses and in the time intervals between them. However, when the stage of rapid increase in energy loss begins, the shape of the density fluctuation spectrum changes significantly. The initially narrow spectrum with one peak near the zero frequency broadens, the amplitude of the central peak decreases, and two additional peaks at frequencies of 0.7 and −0.7 MHz appear.

L-2M stellarator plasmas G.M. Batanov, V.D. Borzosekov, S.E. Grebenshchikov, N.K. Kharchev, A.A. Kharchevsky, Yu.V. Kholnov, L.V. Kolik, E.M. Konchekov, A.A. Letunov, A.E. Petrov, N.N. Skvortsova V.D. Stepakhin, D.G. Vasilkov A.M. Prokhorov General Physics Institute RAS, Moscow, Russia After recent upgrade of the electron-cyclotron resonance heating (ECRH) system of the L-2M stellarator it is possible to study plasma dynamics at 100% modulated ECRH. Such modulation implies that heating goes in a form of a sequence of microwave pulses and between pulses high-temperature current-free plasma is confined without auxiliary heating. One of many interesting aspects to study in this operational regime is evolution of energy losses namely the steep increase of energy losses that happens shortly after (~ 1 ms) start of each heating pulse (Fig.1).

Impurity injection into plasma caused by the sputtering of the wall coating in the L-2M stellarator during auxiliary electron cyclotron resonance heating leads to a change in the level of plasma density fluctuations with frequencies above 0.25 MHz: suppression of long-wavelength (k ⊥ = 2 cm–1) density fluctuations in the edge plasma, intensification of short-wavelength (k ⊥ = 30 cm–1) and long-wavelength (k ⊥ = 1 cm–1) fluctuations at the midradius of the plasma column, and intensification of short-wavelength fluctuations (k ⊥ = 20 cm–1) in the plasma center (including the gyroresonance region). At the same time, the level of fluctuations with frequencies below 0.25 MHz remains unchanged. In the edge plasma, a decrease in the plasma potential and suppression of its fluctuations is observed during impurity injection, which also causes an increase in MHD activity.

The experiments on ECR plasma heating were carried out at the L-2M stellarator at very high volume power density (up to 3.0 MW/m3). Under these conditions, non-monotonous hollow density profiles were measured. At the maximum heating power of P = 0.75 MW, the concavity in the axial region becomes drastic ne(0)/nemax = 0.5. In these experiments, plasma temperature profiles measured in the axial plasma region r/ap < 0.4 occur to be rather flat. We have also measured two-slope SXR spectra in the range from 2 to 12 keV. The possible reasons for these observed phenomena were discussed. However, with growing ECRH power up to PECRH = 0.75 MW, we have not observed dramatic deterioration of plasma confinement at the L-2M stellarator. The measured energy lifetime is generally consistent with that determined from the international LHD scaling.

currentless plasma of the L-2M stellarator after switching-on of auxiliary ECR heating G.M. Batanov, M.S. M.S. Berezhetskii, V.D. Borzosekov, S.E. Grebenshchikov, I.A. Grishina, N.K. Kharchev, A.A. Kharchevskii, Yu.V. Khol’nov, L.V. Kolik, E.M. Konchekov, N.F. Larionova, A.A. Letunov, V.P. Logvinenko, D.V. Malakhov, A.I. Meshcheryakov, A.E. Petrov, K.A. Sarksyan, S.V. Shchepetov, N.N. Skvortsova, V.D. Stepakhin, I.Yu. Vafin, D.G. Vasilkov A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow

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 m 3 /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.

The characteristics of a new MIG-3 gyrotron complex for creating and heating plasma in the L-2M stellarator are presented. The first experimental results using the complex are reported. The complex consists of two three-electrode GYCOM gyrotrons of the new generation with electron beam energy recuperation, a high-voltage modulator that enables both separate and simultaneous operation of the two gyrotrons, and a control/data-recording unit. The total specific power to be inserted into plasma reaches 5 MW/m3 when both gyrotrons in operation.

Results of testing of a possible method for location of water microleakages in the cooling system of the first wall and vacuum chamber of ITER are presented. The method consists in spectroscopic detection of the emission lines of atoms and ions of the Xe additive dissolved in water. These lines are excited when the water with dissolved Xe contacts the plasma. The high electron cyclotron resonance heating (ECRH) power deposited in a relatively small plasma volume in the L-2M stellarator ( P = 0.5 MW, V = 0.24 m 3 , and the specific heating power ∼2 MW/m 3 ) makes it possible to achieve plasma parameters close to those in the edge plasma of ITER for different operating modes, including the H-mode with an edge transport barrier. In test experiments, several lines of Xe ions were revealed suitable for detection of xenon in plasma with parameters close to those in the edge plasma of ITER at leakage rates at a level of ∼10 −6 Pa m 3 s −1 and spatial resolution of ∼0.5 cm.

Possible parameters of a plasma in a compact torsatron that is to be constructed at the Prokhorov Institute of General Physics, Russian Academy of Sciences (the L-5 project) are discussed. The properties of the original vacuum configuration created by the external coils are described. The equilibrium of a plasma with a free boundary and the stability of local MHD modes are investigated. The effective magnetic field ripples and the structural factor of the bootstrap current in the 1/ν regime are calculated, as well as collisionless losses of trapped α-particles. The dependence of these properties on the relative plasma pressure is examined. It is shown that the maximum possible <β> (the ratio of the gas-kinetic plasma pressure to the magnetic field pressure, averaged over the volume of the plasma column) consistent with equilibrium exceeds 2.0%. The power of the external sources for plasma heating in the anticipated operating modes is estimated using the present-day scalings. The efficiency of different methods for calculating the magnetic fields and, accordingly, the magnetic surfaces created by the external coils is analyzed in the Appendix.

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 T e in this region. The threshold power for the transition into the ETB confinement mode with an increased electron temperature gradient is P thr ▿Te = (60 ± 15)n e [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 P thr ▿Te = (67 ± 9)n e [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 T e . The value of P thr 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 τ E 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.