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 validation of spectroscopic method for localization of the water plume around a coolant leak in ITER plasma chamber based on the excitation of coolant particles in plasma near wall is presented. The sensitivity of OH hydroxyl and Xe as minor admixtures to cooling water detection in different types of plasma discharges relevant to ITER tested in stellarator L-2M and hollow cathode glow discharge. The limit of sensitivity of water detection in hot plasma using OH* hydroxyl band is estimated as 3×10−5Pam3s−1and ∼1×10−6Pam3s−1 for Xe. For glow discharge conditions measured sensitivity of OH* detection is equal to 3×10−5Pam3s−1.

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.

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 H α and D α 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.

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.

This paper reports on studies of short-wave turbulence in the plasma of the 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 short-wave 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) × 1013 cm−3. The energy confinement time was ∼3.5 ms at P0 = 100 kW and was reduced to ∼2 ms at P0 = 200 kW. When the limiter was introduced inside the plasma to a depth of 2 cm from the last closed flux surface, τE decreased by a factor of 1.3–1.4. Plasma density fluctuations were measured from the scattering 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 ⩽ Θ ⩽ π/2 (24 cm−1 ⩽ k⊥ ⩽ 44 cm−1). The short-wave turbulence was studied for two radial positions of the scattering region: r/a = 0.3–0.4 and r/a = 0.5–0.6. Short-wave turbulence exhibits features of strong plasma turbulence. It is experimentally established that a change in the energy confinement time in the L-2M stellarator correlates with the level of short-wave turbulence.

A novel method for measuring the particle confinement time by using spectroscopic data has been developed. The electron influx into plasma is estimated from the intensity of the Hα line. The first results obtained by this method in the L-2M stellarator are discussed.

Results are presented from experimental studies of variations in the plasma parameters during the excitation of a multiaxis magnetic configuration by the induction current (up to 17 kA) in the basic magnetic configuration of the L-2M stellarator in the regime of ECR heating at a microwave power of ∼200 kW (∼1 MW m−3) and an average plasma density of (1–2) × 1019 m−3. The current direction was chosen to reduce the net rotational transform (the so-called “negative“ current). The current was high enough for the rotational transform to change its sign inside the plasma column. Computer simulations of the L-2M magnetic structure showed that the surface with a zero rotational transform is topologically unstable and gives rise to magnetic islands, i.e., to a multiaxis magnetic configuration. Magnetic measurements showed that, at negative currents above 10 kA, intense bursts of MHD oscillations with a clearly defined toroidal mode number n = 0 were observed in the frequency range of several kilohertz. Unfortunately, the experimental data are insufficient to draw the final conclusion on the transverse structure of these oscillations. The radial temperature profiles along the stellarator major radius in the equatorial plane were studied. It is found that the electron temperature decreases by a factor of 1.3 in the plasma core (r/a ≤ 0.6) and that the temperature jump is retained near the boundary. A change in turbulent fluctuations of the plasma density during the excitation of a negative current was studied using wave scattering diagnostics. It is found that the probability density function of the increments of fluctuations in the plasma core differs from a Gaussian distribution. The measured distribution is heavy-tailed and broadens in the presence of the current. It is found that the spectrum of turbulent fluctuations and their Doppler shift near the plasma boundary are nonuniform in the radial direction. This may be attributed to the shear of the poloidal velocity. The experimental results indicate that the formation of regions with a zero rotational transform in the plasma core somewhat intensifies plasma transport.

The spatial distributions of the electron temperature and density, the effective and average ion charges, and the thermal and directed ion velocities in current sheets formed in two-dimensional magnetic fields and three-dimensional magnetic configurations with an X line were studied using spectroscopic and interference holographic methods. The main attention was paid to studying the time evolution of the intensities of spectral lines of the working-gas (argon) and impurity ions under different conditions. Using these data, the electron temperature was calculated with the help of an original mathematical code based on a collisional-radiative plasma model incorporating the processes of ionization and excitation, as well as MHD plasma flows generated in the stage of the current-sheet formation. It is shown that the electron temperature depends on the longitudinal magnetic field, whereas the ion temperature is independent of it. The effective ion charge of the current-sheet plasma was determined for the first time.

An edge transport barrier is now one of the most important subjects of controlled fusion research. The edge transport barrier is located in the plasma region where hydrogen atoms readily penetrate, so the intensity of the H(alpha) (D(alpha)) line is high enough. A new diagnostic method uses the well-known property of hydrogen atoms that the ratio of the ionization rate S(i) to the excitation rate S(v) for the H(alpha) line is nearly constant over a wide range of plasma temperatures and densities. An expression has been derived that relates the radial profiles of the plasma density and H(alpha) intensity. The use of charge coupled device detectors makes it possible to measure the radial profile of H(alpha) line intensity with a resolution approximately 0.1 cm; a high intensity of the H(alpha) line ensures a high time resolution approximately 1 ms. A high resolution is thus achieved for the density profile calculated from the H(alpha) intensity profile. The method was tested when studying the plasma density profile in the region of edge transport barrier in the L-2M stellarator. It has been shown that the density gradient varies during the barrier formation and that a fine structure of the density profile correlates with a character of the plasma transport near resonance magnetic flux surfaces.

Results are presented from studies of the effect of the discharge parameters (in particular, plasma density and heating power) and the characteristics of the magnetic configuration (e.g., rotational transform) on the confinement of a low-pressure plasma during electron-cyclotron resonance heating in the L-2M stellarator. An analysis shows that the plasma energy in the steady-state phase of a discharge is fairly well described by the product of power functions of the plasma density, heating power, and rotational transform: \(W = W_0 n_e^{\alpha _n } P^{\alpha _p } \iota ^{\alpha _\iota } \). The energy scalings constructed in terms of the parameters in the initial stage of free plasma decay and those in the steady-state phase are close to one another. The dynamic analysis of the plasma energy decay is now under way.

Variation of parameters of ECR heated p lasma was s tudied in the L-2M stellarator under conditions that t he basic magnetic configuration was modified by exciting induction current. Experiments were carried out at ECRH power of ~200 kW (~1 MW m -3 ) and average plasma density of ~2·10 19 m -3 . The direction of the current was chosen such that the total rotational transform decreased, and its value was large enough (up to 17 kA) for rotational transform to change sign in the inner layers of the plasma column. Computer modeling predicts the formation of a multi-axis magnetic structure in the inner layers of the plasma c olumn. Magnetic probe measurements show the presence of bursts in signals of Pfirsh-Schluter currents and variations of the spectrum and mode of MHD oscillations of the plasma column. The appearance of the n = 0 mode at currents above 10 kA is correlated to the appearance of the region where ι/2π = 0. It i s s hown that, in the presence of the induction current, the e lectron temperature in the region r/a ≤ 0.6 is lower by a factor of 1.3, whereas a characteristic jump in the temperature in the e dge plasma remains; t he density g radient at the plasma e dge decreases. The behavior of turbulent density fluctuations was s tudied b y using diagnostics of scattering o f probing radiation. It i s found that the probability density functions for increments of density fluctuations have heavier tails in the presence of a multi-axis structure. The spectral characteristics of turbulent fluctuations in the e dge plasma and the poloidal plasma velocity were found to vary with radius. The experimental evidence suggests that the formation of the magnetic island structure in the c ore plasma leads to more intense transport i n both core a nd edge plasma, but t he change in transport i s not catastrophic.

Представлены результаты изучения влияния параметров плазмы (в частности плотности плазмы и мощности нагрева) и магнитной конфигурации (угла вращательного преобразования) на удержание плазмы низкого давления при электронно-циклотронном резонансном нагреве в стеллараторе Л-2М. В результате анализа установлено, что энергия плазмы в стационарном состоянии может быть хорошо описана произведением степенных функций плотности плазмы, мощности нагрева и угла вращательного преобразования: W = . Энергетические скейлинги, построенные по параметрам начальной фазы свободного распада плазмы и параметрам стационарной фазы, близки друг к другу. Начаты исследования по динамическому анализу распада энергии плазмы.

Boronization of the vacuum chamber of the L-2M stellarator has resulted in modification of the electron temperature profile. In particular, a well-defined jump in the electron temperature to T-e similar to 100 eV in a narrow region Delta r/r similar to 0.05 is observed in the temperature profile at the plasma edge. In the present paper, the value and shape of the jump in T-e are studied at different values of plasma parameters and ECR heating power. A jump in T-e is absent at a power of P similar to 100 kW, whereas at P similar to 200 kW the electron temperature drops from 150 eV to zero within Delta r similar to 0.5 cm. The value of threshold power for the formation of a jump in T-e at n(e) similar to 1.7 x 10(19) m(-3) lies Within the range P similar to 100-160kW. In terms of power per particle this power threshold is P/V/N-e similar to 0.2-0.3Mw/m(3)/10(19) m(-3), the value of which coincides with threshold power for ETB formation found recently in the CHS stellarator. When the helical-field strength is 25% or 50% below its standard value, a jump in T-e at the plasma edge in L-2M is absent.

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.

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.

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 P in =120–400 kW and average plasma densities from n e ≤3×10 19 to 0.3×10 19 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.

Results are presented from a detailed study of the behavior of the electron temperature during the evolution of a current sheet by comparing the data from spectral measurements with the spatiotemporal evolution of the emission intensities of the atomic and ionic lines of the working gas (He) and impurities (C, O) calculated in the collisional-radiative model. It is shown that the electron temperature in the center of the sheet attains a value of T e =110±40 eV; under these conditions, taking into account metastable states affects the calculated results only slightly. The spatial profiles of the electron temperature and the plasma emission in the spectral lines of various atoms and ions across the plasma sheet are calculated as functions of time. It is shown that as the electron temperature grows most of the spectral lines of atoms and ions of the working gas and impurities are depleted in the center of the sheet and the emission region shifts toward the periphery of the sheet. The results obtained confirm the previous conclusion that, in this regime, a hot plasma is formed in the center of the sheet.