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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.

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 m3, and the specific heating power ∼2 MW/m3) 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 m3 s−1 and spatial resolution of ∼0.5 cm.

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.

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.

Results are presented from experimental studies of the influence of the stellarator magnetic field structure on the plasma behavior in electron-cyclotron resonance regimes with a high heating power per electron. The magnetic field structure was changed by varying the induction current I p from −14 to +14 kA. The plasma electrons were heated at the second harmonic of the electron gyrofrequency by an X-mode microwave beam with a power of P ∼ 200 kW, the average plasma density being in the range n e = (0.5–2) × 1013 cm−3. At I p = 0, the rotational transform varies from $$\rlap{--} \iota $$ (0) = 0.2 on the magnetic axis to 0.8 at the plasma boundary. At a positive current of I p = 13.5 kA, the rotational transform was $$\rlap{--} \iota $$ (0) = 0.8 on the axis and $$\rlap{--} \iota $$ (a p) = 0.9 at the plasma boundary. Experiments with a positive current have shown that the radiative temperature first increases with current. When the current increases to I p = 11–14 kA, strong modulation appears in the electron cyclotron emission signals received from all the plasma radii, the emission spectrum changes, and the emission intensity decreases. At a negative current of I p = −(6.5–13.5) kA, the rotational transform vanishes at r/a p = 0.4–0.6. In this regime, the number of suprathermal electrons is reduced substantially and the emission intensity decreases at both low and high plasma densities.

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.

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≤3×1019 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.

The project of an L-5 stellarator is discussed. This multipurpose device of the compact torsatron type is considered as a possible prototype of the principal experimental device of the Plasma Physics Department (the General Physics Institute, the Russian Academy of Sciences) involved in the stellarator program of controlled fusion research. This device is intended to take place of the L-2 (L-2M) stellarator, which is now in operation. A peculiar feature of this project is the extremely low aspect ratio of a toroidal plasma configuration and, at the same time, the experimental flexibility allowing creation of a wide range of magnetic configurations. This allows scanning of main plasma parameters, determining plasma confinement within substantial limits. The latter circumstance makes it possible to carry out studies of magnetic plasma confinement with the purpose to investigate effects of different theoretical limitations on plasma stability and equilibrium, and also to study efficiency of some methods prop...

This paper describes a series of experiments performed in TJ-II stellarator with the aim of studying the influence of magnetic configuration on stability and transport properties of TJ-II plasmas. Plasma potential profiles have been measured in several configurations up to the plasma core with the heavy ion beam probe diagnostic. Low-order rational surfaces have been positioned at different plasma radii observing the effect on transport features. Plasma edge turbulent transport has been studied in configurations that are marginally stable due to decreased magnetic well. Results show a dynamical coupling between gradients and turbulent transport. Experiments on the influence of magnetic shear on confinement are reported. Global confinement issues as well as enhanced confinement regimes found in TJ-II are discussed as well.

Comparative measurements of the absorbed microwave power are performed using the diamagnetic diagnostics and a multichannel diagnostics receiving the second harmonic electron cyclotron emission from the plasma. The specific features of the experiments and the results obtained are discussed.

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.

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.

The results of an experiment show that in the edge plasma of a L-2M stellarator the radial electric field is one of the determining factors influencing the formation of spatial structures and fluctuational particle fluxes. A point where the poloidal velocity of the fluctuations reverses is observed in an experiment with a high neutral-gas flux from the chamber walls.

Results are reported from an experimental study of the characteristics of the turbulent edge plasma in the L-2M stellarator. Extended coherent structures are observed to form in the poloidal direction. The poloidal coherence length of the fluctuations reaches 20 cm. The coherence of the fluctuations depends appreciably on the poloidal azimuth and is maximum along the outer edge, as is natural for unstable modes in a toroidal plasma.

The results of electron cyclotron heating (ECH) experiments in the L-2M stellarator are presented. The main goal of the experiments is to investigate the physics of ECH and of plasma confinement at high values of the volume heating power density. A current free plasma is produced and heated by extraordinary waves at the second harmonic of the electron cyclotron frequency ( omega 0=2 omega ce). The experimental results are compared with the empirical LHD scaling and with numerical simulations of plasma confinement and heating processes based on neoclassical theory using the full matrix of transport coefficients, including some additional anomalous corrections

The H‐1 heliac recently brought into operation is a medium‐sized 3‐field‐period heliac with major radius R0=1 m, plasma mean minor radius 〈a〉≤0.2 m and a wide range of rotational transforms 0.6≤ι–(0)≤2.0. Electron beam mapping of the vacuum magnetic field was performed using new type of a fluorescent target (movable fluorescent rod array having a transparency about 98%). Up to 150–200 toroidal transits were observed at each electron gun position. The spatial resolution of the system was about 3 mm. Electron collector probes were used for monitoring the positions of the magnetic surfaces in different toroidal field periods. Visible paths of the electron beam due to the excitation of the background gas (p∼10−4 Torr) were used for identification of the toroidal transit numbers. This newly developed method gives an accuracy in measurement of the rotational transform of about 1.5%. Experimental surfaces and measured ι/ profiles show very good agreement with the computer model results.

The results of an experimental study of the magnetic structure in the H-1 heliac are presented. Electron beam magnetic mapping has confirmed the existence of closed nested flux surfaces, in good agreement with a computer model. Measurements over a wide range of helical winding currents demonstrated a variety of attainable magnetic configurations within a rotational transform range of 0.6 ≤ t0 ≤ 1.8. The observed islands can be attributed to deduced small errors in the coil alignment in H-1. A magnetic island study and a correction in the model to fit the experimental observations revealed the error sources in the magnetic field

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.

Comparative studies of the structure of turbulence have been carried out in stellarator plasmas with low (TJ-IU) and medium (L2-M) magnetic shear. In both devices, the value of the radial coherence of fluctuations is bursty. In the plasma edge region, the radial coherence is dominated by low frequency fluctuations ({lt}200 KHZ) in TJ-IU, whereas in L2-M, the radial coherence is due to high frequency fluctuations ({gt} 100 kHz). Numerical calculations of resistive ballooning turbulence show that in the non-linear phase, the correlation length is of the order of the width of the linear mode poloidal components, whereas in the linear phase, the correlation length is related to the width of the envelope. Toroidal coupling effects appear to be relevant in the nonlinear phase in the proximity of instability thresholds. The electrostatic Reynolds stress shows a radial gradient close to the velocity shear location in the TJ-IU torsatron, thus suggesting that this mechanism can drive poloidal flows.

The results of experiments on plasma confinement at ECRH power density up to 1.5 MW/m3 are reported. It is shown that the results of measurements of the diamagnetism and poloidal rotation velocity of the plasma in the peripheral region agree with numerical calculations using a neoclassical model with allowance for anomalous losses. The correlation has been found between the low-frequency modulation of the gyrotron power radiation and the modulation of the radiation scattered by the turbulent plasma density fluctuations.

Transitions to the regime with better confinement in the L-2M stellarator are presented. Transitions are indicated only at sufficiently high plasma densities, and for a given value of average density they appear only at higher heating powers. Each transition is easily identified by a sudden fast (

The H‐1 heliac recently brought into operation is a medium‐sized 3‐field‐period heliac with major radius R0=1 m, plasma mean minor radius 〈a〉≤0.2 m and a wide range of rotational transforms 0.6≤■(0)≤2.0. Electron beam mapping of the vacuum magnetic field was performed using new type of a fluorescent target (movable fluorescent rod array having a transparency about 98%). Up to 150–200 of toroidal transits were observed at each electron gun position. The spatial resolution of the system was about 3 mm. Electron collector probes were used for monitoring the positions of the magnetic surfaces in different toroidal field periods. Visible paths of the electron beam due to the excitation of the background gas (p×10−4 Torr) were used for identification of the toroidal transit numbers. This newly developed method gives an accuracy in measurement of the rotational transform of about 1.5%. Experimental surfaces and measured ■ profiles show very good agreement with the computer model results.

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.

The results of experimental investigations on the possibility of trapping injected plasma by a rising magnetic field are presented. It is shown that a plasma column of prolonged duration is formed, localized and well-separated from the walls of the vacuum chamber. The plasma density is similar 1012 cm-3.

The authors give experimental results on the capture of an injected plasma by a rising magnetic field. It is shown that a long-lived plasma pinch is formed, localized at a distance from the wall of the vacuum chamber: the plasma density is ~1012 cm−3.

The first experimental campaign of the TJ-II stellarator has been conducted using electron cyclotron resonance heating (f = 53.2 GHz, P-ECRH approximate to 250 kW) with a pulse length of Delta t approximate to (80-200) ms. The flexibility of the device has been used to study five different configurations varying plasma volume and rotational transform. In this paper, the main results of this campaign are presented and, in particular, the influence of plasma-wall interaction phenomena on TJ-II confinement is briefly discussed.

ECR (electron cyclotron resonance) heated plasmas have been studied in the low magnetic shear TJ-II stellarator (R = 1.5 m, a < 0.22 m, B = 1 T, f = 53.2 GHz, P-ECRH = 300 kW, power density = 1-25 W cm(-3)). Recent experiments have explored the flexibility of the TJ-II across a wide range of plasma volumes with different rotational transforms and rational surface densities. In this paper, the main results of this campaign are presented and, in particular, the influence of iota and rational surfaces on plasma profiles is discussed.

In stellarators, average equilibrium β can be derived from measurements of the magnetic flux produced by the Pfirsch-Schluter current. This method has been compared with the conventional diamagnetic method. The important advantages of using this method are much higher relative signal ratio and easier installation of the diagnostic in any stellarator devices.

The first results of ECH experiments in the L-2M stellarator are presented. The main goal of the experiments is to investigate the physics of ECH and plasma confinement at very high values of the v...

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 (B v /B 0 =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 (B v/B 0∼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.