Studies of plasma produced and confined in quasi-stationary L-2 and L-2M stellarators are presented. In these facilities, plasma was produced by the non-inductive electron cyclotron resonance (ECR) microwave heating in the power range of P = 0.05–1 MW. The radial structure of the near-separatrix region (the relative radius of 0.8–1) and fluctuations of plasma parameters in the mode without changing macroparameters are considered. Fluctuations of the near-wall plasma parameters—density, electric potential, and magnetic field—and their evolution during discharges are analyzed. The structure of the electric field and heat flux in the near-wall plasma measured by Langmuir probes is analyzed. The relation between changes in fluctuating plasma parameters and possible small-scale instabilities is analyzed. The mechanisms of the development of permutation, peeling and temperature gradient edge instabilities are considered. A comparison is made between the simulation of the energy transfer in plasma using neoclassical models taking into account anomalous energy losses and that based on canonical pressure profiles. The possibility of using a quasi-stationary stellarator as a source of plasma flows with three-dimensional geometry for materials science is considered.

Results of experiments carried out with a quasi-stationary L-2M stellarator in the regime of electron cyclotron resonance heating by means of two gyrotrons are presented. The data on increase in the plasma energy lifetime by means of modulation (profiling) of the microwave pulse are obtained. The first gyrotron operating at fixed power was used for ionization and initial heating of plasma, while the second gyrotron initiated a stationary discharge with a duration of 10 ms. It is demonstrated that the lifetime can be increased by varying the second-gyrotron power in the range of 50–200 kW. A fourfold increase in the lifetime was obtained when the second-gyrotron power was reduced to 50 kW. This work is of interest as a method of studying hot plasma confined in the toroidal magnetic system of a stellarator.

The evolution of fluctuating signals from electrostatic and magnetic probes and reflectometry on the L-2M stellarator is presented. The L-2M facility is a quasi-stationary toroidal magnetic trap in which plasma is generated and heated by high-power pulsed microwave radiation. Pulses with transitions to improved confinement modes accompanied by an increase in energy, increase in plasma density, and restructuring of the peripheral electric field, were analyzed. Spectral analysis of signals is carried out using Fourier analysis and various wavelets. The possible influence of magnetohydrodynamic and kinetic instabilities on the development of transient processes is considered.

The behavior of plasma with a negative current in the range of 5 kA ≤ | J | ≤ 16 kA in the L-2M stellarator is studied. Plasma is created and heated by means of electron cyclotron resonance heating (ECRH). Here, the definition “negative” stands for a toroidal current directed in such a way that it reduces the initial rotational transform µ in absolute value. It is shown that, only at relatively small values of the negative current (| J | ~ 6 kA), the increase in the shear leads to a reduction in electromagnetic turbulence. As | J | rises, the average level of magnetic fluctuations in the stationary phase of the discharge gradually increases. After ECRH is switched off (i.e., when plasma cools down), there is a time interval in which the amplitude of magnetic field fluctuations increases. No coherent three-dimensional modes (e.g., double tearing modes) that could accompany the penetration of the current into the plasma were observed. As for axisymmetric perturbations with m ≠ 0, analysis of the experimental data shows that they are absent at | J | ≤ 8 kA, appear in some discharges at | J | = 9 kA, and are observed in each discharge at | J | ≥ 10 kA. The frequency of these oscillations is on the order of 10 kHz, the poloidal mode number is m = 2, and the duration is 4–6 ms. The absence of oscillations with m = 1 and the small value of the external magnetic field for the m = 2 perturbation indicate that the radial size of the magnetic island is less than 1 cm. This, in turn, suggests that the radial profile of the plasma current changes with increasing | J |.

Fluctuations of the density in a transient process at electron-cyclotron heating have been measured by the methods of backscattering and small-angle scattering of gyrotron radiation by fluctuations of the plasma density. Estimates have indicated the increase and decrease in the turbulent heat fluxes at the switch-on and switch-off of additional electron-cyclotron heating, respectively. The possibility of excitation of vortices of drift waves and their relation with fast anomalous heat transport at electron-cyclotron heating has been discussed.

Long-range spatial correlations in the turbulent plasma of the L-2M stellarator were revealed experimentally, and their relation to the geometry of magnetic surfaces was analyzed (Plasma Phys. Control. Fusion 50 , 045001 (2008)). The operation modes of the facility in which fast transport transitions in plasma are possible were studied. Upon these transitions, the turbulence level is found to decrease substantially. It is shown that long-range spatial correlations are typical of relatively narrow frequency ranges. In particular, before a transport transition, such frequency ranges are f ∼ 30–40 kHz and f ∼ 1–3 kHz. After the transition, long-range spatial correlations in the frequency range of f ∼ 30–40 kHz disappear due to a significant decrease in the turbulence level in this frequency range. At the same time, correlations in the low frequency range are retained and new correlations at frequencies of f ∼ 6-12 kHz occur. It is found that global electromagnetic oscillations in the frequency range of f ∼ 1–3 kHz are related to the m / n = 0/0 perturbation and its toroidal satellites (here, m and n are the poloidal and toroidal mode numbers, respectively). It is also shown that, after the transport transition, a three-dimensional localized electromagnetic mode at the frequency of the geodesic acoustic mode governed by the average magnetic field curvature is excited. At higher frequencies typical of a geodesic acoustic mode related to the three-dimensional curvature of the magnetic field, no long-range spatial correlations were observed both before and after the transport transition.

In some cases, the phase shift between fluctuations of the electric potential and plasma density helps to identify the instability that governs the turbulent state. In this paper, the basic experimental and theoretical results that denote the possibility (or impossibility) of such identification are briefly discussed. The experimental data based on measurements of the phase shift between the floating potential and ion saturation current fluctuations in the L-2M stellarator—a system with externally imposed magnetic surfaces—are presented (Shchepetov, Kholnov, Fedyanin, et al., Plasma Phys. Controlled Fusion 50, 045001 (2008)). It is shown that the observed phase shift Ω varies in a wide range from π to 0, gradually decreasing with deepening inside the plasma. A number of arguments are presented suggesting that Ω ≈ π can indicate that the process is nonlocal, i.e., oscillations at a given spatial point are driven and mainly determined by the processes localized outside of the observation point. We note that, within the framework of the magnetohydrodynamic theory, plasma was definitely unstable with respect to resistive interchange modes in all cases under study. It is demonstrated experimentally that the widespread notion that the phase shift Ω ≈ π/2 is characteristic of only resistive interchange modes is hardly universal. The experimental results are analyzed on the basis of analytical estimates.