K. A. Sarksyan

High power density electron cyclotron experiments in the L2M stellarator

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

Backscattering of gyrotron radiation and short-wavelength turbulence during electron cyclotron resonance plasma heating in the L-2M stellarator

Backscattering of gyrotron radiation (θ = π) by short-wavelength density fluctuations (k⊥ = 30 cm−1) in the plasma of the L-2M stellarator was studied under conditions of electron cyclotron resonance (ECR) plasma heating at the second harmonic of the electron gyrofrequency (75 GHz). The scattering of the O-wave emerging due to the splitting of the linearly polarized gyrotron radiation into the X- and O-waves was analyzed. The signal obtained after homodyne detection of scattered radiation is a result of interference of the reference signal, the quasi-steady component, and the fast oscillating component. The coefficients of reflection of the quasi-steady component, R=2(Y), and fast oscillating component, R∼2(Y), of scattered radiation are estimated. The growth of the R∼2(Y) coefficient from 3.7 × 10−4 to 5.2 × 10−4 with increasing ECR heating power from 190 to 430 kW is found to correlate with the decrease in the energy lifetime from 1.9 to 1.46 ms. The relative density of short-wavelength fluctuations is estimated to be 〈n∼2〉/〈ne2〉 = 3 × 10−7. It is shown that the frequencies of short-wavelength fluctuations are in the range 10–150 kHz. The recorded short-wavelength fluctuations can be interpreted as structural turbulence, the energy of which comprises ∼10% of the total fluctuations energy. Simulations of transport processes show that neoclassical heat fluxes are much smaller than anomalous ones. It is suggested that short-wavelength turbulence plays a decisive role in the anomalous heat transport.

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.

Application of microwave discharge for the synthesis of TiB2 and BN nano- and microcrystals in a mixture of Ti-B powders in a nitrogen atmosphere

Synthesis of titanium diboride and boron nitride nano- and microcrystals by means of a pulsed microwave discharge in a mixture of Ti-B powders in a nitrogen atmosphere is considered. For this purpose, a new type of reactor with a free surface of the powder mixture was used. The reactor design permits free expansion of the reaction products into the reactor volume and their deposition on the reactor walls. Conditions for the synthesis of TiB2 and BN compounds were studied as functions of the energy input in the discharge, the powder component ratio, the rate of the nitrogen flow through the reactor, and the structure and phase composition of the compounds deposited on the reactor walls. The synthesis of boron nitride and titanium diboride in crystal structures is proven. An important role in the process of synthesis is played by the heating of the mixture due to the titanium diboride synthesis reaction, its behavior in the bulk of the reactor, and the titanium concentration in the powder mixture. It is also found that, as the number of discharges in the bulk of the reactor increases, a dust cloud forms. The luminescence of this cloud indicates that the initiated discharge proceeds not only on the powder surface and in the powder bulk, but also in the reactor volume.

A new MIG-3 gyrotron complex for creation and heating of plasma in the L-2M stellarator and the first experimental results

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.

Statistical analysis and modelling of turbulent fluxes in the plasma of the L-2M stellarator and the FT-2 tokamak

A comparative analysis is done of data on turbulent fluxes measured in the edge plasma in the L-2M stellarator and the FT-2 tokamak. This analysis is performed using the estimation-maximization algorithm to determine finite mixtures of normal distributions that give the best fit to the probability distributions (PDFs) of the increments of turbulent fluxes. The resulting PDFs indicate non-Brownian motion of particles, and the weight of rare transport events (heavy tails of PDFs of fluxes) can be calculated. The number of the normal processes in model mixtures, as well as their weights in the PDFs of the increments of the turbulent flux, change during L–H transition in FT-2. Characteristically, the number of normal processes for the H-regime of FT-2 is the same as for the L-2M stellarator, and the parameters of Gaussians are also similar. This fact may be interpreted as an indirect indication that the L-2M stellarator operates in the mode of improved plasma confinement (which is consistent with the neoclassical theory for stellarators).

Stability and variations of plasma parameters in the L-2M stellarator during excitation of the induction current in the regime of ECR plasma heating

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.

Low-Frequency Structural Plasma Turbulence in the L-2M Stellarator

Experiments on the L-2M stellarator have shown the occurrence of steady-state low-frequency strong structural (LFSS) turbulence throughout the entire plasma column. A key feature of LFSS turbulence is the presence of stochastic plasma structures. It is shown that different types of LFSS turbulence are correlated throughout the entire plasma volume. Stable non-Gaussian probability density distributions of all of the fluctuating plasma parameters are measured. The characteristic spatial and time scales of LFSS turbulence, which is responsible for non-Brownian diffusion in plasma, are determined.

New approach to the probabilistic-statistical analysis of turbulent transport processes in plasma

It is proposed to apply the statistical analysis of the increments of fluctuating particle fluxes to examine the probability characteristics of turbulent transport processes in plasma. Such an approach makes it possible to pass over to the analysis of the dynamical probability characteristics of the process under study. It is shown that, in the plasmas of the L-2M stellarator and the TAU-1 linear device, the increments of local fluctuating particle fluxes are stochastic in character and their distributions are scale mixtures of Gaussians. In particular, in TAU-1, the increments obey a Laplacian distribution (which is a scale mixture of Gaussians with an exponential mixing distribution). A mathematical model is proposed to explain such distributions. Possible physical mechanisms responsible for the random character of the increments of fluctuating particle fluxes are discussed.

Stochastic Structures in Low-Frequency Plasma Turbulence: Measurement of Characteristics and Determination of General Features

Results are presented from the experimental and statistical studies of low-frequency turbulence in a magnetized plasma. It is shown that, for all types of driving instability (drift, ion-acoustic, MHD instability), this turbulence is accompanied by the formation of stochastic structures demonstrating a statistically consistent behavior and similar correlation, spectral, probability characteristics. The stochastic structures that are existing in the state of dynamic equilibrium and non-random interaction determine all common features of very different turbulent processes: ionacoustic nonlinear solitons, drift vortices, and MHD spatial structures. It follows that the structural turbulence is a non-Gaussian probability process with the long memory, i.e., a self-similar probability process.