N. N. Skvortsova

Ballistic transport phenomena in TJ-II

Perturbative transport experiments have been performed at the stellarator TJ-II. Both the inward propagation of edge cooling pulses induced by the injection of nitrogen, and the outward propagation of heat pulses due to spontaneous spikes of the central temperature have been analysed. It has been found that the observed propagation is incompatible with diffusive transport models. Simultaneous inward and outward propagation eliminates an explanation in terms of a pinch. A numerical simulation with a resistive interchange turbulence model suggests that the observed propagation is related to the successive destabilizations of pressure gradient driven modes associated with rational surfaces.

Stochastic models of structural plasma turbulence

Five years ago, a series of seminars on Stochastic Structures in Plasma Turbulence was initiated by physicists and mathematicians from various academic centers in Russia aimed at the construction of adequate probabilistic models which can explain the departure of the experimental data from the classical theory. This book contains both a selection of most important reviews published in 2003 in the Russian version of the seminar proceedings as well as new papers dealing with recent investigations in plasma physics and financial mathematics.The collection contains papers presenting various viewpoints on structural plasma turbulence. The collection begins with an article devoted to the research on ionacoustic turbulence in current-carrying magnetized plasma. Other topics treated in this volume are: low-frequency strong structural turbulence in toroidal plasma confinement systems; experimental time samples of particle fluxes in low-frequency strong structural (LF SS) turbulence; diffusion processes in LF SS turbulence; multifractal models of plasma turbulence. One of main topics discussed in the papers of the collection is the heavy-tailedness of the probability distributions of the characteristics of the processes observed in turbulent plasmas. A special paper gives a comprehensive review of the properties of the so-called fractionally stable distributions which appear as limits for continuous-time random walks which are often used for modeling chaotic processes. Much attention is paid to mixtures of normal distributions and hidden Markov models.A new powerful method is proposed for the statistical analysis of chaotic stochastic processes, the so-called method of moving separation of mixtures. This method was originally designed for the study of the evolution of financial indexes, but proved to be very efficient for the analysis of time series representing data accumulated in experiments with turbulent plasmas. The corresponding theory is developed which proposes a new multidimensional interpretation of volatility of chaotic stochastic processes.

Statistical properties and radial structure of plasma turbulence in the boundary region of the L2-M stellarator

The structure of fluctuations and turbulent transport have been investigated in the plasma boundary region of the L2-M stellarator. Normalized fluctuation levels are in the range (3-20)% and fluctuations are dominated by frequencies below 300 kHz. In the edge plasma region located inside the last closed magnetic flux surface the radial coherence of fluctuations is due to high-frequency fluctuations (>100 kHz). The poloidal coherence is dominated by low frequencies. Linear coupling of resistive interchange modes is considered a candidate to explain the existence of highly radially correlated fluctuations in the high-frequency range.

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.

Response of a gyrotron to small-amplitude low-frequency-modulated microwaves reflected from a plasma

Experiments in the L-2M stellarator revealed the intense noise modulation of the gyrotron power and the change in its mean value under the action of the noise modulation of radiation reflected from the plasma column. The effect observed is explained in terms of the resonant locking of the gyrotron self-oscillations due to wave reflection from the fluctuating plasma load.

Effect of ECRH regime on characteristics of short-wave turbulence in plasma of the L-2M stellarator

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.

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).