Vol 45, No 8 (2019)

Results of experimental studies of toroidal Alfvén eigenmodes (TAEs) in the Globus-M spherical tokamak (R = 36 cm, a = 24 cm) are reported. The experiments were carried out in a wide range of plasma parameters at a magnetic field of up to 0.5 T and plasma current of up to 250 kA. Auxiliary plasma heating was performed by tangential injection of a deuterium beam with a power of P_b= 0.75 MW and particle energy of E_b = 28 keV into deuterium plasma. The experiments have shown that the TAE-induced loss of fast particles decreases with increasing plasma current and magnetic field. Using multifrequency Doppler backscattering diagnostics, it is established that the TAEs are localized at the plasma periphery. Results of simulations of the Alfvén continuum and TAE structure by means of the modified KINX and CAXE codes agree satisfactory with the experimental data on the TAE frequencies and localization.

In experiments on multipulse on-axis electron cyclotron resonance heating (ECRH) of plasma by a series of microwave pulses at the L-2M stellarator, several phases of plasma energy loss were observed: the short stage of low-energy loss, the stage of rapid increase in energy loss, the quasi-steady stage, and the relaxation stage between the heating pulses. In the stage of rapid increase in energy loss, the energy loss power is two or more times higher than that in the relaxation stage at the same energy of the plasma column. Short-wavelength plasma density fluctuations were measured using both the ordinary and extraordinary microwave collective scattering technique. It is found that, in the quasi-steady stage, the amplitude of density fluctuations is much lower than that in the preceding heating stages. The fluctuation amplitude lowers just after the restructuring of the density profile and establishment of a steady-state hollow density profile due to the density pump-out effect. The amplitude of large-scale density fluctuations at the plasma periphery recorded by a Doppler reflectometer remains unchanged during the ECRH pulses and in the time intervals between them. However, when the stage of rapid increase in energy loss begins, the shape of the density fluctuation spectrum changes significantly. The initially narrow spectrum with one peak near the zero frequency broadens, the amplitude of the central peak decreases, and two additional peaks at frequencies of 0.7 and −0.7 MHz appear.

Current interest in ion cyclotron resonance (ICR) heating by the magnetic beach method is motivated by the experiments carried out with the VASIMR plasma thruster. The method implies excitation of Alfvén eigenmodes of a plasma column (or a plasma jet in the case of plasma thruster) outside the ICR zone. In the present work, it is shown that, under conditions typical of, e.g., plasma thrusters, most of the electromagnetic energy deposited in the plasma can be transferred to the so-called Alfvén continuum, rather than to the discrete spectrum of Alfvén eigenmodes. Due to the phenomenon of the Alfvén resonance, continuous-spectrum oscillations transform into short-wavelength lower hybrid oscillations, which differ from the Alfvén oscillations in their polarization. These oscillations efficiently interact with electrons, thereby heating them.

Parametric excitation of surface waves by an s-polarized electromagnetic wave incident obliquely on a semi-infinite supercritical plasma is considered. The growth rates and thresholds of the instability associated with the decay of the pump wave into two surface electromagnetic waves are calculated as functions of the incidence angle and plasma electron density. It is shown that the instability growth rate depends linearly on the electric field of the pump wave and is maximal at a plasma electron density slightly exceeding the critical density.

The electron transport coefficients in weakly ionized nonuniform nonequilibrium plasmas of water vapor and fuel–oxygen mixtures are calculated as functions of the degree of fuel oxidation. The calculations are performed for hydrogen and hydrocarbon fuels. It is shown that the production of water vapor during fuel oxidation strongly affects the electron transport coefficients. Using the calculated coefficients, the parameter ranges are determined in which thermocurrent instability can develop in the gas mixtures under study. The domain of reduced electric fields at which this instability develops extends with increasing the degree of fuel oxidation and becomes the largest under complete fuel oxidation.

A vircator-based microwave oscillator with a nonlaminar electron beam and an extended matched section of a helical slow-wave system was studied numerically and experimentally. It is shown that the oscillator efficiency in the frequency range of 1–3 GHz reaches 20%.

The problem of propagation of an electromagnetic wave in plane-stratified magnetoactive plasma is analyzed. A matrix algorithm of approximate solution of a set of wave equations is proposed. The algorithm consists in successive finding of the medium-inhomogeneity-induced corrections to the local roots of the dispersion relation and local polarization vectors. The set of field equations is reduced to a set of algebraic equations. The proposed algorithm is convenient for numerical calculations. In contrast to the classical geometrical-optics approximation, the proposed algorithm allows one to take into account the weak effect of linear mode interaction. Examples of numerical calculations of the power reflection coefficient of whistler waves incident on the ionosphere from above are presented. The proposed matrix algorithm can be useful to find the coefficients of reflection and linear transformation of waves in a smoothly inhomogeneous ionosphere.

With the purpose to create new methods for monitoring the parameters of low-temperature nonequilibrium plasma, a numerical drift-diffusion model of an inductive RF discharge in argon is developed and a study is made of ion transport onto the surface of the processed material. The model was tested against the available experimental and theoretical data. The calculations were performed for an inductive discharge in argon with parameters typical of modern plasmachemical reactors (a frequency of 13.56 MHz and a gas pressure in the chamber of 10 mTorr). The plasma density, electron temperature, and ion flux onto the processed surface are calculated; the gas temperature is found as a function of the input RF power; and the discharge parameters are determined as functions of the gas flow rate.