Plasma Physics Reports

This work reviews the current state of research on the three main components of the turbulence spectrum observed in modern tokamaks. These components differ in frequency range and correlation properties, have relatively wide frequency intervals, and sizes larger than the ion Larmor radius. These are broadband (BB), quasi-coherent (QC), and stochastic low-frequency (SLF) fluctuations. BB fluctuations have the broadest frequency range, from 0 to 200–400 kHz, and make the main contribution to the total amplitude of density fluctuations. The characteristic BB sizes are close to those predicted by theory for the ion temperature gradient (ITG) and trapped electron mode (TEM) instabilities. BB are the least correlated fluctuations; in the T-10 tokamak plasma (R/a = 1.5/0.3 m), their typical radial and poloidal correlation lengths are approximately 1 and 2 cm, respectively, while the correlation length along the magnetic field line is less than 2.5 m. Quasi-coherent fluctuations (QC) are manifested as local maxima in the frequency spectra of density oscillations; however, they are most clearly visible in the coherence spectra, since they have radial and poloidal correlation lengths significantly longer than BB. In the T-10 tokamak plasma, correlations along the magnetic field line over a length of up to 10 m were observed for QC modes. Two types of such fluctuations were observed at T-10: low-frequency (LFQC) and high-frequency (HFQC). The characteristic poloidal sizes and dependences on discharge parameters in the T-10 experiments show that the properties of LFQC and HFQC are close to those of ITG and TEM, respectively. The poloidal rotation of QC coincides with the drift [E×B] rotation in magnitude and direction. A relation between the characteristics of these modes and the discharge parameters is demonstrated with a change in density. The magnetic component in QC was demonstrated at T-10 and DIII-D. Gyrokinetic simulation of these experiments showed that the properties of QC are close to the micro-tearing mode (MTM). Additional evidence for the MTM nature of QC is the strong dependence of their spectra on the current profile, the discrete mode structure, and the absence of QC in stellarator plasma. Stochastic low-frequency fluctuations (SLF), excited in the range from 0 to 70 kHz, are the least studied. In the T-10 tokamak, plasma on the side of a low magnetic field, these fluctuations can rotate in the direction opposite to QC. SLFs are uncorrelated along the magnetic field line at the LFS, but correlated at the HFS. SLF fluctuations have a magnetic component. Density and potential fluctuations have different radial and poloidal sizes and are uncorrelated with each other. This suggests the existence of two independent types of fluctuations in the SLF frequency domain. Experiments at DIII-D, by comparing spectra in the L, I, and H modes, showed that fluctuations in the SLF region (up to 70 kHz) can be associated with particle transport, while high-frequency fluctuations, such as QCM and BB, can be associated with heat transport.

Currently, the National Research Center "Kurchatov Institute" is developing an ion-cyclotron resonance (ICR) heating system for the T-15MD tokamak. The ICRF (ion cyclotron range of frequency) system is to be used to heat the ion component of the plasma and generate a non-inductive current (drag current). The total power of the system is 6 MW with a pulse duration of up to 30 s. Under these parameters, reflected power could lead to failure of the ion-cyclotron resonance heating (ICRH) of the system. Therefore, matching the load (plasma) to the generator requires special attention. This work presents a numerical study of the antenna-plasma coupling efficiency for a three-loop antenna developed for ICRF plasma heating in the T-15MD tokamak. The dependence of the antenna impedance on plasma parameters, its position, and the phasing of the antenna excitation currents is studied.

For effective ion cyclotron resonance (ICR) plasma heating using the “magnetic beach” method in electrodeless plasma thruster, it is necessary to precisely optimize the geometric parameters and shape of the RF antenna. This article presents a full-wave simulation of the excitation of magnetized plasma column eigenmodes by ion-cyclotron (IC) antennas, accounting for the radial inhomogeneity of the plasma column and the spatial dispersion of the electron and ion dielectric response. The simulations calculated both the total power deposited by the antennas into the plasma and its division between the plasma column’s Alfvén eigenmodes and the Alfvén continuum. The calculations were performed for the three most popular antenna types used for ICR plasma heating for the PS-1 setup. The efficiency of these RF antenna types was compared, and the optimal lengths for each were determined.

In experiments on plasma heating during the passage of a relativistic electron beam with a current 10 kA through a plasma column, the electric and magnetic fields of the beam have to be compensated by the current induced in the plasma, i.e., the so-called charge and current neutralizations of the beam have to occur. In the earliest experiments on injection of high-current relativistic electron beams into a plasma with a magnetic field, the effects of plasma neutralization of the electron beam’s internal electromagnetic field were observed. At the same time, the dynamics of these neutralization processes also depends on the ratio between the duration of the leading edge of the beam current and the flight time of its electrons along the plasma column length. This work describes the results of two substantially different series of experiments on the neutralization of an electron beam with a current of about 10 kA in a plasma column under a strong (4 T) leading magnetic field. In the first series of experiments, the leading edge of the beam current was ≈5 ns and its duration was about 50 ns. In the second series of experiments, the leading edge of the current beam was 0.1 μs and its duration was ≈5 μs.

This work presents a detailed analysis of oscillographic data obtained in experiments on the nanosecond explosion in air of thin wires made of Ag, W, Pd, and Ti - metals belonging to different classes (copper, tungsten, and nickel groups) in terms of their electrical and thermal properties. Features of electrical signals characteristic of explosion scenarios corresponding to each of the groups are discussed. The experiments are conducted on a capacitor- type setup with a total stored energy of up to 20 J (C = 0.1 μF, I_max = 10 kA, dI/dt ~ 50 A/ns) at a charging voltage of 13 kV. This work is the first stage of a study aimed at establishing the mutual influence of the generator parameters serving as the source of explosion energy and the loads used.

Stationary states of electron beam with arbitrary velocity spread in a vacuum diode are studied. The velocity distribution function of beam electrons is assumed to be constant over a certain velocity range. The limiting current as a function of the velocity spread was obtained. It was ascertained that, within a certain range of parameters, stationary states with two types of virtual cathodes can exist.

This work presents a laboratory experiment on the formation of dusty plasma and the visualization of flows of charged dust particles ranging in a diameter from 10 to 100µm. These particles consist of silicon dioxide, a component of lunar regolith. The influence of near-surface plasma simulated using an electrostatic field and ultraviolet radiation (UV) on the dynamics of regolith-analog particles is examined. Particle trajectories and changes in the surface topography of the particles are visualized, and estimates of their takeoff velocities are obtained. It is shown that the pattern of the particle motion in dusty plasma depends on the presence of UV radiation and the size of the particles themselves. The results of this study are of interest for understanding the physical processes occurring near the surface of the Moon and other atmospheres less bodies in the Solar System, such as Mercury, asteroids, the moons of Mars, etc.

A self-consistent model combining the kinetic Vlasov equation governing the distribution function of the resonant particles and the Ampere-Maxwell law is presented. The nonresonant particles are taken into account via the dielectric permittivity in the linear approximation. The results of numerical simulations demonstrating the nonlinear evolution of the broad spectrum of waves induced by an unstable distribution of electrons in homogeneous and inhomogeneous plasma are analyzed. The frequency-time analysis of the electric field reveals the presence of quasi-periodic elements with increasing frequency. It is demonstrated that temporal modulation of the amplitude of the electric field appears due to selection of harmonics with equally spaced wavenumbers in the initial spectrum of the waves. This choice of harmonics gives rise to spatial periodic structures of the electric field that transform into temporal modulation seen in the spectrograms upon propagation.

Traditional microwave resonator methods for plasma diagnostics face fundamental limitations at high plasma densities (n_e > 10¹¹ cm^-3) due to the response nonlinearity and resonant peak broadening. The search for alternative approaches that provide accurate measurements over a wide range remains a topical issue. In this work, the feasibility of using the TM₁₁₀ (E₁₁₀) mode of a cylindrical resonator (radius R = 45 mm, length H = 30 mm) to measure the electron density in a gas-discharge plasma excited by a surface electromagnetic wave is studied experimentally and numerically. It is found that the coupling coefficient C_v for the TM₁₁₀ mode remains constant in the range n_e = 10¹¹–10¹² cm^-3, ensuring a linear dependence of the resonant frequency shift on the plasma density. The TM₁₁₀ mode demonstrates resistance to the resonant peak degradation at high plasma densities n_e, in contrast to the TM₀₁₀ mode, where the signal suppression was observed already at n_e ≈ 3·10¹¹ cm^-3. The obtained measurement results are in good agreement with the results of plasma density measurements using the transmitted wave method and the integrated plasma luminosity, as well as with literature data. The proposed modification of the method is suitable for noninvasive diagnostics of the longitudinal electron density distribution of gas discharge plasma, including plasma antennas.