Plasma Physics Reports最新文献

Plasma flow and acceleration in the converging-diverging magnetic field configuration, such as magnetic nozzle in electric propulsion and open magnetic mirrors for fusion applications are considered. This work analyses plasma acceleration in the magnetic nozzle with an emphasis on the electromagnetic effects and centrifugal forces due to plasma rotation. Intrinsic coupling of the azimuthal rotation and azimuthal magnetic field is analyzed, and additional plasma acceleration due to the conversion of the energy of the azimuthal magnetic field and azimuthal rotation is demonstrated. For large expansion in the diverging magnetic field plasma flow velocities may approach and exceed the Alfvén velocity. In these regimes, stationary solutions for the transonic and trans-Alfvénic flows have been obtained that demonstrate the existence of the unique regular solution passing through all critical points within the MHD theory, i.e., the points where the plasma flow is equal to the signal velocities of the MHD modes: slow and fast magnetohydrodynamic waves and Alfvén wave. The time-dependent initial value simulations show that stationary equilibrium flows are robust and stable, so that time-dependent solutions converge toward stationary solutions.

A three-dimensional numerical simulation of the interaction between a high-velocity aluminum plasma jet and the geomagnetic field has been performed, based on the injection scenario derived from the North Star II experiment. The simulation results demonstrate the excitation of Alfvén waves, expulsion of the magnetic field, dynamics of the diamagnetic cavity, deceleration of the jet, and induced motion in the background plasma. The calculated parameters show agreement with experimentally measured ion densities.

Numerical experiments were performed using the Lee model code to study the soft X-ray characteristics versus pressure at different operating voltages of the NX2 plasma focus device for two types of anodes (cylindrical and tapered). It was observed that the computed yield increased by 5 times when switching from the cylindrical anode to the tapered one, which agreed with the experimental results. Later, numerical experiments were conducted to study the effect of the different tapering conditions (anode radius at the end of the taper and axial distance at the start of the taper) on the soft X-ray yield, interesting results were reached. We found that the yield is not related to the axial distance for the taper start, but is largely related to the end anode radius of the taper, as there is an optimum end anode radius of the taper that gives the highest yield for soft X-ray. The convergence in previous computed and measured results was taken advantage to conduct numerical experiments on the PF-24 device to find the soft X-ray characteristics and the maximum soft X-ray yield corresponding to the optimum pressure for different anode shapes (cylindrical and tapered). In addition, numerical experiments were conducted to find the optimum conditions of geometric dimensions of the cylindrical anode (anode radius, anode length), and tapering conditions (anode radius at the end of the taper and axial distance at the start of the taper) which give the maximum soft X-ray yield for PF-24 device. The results obtained for the PF-24 device behave identically to the behavior of the results obtained for the NX2 device.

Results of the particle-in-cell simulation that illustrate evolution of parameters of laser-produced plasma formed upon irradiation of steel foils with a thickness of 1–5 µm by femtosecond laser pulses with intensity of ≥5 × 10²¹ W/cm² are presented. Analytical estimates for analysis of energy dissipation in a foil of finite thickness are obtained. Numerical simulations are compared with the results of the recent experiment in which diagnostics of plasma parameters was carried out by methods of X-ray spectroscopy. The results of simulation agree with the experimental results and confirm that a microscopic-sized plasma source with energy density exceeding 1 GJ/cm³ and lifetime of about 500 fs can be formed as a result of action of high-contrast femtosecond laser pulses of ultrarelativistic intensity. In addition, simulations demonstrate that a plasma source with a volume of ~1 µm³ and lifetime of ~5 light periods that has parameters close to those existing inside the Sun, i.e., the temperature of ~1–3 keV and energy density of ≥10 GJ/cm³ (pressure of ≥100 Gbar), can be formed under the same parameters of the experiment.

The applicability of several methods for processing experimental signals of local magnetic probes is analyzed for the purpose of determining the poloidal mode number m of MHD perturbations in plasma of tokamak with noncircular cross-section. These methods are the two-dimensional Fourier transform, the -Hilbert transform method, the method of singular value decomposition, and the method of phase matrices. The methods listed were applied under conditions of the D-shaped cross-section of the T-15MD tokamak vacuum vessel. They showed good results in determining the poloidal wave numbers of MHD perturbations for both synthetic and experimental signals of local magnetic probes. The results of processing the experimental data from local magnetic probes were in good agreement with the data of soft X-ray diagnostics of the T‑15MD tokamak.

Phobos and Deimos are atmosphereless celestial bodies with a weak gravity. Their surfaces consist of small regolith grains that are not connected to one another and have appeared due to bombardment with micrometeorites. Their weak gravity makes these bodies interesting for piloted flights and increases the role of dust, since even a small perturbation leads to the creation of massive dust clouds above their surfaces. The surfaces of these satellites of Mars are charged by the electromagnetic radiation from the Sun and the plasma of the solar wind. The dust grains located at the surface and in the near-surface layer absorb the photons, photoelectrons, electrons, and ions of the solar wind, and, as a result, they obtain electric charge. The action of the electrostatic force under the weak gravity conditions leads to the detachment of dust grains from the surface and the creation of a dusty plasma system together with the electrons and ions. In the dusty plasma system above the surfaces of Mars’s satellites, dust acoustic waves can propagate. In this work, we consider nonlinear periodic and solitary dust acoustic waves with an arbitrary amplitude, which can propagate near the surface of Phobos and Deimos, and discuss the possibility of observing these structures.

A numerical model is constructed for calculating the evolution of the ion distribution function in an axially symmetric open trap. A single force tube with uniform distribution of parameters over the cross section is considered. The phase space is parameterized by the total energy and magnetic moment. The Rosenbluth–Trubnikov potentials are calculated from the distribution functions averaged over angles. A Maxwellian distribution of electrons with the same temperature is assumed throughout the computational domain.

The results of one of the first experimental campaigns on the experimental stand GPI-2 (helicon plasma source, 2 kW) on studying the plasma material interaction, which is designed and manufactured at the National Research Center “Kurchatov Institute”, are presented. Plasma parameters have been measured using a set of probe diagnostics. The optimal magnetic field values and working gas flow rates for this stand configuration are selected based on the experimental data, and the possibility of obtaining plasma with a density of about 10¹¹ cm^–3 is demonstrated. The effect of the RF antenna position in a decreasing magnetic field relative to its maximum has been studied.

The probability of heterogeneous recombination of hydrogen atoms, γ_H, on the surface of a Pyrex tube in a direct current glow discharge in pure hydrogen at medium pressure (2–7 Torr) has been measured as a function of the discharge pressure and current for two wall temperatures. It has been found that the recombination probability does not depend on the discharge pressure and current under the condition of preliminary training of the tube in a hydrogen discharge. The γ_H value decreases during the tube training with a characteristic time of reaching a steady-state value of ~30 min. Analysis of a possible recombination mechanism using quantum chemical methods shows that the recombination of hydrogen atoms on the Pyrex surface is associated with OH radicals and oxygen vacancies on the surface, and the dynamics of γ_H can be explained by the decay of surface OH radicals during tube training.

An estimate was made of the operational and measurement range for the “Reflectometry for the Electron Density Profile Measurements” diagnostics that is being developed for the T-15MD device. The latest changes related to the design of the diagnostics are discussed, parameters of the diagnostics components are presented, and the expected signal-to-noise ratio is given. Also, an assessment of the maximum tolerable level of density fluctuations for diagnostics is presented and compared with measurement results obtained on the other devices. This study is of interest for professionals who develop microwave diagnostics for fusion machines.