Plasma Physics Reports最新文献

Results of experimental study and numerical simulation of the fractional content of argon and copper ions in plasma generated in a direct-current planar magnetron with a copper target in the gaseous (at argon pressure at the level of 0.1 Pa) and vacuum (at residual-gas pressure of 0.004 Pa) operation modes are presented. It is demonstrated that fractions of copper ions in the gaseous and vacuum operation modes at the discharge current, sufficiently high for sustaining the self-sputtering regime (10 A), are close to each other and are equal to 97 and 100%, respectively. The results of experiments and numerical estimates are indicative of the possibility of achieving a stable continuous discharge and obtaining a flow of metal ions in high vacuum in a planar magnetron without effects of thermal evaporation or sublimation of the copper target.

The steady states of a diode in which electrons and positrons come from opposite electrodes with relativistic velocities have been studied. Such a situation may occur, e.g., in the plasma of pulsar diodes. Both the mode when charged particles reach the opposite electrode and the mode when a portion of particles is reflected by the potential barriers within the inter-electrode gap are considered. It is established that with a fixed potential difference between the electrodes, the solutions are determined by only three dimensionless parameters: the inter-electrode distance (delta ), the relativistic factor of the entering electrons ({{gamma }_{0}}), and the electric field strength at the left electrode ({{varepsilon }_{0}}). The whole variety of solutions is represented by a set of curves in ({ {{varepsilon }_{0}},delta } ) plane. It turned out that these curves qualitatively coincide with the branches of the solutions in the non-relativistic case. The relativistic factor effect on structure of the solution branches has been analysed.

The theory of the ponderomotive forces in space plasma represents an essential part of the solar-terrestrial physics. The problem that appears when the classical expression governing the ponderomotive force is used for analysis of ion acceleration in the Earth’s magnetosphere under the influence of the ion-cyclotron wave is described. It consists in the fact that the force tends to infinity when the wave frequency approaches the local ion gyrofrequency. It is demonstrated that the singularity appears due to idealizations unavoidably adopted upon construction of the standard theory of ponderomotive forces. Two limiting cases are analyzed: the case of a wide opacity band for the ion-cyclotron waves and the case of narrow opacity band. Two approaches for regularization of the ponderomotive force in the vicinity of the pole of the refractive index for the ion-cyclotron wave are proposed. One of them removes the singularity at the pole at the boundary of the wide opacity band while the other removes it at the boundary of the narrow opacity band. Examples of situations in which the singularity in the ponderomotive force calculated within the framework of the standard theory has to be taken into account when studying wave phenomena in the near-Earth plasma are pointed out.

The measurements of the RF power absorption efficiency have been performed in the pressure range of argon in the range of 1 × 10^–3–0.7 Torr, oxygen pressures in the range of 8 × 10^–4–0.05 Torr, nitrogen pressure 3 × 10^–3–4 × 10^–2 Torr at the operating frequency of 4 MHz. In argon the efficiency of RF generator power coupling to plasma in the considered pressure range weakly increases with increasing argon pressure. In oxygen and nitrogen plasma at powers below 500 W the RF power coupling efficiency depends non-monotonically on pressure. The admixture of even small amount of oxygen to argon discharge leads to the decrease of RF power coupling efficiency at high Ar pressure and RF generator power below 400 W. A decrease in the fraction of RF generator power absorbed by the plasma under pressure growth leads to a decrease in the plasma density and discharge disruption. Thus, the area of existence of the discharge appears to be limited on the high-pressure side. The higher is the RF generator power, the wider is the range of pressures at which the RF inductive discharge can exist. The numerical simulation showed that the observed experimental facts were due to the peculiarities of the dependence of equivalent plasma resistance on electron density and frequency of electron collisions with heavy particles.

Instability of axisymmetric electrostatic oscillations in an inhomogeneous beam of electrons confined by an axial magnetic field is studied within the framework of an ideal hydrodynamic model. The equation of perturbation of the beam electrostatic potential is derived in the linear approximation. It is demonstrated that the effects of differential beam rotation in self inhomogeneous electric field destabilize the Trivelpiece–Gould modes. The spatial structure of the unstable eigenmodes is calculated analytically and numerically for model profiles of the beam density, and their spectrum is analyzed.

The X-ray spectral diagnostics of laser plasma of heavy elements that does not contain few-electron ions is considered. The X-ray spectral method for measuring the temperature and density of laser plasma of krypton cluster targets is developed. It is based on analyzing resonance spectral lines of Ne-like krypton ions Kr XXVII and their two-electron satellites arising due to transitions in Na- and Mg-like Kr XXVI and Kr XXV ions. Using the results of the calculations performed, it is possible to determine the ion density of plasma in the range of 10¹⁷–10²¹ cm^–3 and the electron temperature in the range of 200–1000 eV. It is shown that one spectrometer with the spherically curved quartz crystal, which has a sufficiently high spectral and spatial resolution, can be used for experimental recording all required diagnostic spectral lines of krypton ions.

The results of experiments on the usage of a low-temperature plasma (LTP) jet to activate the nutrient liquid medium αMEM containing mesenchymal stem cells (MSCs) isolated from the bone marrow of Wistar rats are presented. The LTP jet had been created by an axially symmetric barrier discharge with a thin rod electrode located inside a quartz tube along its axis. The tube has been purged with argon at a flow rate of about 25 m/s at the tube output. The conditions under which LTP activation of the αMEM medium can accelerate MSC proliferation have been studied. It turns out that the final effect of the activated liquid medium on cells strongly depends on the purity of argon used to form the plasma jet. A small admixture of oxygen in argon at a level of 700 ppm leads to the formation of active oxygen species in the discharge and in the plasma jet, as well as ozone at a fairly high concentration. Ozone supplied by a jet into a liquid medium dissolves well in it and, as a strong oxidizer, can have a detrimental effect on stem cells. The results on the difference in the composition of active particles in plasma jets in pure argon and in argon with a small admixture of oxygen are presented, as well as the results of microbiological studies on the effect of two types of plasma jets on mesenchymal stem cells.

The feasibility of developing an alternating current generator based on a plasma diode is being explored. Under certain conditions, in a collisionless regime, electron instability can arise in such a diode, leading to an abrupt breakdown of the current flowing through the inter-electrode gap. Technically, a generator utilizing this effect can be implemented by connecting the electrodes through an inductive element. To determine the optimal operating conditions of the generator, it is crucial to first investigate the effect of the external circuit with an inductive element on the stability of the diode’s stationary states. This study theoretically examines the problem for both over-neutralized and under-neutralized regimes. Dispersion equations are derived for both cases. It is established that the presence of external inductance shifts the instability threshold, lowering it below the Pierce threshold. In this scenario, oscillatory instability develops instead of aperiodic one. The ranges of inductance values that allow the instability to develop are also determined.

Ion motion upon propagation and collisions of ion-acoustic solitary waves in the electron–ion plasma and in the negative-ion plasma is analyzed. Ion displacement after wave propagation is calculated for a number of cases, and its dependence on the wave amplitude is found. The back action of the processes that occurs in plasma upon collisions of the ion-acoustic solitary waves on the waves themselves is described. Physical mechanisms that prevent preservation of the wave identity upon their mutual collisions are discussed. The influence of ion displacement on formation of caustics upon collisions of the ion-acoustic solitary waves in the negative-ion plasma is analyzed.

The propagation of the ionization wave along the discharge tube with a precharged wall was studied. Pure inert gases, from neon to xenon, were used, as well as the Ne–Ar mixture, at pressures ~1 Torr. The discharge tubes had a length of 80 cm and a diameter of about 1.5 cm. The ionization wave was excited by a positive- or negative-polarity voltage pulse. The wall charge appeared as a result of the passage of the previous ionization wave initiated in the single-electrode mode. In this mode, the ionization wave is not accompanied by the ignition of the glow discharge. It was established that the time dependence of the longitudinal coordinate of the wave and its instantaneous velocity at each point were determined not by the exciting pulse amplitude but by its excess over the potential of the charged wall. This excess also determines the breakdown voltage in the tube with a precharged wall. This fact confirms the concept of the primary breakdown between the high-voltage electrode and the wall (Nedospasov and Novik, 1960) as the mechanism of the appearance of the ionization wave during the ignition of the glow discharge. Estimates of the characteristic lifetime of the wall charge were obtained.