Fizika plazmy

The work presents the results of the first calculations of the T-15MD tokamak near-wall plasma in the SOLPS-ITER code taking into account the effects of drifts and currents. The modes with the power passing through the separatrix PSOL = 6 MW and different H gas puff intensities, corresponding to the average electron density on the separatrix, nesep = (2–4.5)·1019 m−3, are considered. The same as in other tokamaks of similar size, E × B drift leads to the flow of hydrogen from the outer divertor to the inner one, which changes the load distribution between the divertor targets. Drifts also affect the flow of the carbon impurity. As a rule, when describing the dependence on the H gas puff, either n esep or the total amount of hydrogen in the scrape off layer (SOL), Ntot, is used as a parameter characterizing the discharge. In this case, these quantities are considered as equivalent plasma characteristics in the SOL. It is shown that, from the point of view of estimating the influence of drifts, these quantities are not equivalent: the dependence of some divertor parameters on nesep does not change with the inclusion of drifts, but the dependence on Ntot can change. It is also seen that drifts lead to a more pronounced maximum in the dependence of the saturation current on the electron density, Isat(nesep). This is explained by changes in the emission of the carbon impurity and the power of the recombination source of hydrogen in the divertor.

For typical conditions of the ECRH experiments at the TCV tokamak, a new scenario of low-threshold decay of the extraordinary wave, the frequency of which corresponds to the second harmonic of electron cyclotron frequency at the axis of the plasma column, is considered. Within the scenario, the parametric decay of the pump wave is accompanied by the excitation of a two-dimensional eigenmode of the ion Bernstein (IB) wave and a traveling (non-localized) electron Bernstein wave. It is shown that the saturation of this parametric decay instability occurs at a relatively low level, as a result of stochastic amplitudedependent damping of the IB-wave. It was discovered that there is a “second threshold” in the pump wave power, above which the stochastic damping only reduces the growth rate of primary instability, and the saturation occurs at considerably higher level, as a result of a cascade of secondary decays of the primary IB-wave, accompanied by the excitation of eigenmodes of the plasma waveguide of the IB-wave.

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.

The paper studies ion motion during propagation and collisions of ion-acoustic solitary waves in electronion plasma and in plasma with negative ions. In a number of cases, the displacements of ion locations after the passage of the wave are calculated and their dependecies on the wave amplitude are found. The reverse influence of processes occurring in plasma during collisions of ion-acoustic solitary waves on the waves themselves is described. Physical mechanisms preventing the preservation of the identity of such waves during their mutual collisions are discussed. The effect of ion displacement on the formation of caustics during collisions of ion-acoustic solitary waves in plasma with negative ions is considered.

The influence of non-reciprocity of interparticle forces on the energy transfer in a planar plasma crystal is theoretically studied. It is shown that the energy exchange between the dust component and the surrounding plasma plays a significant role. The distribution of the average kinetic energy of particles in the vicinity of a point source of random noise is calculated.

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 positiveor 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.

Lightning is represented as a multistable system demonstrating the ability to self-regulation by maintaining its own electroneutrality. Within the framework of the description of the lightning channel using telegraphic equations, a nonlinear parabolic equation is obtained for the nonlinear voltage dependence of the rate of change of the plasma cord charge. The analysis of the model shows that the lightning channel alternately develops in one of two modes, each of which is characterized by damping of the longitudinal current from one end of the lightning to the other. The transition between the modes is realized by excitation of a fast switching wave. Lightning development within each mode is accompanied by recharging of the leader system sheath and movement of the point of zero charge of the sheath (called the lightning reversal point) in the direction of longitudinal current growth. The movement of the reversal point is caused by the change of the mean potential of the discharge tree in the process of sheath recharge and explains the observed dynamics of lightning transients.