Vol 45, No 1 (2019)

The phenomenon of large-scale discharge oscillations in Morozov’s stationary plasma thruster (SPT) is physically interpreted by analyzing global modes of gradient drift instability. The problem is solved using an ideal two-fluid hydrodynamic plasma model that includes the effects of stationary electron flow, electron inertia, and spatial inhomogeneities of the magnetic field and plasma density along the accelerating channel. The frequencies and axial structure of unstable eigenmodes are calculated for typical parameters of the SPT-100 thruster. The obtained spectrum is characterized by a finite set of long-wavelength azimuthal modes in the lower hybrid frequency range, which are predominantly localized in the near-anode region of the thruster. It is shown that the eigenmodes can form wave packets the main characteristics of which in the linear stage of instability coincide with the parameters of the experimentally observed large-scale azimuthal spoke-like structures. The influence of the thruster geometry (the length and width of the accelerating channel) on the frequency characteristics of oscillations and formation of beatings is investigated.

Morozov’s stationary plasma thrusters (SPTs) operating with xenon have been successfully used for many years in space technology. At the same time, due to the high cost of xenon, interest has arisen in alternate working substances. In a number of works, specific features and characteristics of SPTs operating with krypton, which is produced in greater amounts and is one order of magnitude cheaper than xenon, were studied. It was shown that SPTs operating with krypton under typical SPT conditions have traditional characteristics; however, their thrust efficiency is significantly lower than that of thrusters operating with xenon. One of the main reasons for the reduction in the thrust efficiency is the lower conversion efficiency of krypton atoms into ions, which depends on many factors. The most important among them is the krypton flow rate, which determines the plasma density in the acceleration channel. The influence of this parameter on the characteristics of SPTs operating with krypton has not yet received sufficient study, and most works were carried out using one model and in a limited range of krypton flow rates. This paper presents results of a comparative study of the influence of the xenon and krypton flow rates on the characteristics of different-scale SPTs in a wide range of gas flow rates. The results of this study provide information on the specific features of SPTs operating with krypton for different values of the gas flow rate and different geometries of the exit part of the acceleration channel. This information can be helpful for the development of advanced thrusters operating with krypton.

Designing magnetic systems of Galathea plasma traps on the basis of levitating superconducting magnetic coils requires searching for their stable levitating states. For this purpose, proceeding from the property of superconductors to preserve the trapped magnetic flux, the potential energy of a system of several coaxial superconducting rings (one of the rings is fixed) with given trapped magnetic fluxes is obtained as an analytic function of the coordinates of the free rings along the system axis and angular deviations of their axes from the common axis of the system in a uniform gravity field in the thin-ring approximation. Computations demonstrate that, under certain values of physical parameters (the trapped magnetic flux and the dimensions and masses of the rings), this dependence has local minima, which correspond to stable equilibrium states of the levitating rings. For high-temperature superconducting (HTSC) rings and short-circuited HTSC coils manufactured for experiments on levitation, equilibrium states are found in different cases by calculating the aforementioned dependence of the potential energy. In the case where the magnetic fluxes trapped by the rings have the same signs, the calculated equilibrium levitating states of an HTSC ring in the field of a short-circuited HTSC coil (or an HTSC ring), namely, states that are stable against vertical displacements and angular deviations of the axis from the vertical, are implemented experimentally.

Mathematical models and simulations of plasma processes in scientific and technological projects proposed and, to a large extent, implemented by A.I. Morozov are reviewed. The plasmadynamic models are based on the magnetohydrodynamic (MHD) equations and their generalizations and deal with investigation of plasma flows in the channels–nozzles of the plasma thrusters. The calculations made an important contribution to the theory of an MHD analog of the de Laval nozzle and facilitated successful development and creation of a high-power quasi-stationary high-current plasma thruster. The plasmastatic models in terms of boundary-value problems with the Grad–Shafranov equation were realized in calculations of equilibrium magnetoplasma configurations in traps with current-carrying conductors embedded in plasma. Morozov also named these systems the Galatea traps. The results include calculations of the geometry, quantitative characteristics of the analyzed configurations, and a number of characteristic features in problems related to magnetic plasma confinement. General questions regarding mathematical models of interaction of reaction and diffusion processes are also discussed. The geometry of vacuum magnetic field forming magnetic surfaces for plasma confinement in traps is calculated.

Capillary-porous electrodes for plasma MHD devices are considered. The electrodes can be continuously renewable and allow one to use a scheme of the inverted MHD generator (i.e., MHD accelerator) as a thruster for interorbital flights. Two types of plasma acceleration are considered: (i) Lorentz force acceleration with a primary current perpendicular to the acceleration direction (Faraday scheme) and (ii) acceleration based on the Hall effect. In the first case, the thruster has advantages only at thrust powers exceeding 1 MW, while in the second case, the thrust and specific impulse are comparable with those of the known analogs (or even surpass them) already at powers of 500–1000 kW. The operating conditions of capillary-porous electrodes are formulated.

In 1974, A.I. Morozov and L.S. Solov’ev derived a general system of time-independent equations of two-component ideal plasma hydrodynamics [1]. This system is extremely complicated and cumbersome. For the axisymmetric case, the authors succeeded in writing it in a more compact form by introducing three flux functions for the magnetic field, electrons, and ions. The situation resembles the derivation of the Grad–Shafranov equation, when the problem of static plasma equilibrium is reduced to one second-order equation for the magnetic flux. In this case, in order to close the problem, it was required to specify two arbitrary parameter functions. When considering flows instead of static equilibrium, the number of parameter functions increases. In the present work, the Morozov–Solov’ev equations are used to describe steady-state plasma configurations in toroidal magnetic traps.

Accumulation of cold ions trapped within a space-charge-limited sheath collapses the sheath, causing a transition to the inverse sheath mode. A driving mechanism creating trapped ions is charge-exchange collisions which occur between fast ions and cold neutrals. Due to the complex nature of the temporally evolving sheath, it is difficult to predict how long the transition takes. Depending on the properties of the plasma, emitted electrons, and neutrals, the time scale can range from microseconds to hours. For experimental situations, it is important to understand whether the sheath will transition to an inverse mode within the observation time allotted. In this paper, we establish a theoretical basis for defining transition time of the sheath in terms of plasma properties. Calculations include an analytical approximation for the length of the virtual cathode, the amount of charged particles in each layer of the space-charge-limited sheath, and a time for its transition to the inverse sheath. The theoretical model is then compared to 1D kinetic simulations of a space-charge-limited sheath with charge-exchange collisions present. The results are applied to estimate transition time scales for applications in laboratory plasma experiments, the lunar sheath, and tokamaks.