Weakly ionized plasma in a supersonic plasma flow

The structure of the ion acoustic precursor of a shock wave in a weakly ionized collision-dominated plasma is studied numerically. It is shown that the simultaneous action of the nonlinearity, dispersion, and dissipation leads to the formation of an oscillating profile of the ion density in the precursor. There exist regimes in which the charged-particle density decreases abruptly and simultaneously the number of maxima in its profile within the precursor becomes smaller as the shock wave velocity increases in a jumplike manner. This effect is analogous to the corresponding hydrodynamic effect in narrow shallow channels (the “Houston's horse” effect). In the stage preceding this jumplike process, local regions may appear in which the degree of plasma ionization is elevated. Such plasma “bunches” give rise to the strong reverse action of the charged particles on the neutral component, resulting in the “stretching” of the precursor. This phenomenon is resonant in character and occurs in a narrow range of shock wave velocities.     

Investigation of the helicon discharge plasma parameters in a hybrid RF plasma system

Results of an experimental study of the helicon discharge plasma parameters in a prototype of a hybrid RF plasma system equipped with a solenoidal antenna are described. It is shown that an increase in the external magnetic field leads to the formation of a plasma column and a shift of the maximum ion current along the discharge axis toward the bottom flange of the system. The shape of the plasma column can be controlled via varying the configuration of the magnetic field.     

Role of plasma electrons in the generation of a gas discharge plasma

The role of different ionization mechanisms in penning-type gas discharges used to generate an emitting plasma in plasma electron sources is considered. It is shown that, under certain conditions, a substantial contribution to the process of gas ionization is provided by plasma electrons.     

Interaction of a high-current electron beam with hybrid waveguide and plasma modes in a dielectric Cherenkov maser with a plasma layer

The characteristics of a high-current electron beam-driven microwave amplifier—a dielectric Cherenkov maser—are investigated in the framework of linear theory for the case of a plasma layer present at the surface of the maser slow-wave structure. The dispersion relation for axisymmetric perturbations is obtained for the conventional configuration (a circular dielectric-lined waveguide and a thin annular beam propagating within the vacuum region inside the annular plasma) in the model of a fully magnetized plasma and beam. The results of numerically solving the dispersion relation for different beam and plasma parameters are presented, and an analysis based on these results is given with regard to the features of the beam interaction with the hybrid waves of the system (both hybrid waveguide and hybrid plasma modes). For the hybrid waveguide mode, the dependences of the spatial growth rate on the frequency demonstrate an improvement in the gain at moderate plasma densities, along with narrowing the amplification band and shifting it toward higher frequencies. For the hybrid plasma mode, the interaction with a mildly relativistic (200–250 keV) beam, when the wave phase velocity is close to the speed of light in the dielectric medium, is most interesting and, therefore, has been studied in detail. It is shown that, depending on the beam and plasma parameters, different regimes of the hybrid plasma mode coupling to the hybrid waveguide mode or a usual, higher order plasma mode take place; in particular, a flat gain vs. frequency dependence is possible over a very broad band. The parameters at which the ?3-dB bandwidth calculated for the 30-dB peak gain exceeds an octave are found.     

Wall plasma in a wideband dielectric cherenkov maser

Results are reported of experimental investigations that have revealed the presence of a plasma in the interaction region of a model wideband relativistic microwave amplifier—a dielectric Cherenkov maser. The electrodynamic properties of a hybrid system—a waveguide with an annular dielectric liner and a plasma layer adjacent to its inner wall—are analyzed. Experiments with a high-current accelerator have revealed that the power of the emitted microwaves at the output of the system increases strongly when an external microwave source at different frequencies in the X-band is switched on. However, this effect was found to be hard to reproduce. Indirect evidence is obtained of the fact that, during the transport of an electron beam and under the action of the signal from a high-power pulsed magnetron, the plasma in the system is created at the surface of the dielectric. In the model of a cold magnetized plasma, a dispersion relation is derived for axisymmetric waves in a system with a wall plasma layer. The spectra of the waveguide and plasma modes in the system and the transverse structure of their electromagnetic fields are investigated thoroughly as functions of the plasma density and layer thickness. It is shown that even a very thin layer of a high-density plasma results in a large frequency shift of the dispersion curve of the waveguide mode, in which case the coupling impedance at a fixed frequency decreases sharply. On the other hand, a layer of a moderately dense plasma increases the coupling impedance for the waveguide mode. It is established that, in a configuration with a wall plasma layer, the longitudinal component of the electric field of a plasma mode whose power flux in the dielectric is of a volumetric nature reverses direction across the layer.     

Helicon plasma in a nonuniform magnetic field

The distributions of the electron density in a plasma produced by helicon waves and the correspond-ing wave amplitudes and phases are studied experimentally. The measurements were carried out in an argon plasma at a pressure of 3 mtorr and at an input RF power of up to 600 W. The magnetic field was caried in the range from 0 to 200 G. The efficiency of plasma production in both uniform and nonuniform fields is investigated. It is shown that, in a nonuniform magnetic field, the electron density can be substantially increased (up to 5×10¹² cm^?3) by placing an antenna in the region in which the magnetic field is weaker than in the main plasma.     

Radio-frequency stabilization of a nonequilibrium plasma accelerator

Results of experimental studies of the effect of an external RF field on the excitation of oscillations in a magnetoplasmadynamic plasma accelerator are presented. It is found that applying an RF field can suppress the drift component of low-frequency oscillations in the ejected plasma flow. The experimental data agree with the concept of stabilization of the plasma accelerator by the magnetic component of the field generated by the RF current loop. The conditions under which the RF field stabilizes the generation of the plasma flow are determined, and the factors limiting the stabilization efficiency are revealed.     

Buffalo plasma fibronectin: a physico-chemical study

Plasma fibronectin (FN) of buffalo (Babulis babulis) was purified to apparent homogeneity, using gelatin-Sepharose and heparin-Sepharose affinity columns. It was found to have two subunits of molecular mass 246 kDa and 228 kDa, on SDS-gel. Its immunological cross-reactivity with anti-human plasma FN was confirmed by Western blotting. The amino acid composition was found to be similar to that of human and bovine plasma FNs. Buffalo plasma FN contained 2.23% neutral hexoses and 1.18% sialic acids. No titrable sulfhydryl group could be detected in the absence of denaturant. Reaction with DTNB indicated 3.4 sulfhydryl groups in the molecule, whereas BDC-OH titration gave a value of 3.8 -SH groups in buffalo plasma FN. Stoke's radius, intrinsic viscosity, diffusion coefficient and frictional ratio indicated that buffalo plasma FN did not have a compact globular conformation at physiological pH and ionic strength. Molecular dimensions (average length, 120 nm; molar mass to length ratio, 3950 nm(-1) and mean diameter, 2.4 nm) as revealed by rotary shadowing electron microscopy further supported the extended conformation of buffalo plasma FN. These results show that buffalo plasma FN has similar properties as that of human plasma FN.     

Dynamics of the plasma injected into the gap of a plasma opening switch across a strong magnetic field

A method is proposed to increase the linear charge density transferred through a plasma opening switch (POS) and, accordingly, reduce the POS diameter by enhancing the external magnetic field in the POS gap. Results are presented from experimental studies of the dynamics of the plasma injected into the POS gap across a strong magnetic field. The possibility of closing the POS gap by the plasma injected across an external magnetic field of up to 60 kG is demonstrated.     

Potential of a test charge in a dust-electron plasma

The potential of a slowly moving test charge in a positive-dust-electron plasma is calculated taking into account dust grain charge fluctuations, as well as collisions between neutral atoms and electrons and dust grains. The results should be useful for understanding the shielding in a dusty plasma sheath with levitated grains.     

Photoionization two-stream instability in a collisional plasma

An analysis is made of how electron collisions influence the development of photoionization two-stream instability of a plasma produced in the irradiation of matter by a short X-ray pulse from a free-electron laser. The cases of a weakly ionized and a completely ionized plasma are discussed.     

Dispersion properties of a plasma produced by a short X-ray pulse

A collisionless plasma produced by a short ionizing pulse from an X-ray laser is characterized by an anisotropic monoenergetic electron distribution governed by the classical photoeffect. The dispersion properties of such a photoionized plasma are studied. The spectra of high-frequency plasma waves and their damping, as well as the parameters of the aperiodic instability of a photoionized plasma, are described. The relationship between the electrostatic and magnetic perturbations generated by this instability is investigated, and an analysis is made of how the instability transforms into a purely longitudinal (two-stream-like) instability and into a purely transverse (Weibel-like) instability, depending on the absolute value and direction of the wave vector.     

Kelvin-Helmholtz instability for a bounded plasma flow in a longitudinal magnetic field

Kelvin-Helmholtz MHD instability in a plane three-layer plasma is investigated. A general dispersion relation for the case of arbitrarily orientated magnetic fields and flow velocities in the layers is derived, and its solutions for a bounded plasma flow in a longitudinal magnetic field are studied numerically. Analysis of Kelvin-Helmholtz instability for different ion acoustic velocities shows that perturbations with wavelengths on the order of or longer than the flow thickness can grow in an arbitrary direction even at a zero temperature. Oscillations excited at small angles with respect to the magnetic field exist in a limited range of wavenumbers even without allowance for the finite width of the transition region between the flow and the ambient plasma. It is shown that, in a low-temperature plasma, solutions resulting in kink-like deformations of the plasma flow grow at a higher rate than those resulting in quasi-symmetric (sausage-like) deformations. The transverse structure of oscillatory-damped eigenmodes in a low-temperature plasma is analyzed. The results obtained are used to explain mechanisms for the excitation of ultra-low-frequency long-wavelength oscillations propagating along the magnetic field in the plasma sheet boundary layer of the Earth’s magnetotail penetrated by fast plasma flows.     

Transport barrier as a bifurcation of the equilibrium of a tokamak plasma

The phenomenon of the internal transport barrier in a tokamak plasma is interpreted as a bifurcation of the plasma equilibrium. An expression describing the change in the plasma pressure due to the buildup of the barrier is derived as a functional dependent on the parameters of the original plasma equilibrium and the plasma rotation velocity within the barrier. This expression is applied to a circular cross section tokamak, specifically, the T-10 device.     

Wakefield excitation by a relativistic electron bunch in a magnetized plasma

The excitation of a wake wave by a relativistic electron beam in an unbounded magnetized plasma and a plasma waveguide is studied theoretically. It is shown that, in a waveguide partially filled with a plasma, the energy that the electrons of the accelerated beam can gain is 37 times higher than the energy of the electrons of the beam generating wakefield.     

Numerical simulations of a high-current plasma lens

The dynamic processes by which an electrostatic plasma lens with a wide-aperture ion beam and electrons produced from the secondary ion-electron emission relaxes to a steady state is investigated for the first time by the particle-in-cell method. The parameters of a two-dimensional mathematical model were chosen to correspond to those of actual plasma lenses used in experimental studies on the focusing of high-current heavy-ion beams at the Institute of Physics of the National Academy of Sciences of Ukraine (Kiev, Ukraine) and the Lawrence Berkeley National Laboratory (Berkeley, USA). It is revealed that the ion background plays a fundamental role in the formation of a high potential relief in the cross section of a plasma lens. It is established that, in the volume of the plasma lens, a stratified electron structure appears that is governed by the nonuniform distribution of the external potential over the fixing electrodes and the insulating magnetic field. The stratification is very pronounced because of the finite sizes of the cylindrical fixing electrodes of the lens. It is shown that the presence of such a structure limits the maximum compression ratio for an ion beam to values that agree with those observed experimentally.     

Effect of plasma nonlinearity on the beam-plasma interaction in a hybrid plasma waveguide

A study is made of the characteristic features of the effect of plasma nonlinearity in a slow-wave structure on microwave generation by an electron beam and on electron beam energy losses. Theoretical results on the plasma density variation, the amplitude of the excited microwaves, and the velocity distribution function of the beam electrons are compared with the experimental data. It is shown that the self-consistency between the decreasing plasma density gradient and the spatial variation of the amplitude of an amplified wave in a slowwave structure leads to a significant (severalfold) increase in the efficiency with which the electron beam energy is converted into microwave energy in short pulses. The predictions of the theoretical model developed to describe the non-steady-state beam-plasma interaction agree well with the experimental data.     

Interaction of an ion bunch with a plasma slab

Charge neutralization of a short ion bunch passing through a plasma slab is studied by means of numerical simulation. It is shown that a fraction of plasma electrons are trapped by the bunch under the action of the collective charge separation field. The accelerated electrons generated in this process excite beam?plasma instability, thereby violating the trapping conditions. The process of electron trapping is also strongly affected by the high-frequency electric field caused by plasma oscillations at the slab boundaries. It is examined how the degree of charge neutralization depends on the parameters of the bunch and plasma slab.     

Mass separation of a multicomponent plasma flow in a curvilinear magnetic field

The motion of a metal plasma flow of a vacuum-arc discharge in a transportation plasma-optical system with a curvilinear magnetic field is studied experimentally and numerically. The flow position at the output of the system is shown to depend on the cathode material, which determines the mass-to-charge ratio of plasma ions. As a result, the flow with a greater ion mass-to-charge ratio moves along a trajectory with a larger radius. A similar effect is observed in the case of a multicomponent plasma flow generated by a composite cathode. The results of two-fluid MHD simulations of a plasma flow propagating in a curvilinear magnetic field agree qualitatively with the experimental data.     

Effect of the radial plasma profile on the surface wave structure in a plasma waveguide

A study is made of the effect of the radial plasma profile on the spectra and fields of the surface waves in a plasma waveguide. It is shown that the surface wave is localized in the region where the plasma permittivity vanishes. In waveguides with smoother radial plasma profiles, the region where the surface wave can exist is narrower and may even disappear.