Thermal nonlinearity of potential surface waves at a plasma-metal interface

A study is made of the effect of the heating of plasma electrons in the field of a potential surface wave on the wave dispersion properties. The wave is assumed to propagate along the boundary between a metal and a finite-pressure plasma. Different mechanisms for electron energy losses are considered in the weak heating approximation. The spatial distribution of the plasma electron temperature under nonlocal heating conditions is derived on the basis of the electron energy balance equation. Expressions for the spatial damping rate and the nonlinear shift of the wavenumber are analyzed for different values of the plasma parameters. The results obtained are valid for both semiconductor and gaseous plasmas.     

Amplification of surface waves in a plasma waveguide by a straight relativistic electron beam in a finite magnetic field

The problem of the excitation of electron waves in a thin-walled annular cold plasma in a cylindrical waveguide by a straight relativistic electron beam in a finite magnetic field is considered. The dispersion properties of a waveguide system with parameters close to the experimental ones are investigated. It is shown that the growth rate of the excited high-frequency plasma wave is comparable to that of the low-frequency wave, which is weakly sensitive to the strength of the longitudinal magnetic field.     

Solitary fast magnetosonic waves propagating obliquely to the magnetic field in cold collisionless plasma

Solutions describing solitary fast magnetosonic (FMS) waves (FMS solitons) in cold magnetized plasma are obtained by numerically solving two-fluid hydrodynamic equations. The parameter domain within which steady-state solitary waves can propagate is determined. It is established that the Mach number for rarefaction FMS solitons is always less than unity. The restriction on the propagation velocity leads to the limitation on the amplitudes of the magnetic field components of rarefaction solitons. It is shown that, as the soliton propagates in plasma, the transverse component of its magnetic field rotates and makes a complete turn around the axis along which the soliton propagates.     

High-frequency surface waves at a plasma-metal interface: II. Dispersion of a nonlinear wave

A study is made of the dispersion properties of nonlinear surface waves propagating along a plasma-metal interface under conditions corresponding to the formation of a space charge sheath that equalizes the electron and ion fluxes to the wall. Oscillations of the plasma boundary under the action of the surface wave field are taken into account. It is shown that these oscillations are the main nonlinear mechanism for generating wave field harmonics and are analogous to the nonlinearity in the current-voltage characteristic of the space charge sheath. The effect of the nonlinearity on the dispersion properties of surface waves due to the relationship between the sheath thickness and wave amplitude is calculated with allowance for harmonic generation. The energy transported by surface waves under conditions typical of RF and microwave discharges is calculated.     

Emission of low-frequency electromagnetic waves by a short laser pulse propagating in a plasma with density fluctuations

It is shown that a short laser pulse propagating in a plasma with electron density fluctuations can emit electromagnetic waves with frequencies much lower than the laser carrier frequency. Emissions with frequencies close to the plasma frequency and the doubled plasma frequency in a nonisothermal plasma, as well as emission generated in a turbulent plasma, are examined. The effects in question are related to the transformation of the laser pulse wakefield into electromagnetic radiation by electron density fluctuations. The phenomenon under study opens new possibilities for diagnostics of both plasma fields excited by laser pulses and electron density fluctuations in a plasma.     

Concerning arc discharge in a transverse magnetic field

The retrograde motion of an arc in a transverse magnetic field is attributed to the onset of a tangential flow of gas or vapor. The physics of a polarized plasma jet conducting the current between the cathode and anode is discussed.     

Propagation of MHD waves in a plasma in a sheared magnetic field with straight field lines

The propagation of MHD plasma waves in a sheared magnetic field is investigated. The problem is solved using a simplified model: a cold plasma is inhomogeneous in one direction, and the magnetic field lines are straight. The waves are assumed to travel in the plane perpendicular to the radial coordinate (i.e., the coordinate along which the plasma and magnetic field are inhomogeneous). It is shown that the character of the singularity at the resonance surface is the same as that in a homogeneous magnetic field. It is found that the shear gives rise to the transverse dispersion of Alfvén waves, i.e., the dependence of the radial component of the wave vector on the wave frequency. In the presence of shear, Alfvén waves are found to propagate across magnetic surfaces. In this case, the transparent region is bounded by two turning points, at one of which, the radial component of the wave vector approaches infinity and, at the other one, it vanishes. At the turning point for magnetosonic waves, the electric and magnetic fields are finite; however, the radial component of the wave vector approaches infinity, rather than vanishes as in the case with a homogeneous field.     

Effect of the magnetic field curvature on magnetic islands in tokamaks

The effect of the magnetic field curvature on magnetic islands in a tokamak is analyzed. It is demonstrated that the original investigation of this effect by Kotschenreuther et al. (1985) is inconsistent: on the one hand, the authors made the correct assumption that this is an ideal effect and, on the other hand, they described it in terms of the parameters characteristic of the “resistive ordering” approach, which is incompatible with the ideal approximation. More recent studies of the magnetic curvature effect have produced further ambiguities; as a result, a branch of the theory of magnetic islands has arisen that is based on the supposition that the effect under discussion can be described in terms of the Glasser-Greene-Johnson parameter D_R. This branch is shown to be erroneous, because the parameter D_R describes the plasma response to magnetic field perturbations on spatial scales of about the dimension of the linear resistive layer, while the characteristic spatial scale of the magnetic islands is much longer. It is concluded that the correct theory developed here for the magnetic curvature effect makes more optimistic predictions about its stabilizing role.     

Quenching of an electric arc in a vacuum gap with a uniform transverse magnetic field

The breaking ability of a vacuum arc interrupter with a uniform transverse magnetic field formed by a system of permanent magnets was investigated experimentally. The vacuum interrupter with a 0.5-μF shunting capacitor switched off a dc current of up to 150 A at magnetic fields of 100–180 mT. At magnetic fields of 120–160 mT, the breaking ability of the vacuum interrupter was increased to 300 A by introducing a nonuniformity in the magnetic field distribution near the contact surface.     

Antibiotics production in a fluidized bed reactor utilizing a transverse magnetic field

A mathematical model is developed to describe the performance of a three-phase fluidized bed reactor utilizing a transverse magnetic field. The model is based on the axially dispersed plug flow model for the bulk of liquid phase and on the Michaelis-Menten kinetics. The model equations are solved by the explicit finite difference method from transient to steady state conditions. The results of the numerical simulation indicate that the magnetic field increases the degree of bioconversion. The mathematical model is experimentally verified in a three-phase fluidized bed reactor with Penicillium chrysogenum immobilized on magnetic beads. The experimental results are well described by the developed model when the reactor operates in the stabilized regime. At low and relatively high magnetic field intensities certain discrepancy in the model solution is observed when the model over estimates the product concentration.     

Flow of a plasma of multielectron elements along a magnetic field

An analysis is made of a flow of Ar plasma imitating plasma flows in ion separation systems such as systems for processing spent nuclear fuel or ion cyclotron resonance isotope separation systems. It is found that the electron temperature is equalized along the flow by electron heat conduction. When the electron temperature is not too low (T _e ≥ E _ion/10, where E _ion is the ionization energy), multicharged ions are intensely produced along the entire flow. It is shown that this process is accompanied by the flow acceleration. Difficulties in describing a supersonic flow by hydrodynamic equations are pointed out.     

Numerical simulations of spark channels propagating along the ground surface: Comparison with high-current experiment

A numerical model of a spark discharge propagating along the ground surface from the point at which an ∼100-kA current pulse is input into the ground has been developed based on experiments in which the velocity of a long leader was measured as a function of the leader current. The results of numerical simulations are in good agreement with the measured characteristics of creeping discharges excited in field experiments by using a high-power explosive magnetic generator. The reason why the length of a spark discharge depends weakly on the number of simultaneously developing channels is found. Analysis of the influence of the temporal characteristics of the current pulse on the parameters of the creeping spark discharge shows that actual lighting may exhibit similar behavior.     

Nonresonant excitation of surface waves by a normal electric field

A study is made of nonresonant parametric excitation of surface waves by a spatially uniform, time-dependent electric pump field directed perpendicular to a plane plasma-dielectric interface. A set of equations is derived that describes the dynamics of surface wave excitation. Expression for the growth rate in the linear stage of instability is obtained, and the threshold amplitude of the external electric field above which the parametric instability can occur is found. The spectrum of the excited waves is analyzed. Published in Russian in Fizika Plazmy, 2006, Vol. 32, No. 11, pp. 994–998. The article was translated by the author.     

Periodic emission bursts accompanying the interaction of plasma flows propagating across a magnetic field

It is found experimentally that the interaction of plasma flows propagating in opposite directions across a magnetic field is accompanied by periodic local bursts of intense optical and soft X-ray emission. It is shown that periodic variations in the emission intensity from the interaction region are related to the excitation of steady-state self-oscillations in the plasma. By varying the frequency and amplitude of these oscillations, it is possible to satisfy the resonance conditions at certain characteristic plasma frequencies.     

Surface waves in plasma waveguides with a smooth transverse inhomogeneity

A theory of cylindrical surface waves in a circular waveguide filled with a smoothly inhomogeneous plasma is presented. For a special radial profile of the plasma density, dispersion relations for the complex frequencies of surface waves are derived analytically. The dispersion relations are solved numerically (in the long-wavelength limit) and numerically. It is shown that there are two types of surface waves. When passing to the case of a sharply bounded plasma, one of the waves becomes an ordinary surface wave, while the other becomes strongly damped.     

Kinetic theory of the positive column of a low-pressure discharge in a transverse magnetic field

The influence of a transverse magnetic field on the characteristics of the positive column of a planar low-pressure discharge is studied theoretically. The motion of magnetized electrons is described in the framework of a continuous-medium model, while the ion motion in the ambipolar electric field is described by means of a kinetic equation. Using mathematical transformations, the problem is reduced to a secondorder ordinary differential equation, from which the spatial distribution of the potential is found in an analytic form. The spatial distributions of the plasma density, mean plasma velocity, and electric potential are calculated, the ion velocity distribution function at the plasma boundary is found, and the electron energy as a function of the magnetic field is determined. It is shown that, as the magnetic field rises, the electron energy increases, the distributions of the plasma density and mean plasma velocity become asymmetric, the maximum of the plasma density is displaced in the direction of the Ampère force, and the ion flux in this direction becomes substantially larger than the counter-directed ion flux.     

Excitation of surface waves at a plasma boundary by a short laser pulse

The excitation of surface waves by a laser pulse as it crosses a vacuum-plasma interface is considered. Surface waves are excited by a vortex electric current that is generated at the plasma boundary by the ponderomotive force of the pulse. The question is considered of how the duration and transverse dimensions of the pulse affect the spatiotemporal distribution and the spectral and energy parameters of the excited surface waves.     

Separation of the heavy and light ion components in a plasma flow propagating in a curvilinear magnetic field

The propagation of a metal plasma jet in a transport system with a curvilinear magnetic field was studied experimentally. The jet was generated by a pulsed vacuum arc discharge with a composite (W + Fe) cathode. Spatial separation of ions of the cathode material was observed at the exit from the system. The ions of the lighter element (Fe) were concentrated in the inner part of the cathode plasma jet deflected by the magnetic field. The jet is also found to be deflected along the binormal to the magnetic field lines due to plasma drift in the crossed magnetic and electric fields. The degree of mass separation of elements is shown to increase with increasing jet deflection along the binormal. The maximum value of the mass separation efficiency reaches 45, the effective value being 7.7.     

Effect of toroidal magnetic field variations on the spectra of azimuthal surface waves in metal waveguides entirely filled with plasma

The propagation of surface waves transverse to the circular axis of a toroidal magnetized metal waveguide entirely filled with a plasma is studied using perturbation theory. The distribution of the fields of these waves in such a waveguide structure is investigated. It is shown that the toroidicity introduces a second-order correction to the eigenfrequency of the surface waves.