Electromagnetic beam-plasma interactions in a magnetic field

The excitation of plasma oscillations in a thin-walled annular plasma by an annular electron beam in a cylindrical waveguide is considered in the linear approximation. The instability growth rates and spatial amplification coefficients in the beam-plasma system under the conditions of the Cherenkov and anomalous Doppler resonances are obtained and compared with those in a transversely homogeneous system. The contributions from different instability mechanisms are analyzed.     

Acceleration of ions in a beam-plasma discharge in a low magnetic field: Interrelation between the electron and ion energy distributions

Previous experiments revealed the effect of stable acceleration of ions in a plasma-beam discharge in a low magnetic field to energies one order of magnitude higher than the electron thermal energy. To verify the previously proposed mechanisms for this effect, the velocity distribution function of the electrons arriving at the collector and the energy distribution of the ions escaping from the discharge transversely to the axis were measured. It is found that ion acceleration is accompanied by significant electron heating near the discharge axis. The time behavior and longitudinal profile of the intensity of the excited high-frequency oscillations in the frequency range ω ～ ω _pe were studied. The accumulation of regular oscillations in the beam-injection region and their stochastization during the propagation along the system axis were observed. The experimental results correlate qualitatively with the data of previous numerical simulations.     

Mechanism for ion acceleration along the normal to the axis of a beam-plasma discharge in a weak magnetic field

The mechanism responsible for the previously discovered phenomenon of acceleration of an ion flow along the normal to the axis of a beam-plasma discharge in a weak magnetic field is investigated. It is suggested that the ions are accelerated in the field of a helicon wave excited in the discharge plasma column. It is shown theoretically that, under actual experimental conditions, a helicon wave can be excited at the expense of the energy of an electron beam. The spectral parameters and spatial structure of the waves excited in a beam-plasma discharge in the frequency ranges of Langmuir and helicon waves are studied experimentally and are shown to be related to the parameters of the ion flow. Theoretical estimates are found to agree well with the experimental results.     

Design of a magnetic field generator for experiments on magnetic effects in cell cultures

A magnetic field generator constructed of rare earth-cobalt magnets is proposed for examining the biological effects of static magnetic fields (less than 1 T) on tissue cultures. Important quantities of a magnetic field from a biological-effects viewpoint, ie, its strength and the product of strength and gradient, are analysed. A practical procedure for designing the generator with optimum parameters is given. Also, parameters are determined which will yield a sinusoidal spatial field distribution.     

Electrostatic electron vortex structure in a plasma in an external magnetic field

A previously developed method for describing vortex structures is used to construct electrostatic vortices in a plasma in an external magnetic field. An equation for the radial electric field that gives rise to azimuthal electron drift in crossed electric (E _r ) and magnetic (B _z ) fields is derived without allowance for the magnetic field of the electron currents. Two types of the resulting electrostatic vortex structures with a positive and a negative electric potential at the axis are analyzed. The results obtained are compared with experimental data on vortex structures.     

Development of beam-plasma instability during the injection a low-energy electron beam into the ionospheric plasma

Results are presented from an active experiment on the injection of charged particle beams into the ionospheric plasma. The experiment was carried out in 1992 onboard the Intercosmos-25 satellite and the Magion-3 daughter satellite (APEX). A specific feature of this experiment was that both the ion and electron beams were injected upward, in the same direction along the magnetic field. The most interesting results are the excitation of HF and VLF-LF waves and the generation of fast charged particle flows, which were recorded on both satellites.     

Relativistic diamagnetic equilibrium of a thin-walled annular electron beam in an external magnetic field

A self-consistent equilibrium state of a thin-walled annular electron beam in an external magnetic field is investigated with allowance for diamagnetic effect and relativistic effects in the beam rotational motion. An equation for the relativistic angular velocities of the beam rotation is derived in the hydrodynamic approximation. The main parameters of the beam equilibrium state are obtained analytically and are calculated numerically. The parameters of a longitudinally homogeneous, relativistic diamagnetic high-density electron beam are determined.     

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.     

Investigation of the chaotic dynamics of an electron beam with a virtual cathode in an external magnetic field

The effect of the strength of the focusing magnetic field on chaotic dynamic processes occurring in an electron beam with a virtual cathode, as well as on the processes whereby the structures form in the beam and interact with each other, is studied by means of two-dimensional numerical simulations based on solving a self-consistent set of Vlasov-Maxwell equations. It is shown that, as the focusing magnetic field is decreased, the dynamics of an electron beam with a virtual cathode becomes more complicated due to the formation and interaction of spatiotemporal longitudinal and transverse structures in the interaction region of a vircator. The optimum efficiency of the interaction of an electron beam with the electromagnetic field of the vircator is achieved at a comparatively weak external magnetic field and is determined by the fundamentally two-dimensional nature of the motion of the beam electrons near the virtual cathode.     

Electric potential of a macroparticle in beam-plasma systems

Dusty plasma research is one of the most extensively developed areas of modern plasma physics. In spite of the large number of papers devoted to determining the electric potential of dust grains in gas discharge plasmas, this problem is still far from being resolved completely. In this paper, the behavior of the floating electric potential of a macroparticle in an electron beam-plasma system is studied in the framework of the orbit motion limited model taking into account secondary electron emission.     

Role of the Hall flute instability in the interaction of laser and space plasmas with a magnetic field

The interaction of an expanding laser plasma with a uniform external magnetic field is studied over a wide range of experimental parameters (for a plasma energy of up to 300 J and a magnetic induction of up to 8 kG). By analyzing the data from these and other experiments, as well as the results of simulations with the use of a two-fluid Hall plasma model, it was found for the first time that the flute instability of the plasma boundary plays a decisive role in the process of the plasma cloud expansion. It is shown that, when the ion Larmor radius is sufficiently large, this instability can significantly affect the maximum radius of the diamagnetic cavity of the plasma cloud and the deceleration of its front by the magnetic field. A physical model based on the Hall effect is proposed to explain such influence. The model adequately describes data from one-dimensional simulations, as well as from experiments with quasi-spherical laser plasma clouds. The results obtained can be helpful in interpreting the data from active magnetospheric experiments with barium plasma clouds (such as AMPTE) and analyzing the plasma dynamics in future ICF reactors and propulsion systems with a magnetic field for direct conversion of fusion energy into electric energy.     

Impact of magnetic field inhomogeneity on electron cyclotron radiative loss in tokamak reactors

The potential importance of electron cyclotron (EC) emission in the local electron power balance in the steady-state regimes of ITER operation with high temperatures, as well as in the DEMO reactor, requires accurate calculation of the one-dimensional (over magnetic surfaces) distribution of the net radiated power density, P _EC(ρ). When the central electron temperature increases to ∼30 keV, the local EC radiative loss comprises a substantial fraction of the heating power from fusion alphas and is close to the total auxiliary NBI heating power, P _EC(0) ≃ 0.3P _α(0) ≃ P _aux(0). In the present paper, the model of EC radiative transport in an axisymmetric toroidal plasma is extended to the case of an inhomogeneous magnetic field B(R, Z). The impact of such inhomogeneity on local and total power losses is analyzed in the framework of this model by using the CYNEQ code. It is shown that, for the magnetic field B, temperature T _e , density n _e , and wall reflection coefficient R _w expected in ITER and DEMO, accurate simulations of the EC radiative loss require self-consistent 1.5D transport analysis (i.e., one-dimensional simulations of plasma transport and two-dimensional simulations of plasma equilibrium). It is shown that EC radiative transport can be described with good accuracy in the 1D approximation with the surface-averaged magnetic field, B(ρ) = 〈B(R, Z)〉 _ms . This makes it possible to substantially reduce the computational time required for time-dependent self-consistent 1.5D transport analysis. Benchmarking of the CYNEQ results with available results of the RAYTEC, EXACTEC, and CYTRAN codes is performed for various approximations of the magnetic field.     

Effect of the external magnetic field on the critical current for the onset of a virtual cathode in an electron beam

The critical current at which an unsteady oscillating virtual cathode forms in an electron beam is studied as a function of the external magnetic field guiding the beam electrons. It is shown that the critical beam current decreases with external magnetic field and that there is an optimum magnetic induction at which the critical current for the onset of an oscillating virtual cathode in the beam is minimum. For a strong guiding magnetic field, the critical beam current is described by relationships derived under the assumption that the motion of the beam electrons is one-dimensional. Such behavior is explained by the characteristic features of the dynamics of the beam electrons in longitudinal and radial directions in the interaction space at different inductions of the external magnetic field.     

Energy characteristics of beam-plasma interaction in a closed volume

Energy exchange between an electron beam and plasma during a beam-plasma discharge in a closed cavity excited by the electron beam is analyzed using computer simulations by the KARAT code. A method allowing one to analyze the beam-plasma interaction in the quasi-steady stage of the discharge is proposed. Qualitative characteristics of energy exchange (such as beam energy losses and the energy distributions of beam electrons and plasma particles leaving the discharge) both during spontaneous discharge excitation and in the presence of initial beam modulation by regular or noiselike signals are determined. The results obtained enable one to estimate the energy characteristics of a plasma processing reactor based on a beam-plasma discharge.     

Nonlinear dynamics and chaotization of oscillations of a virtual cathode in an annular electron beam in a uniform external magnetic field

Results are presented from a numerical study of the effect of an external magnetic field on the conditions and mechanisms for the formation of a virtual cathode in a relativistic electron beam. Characteristic features of the nonlinear dynamics of an electron beam with a virtual cathode are considered when the external magnetic field is varied. Various mechanisms are investigated by which the virtual cathode oscillations become chaotic and their spectrum becomes a multifrequency spectrum, thereby complicating the dynamics of the vircator system. A general mechanism for chaotization of the oscillations of a virtual cathode in a vircator system is revealed: the electron structures that form in an electron beam interact by means of a common space charge field to give rise to additional internal feedback. That the oscillations of a virtual cathode change from the chaotic to the periodic regime is due to the suppression of the mechanism for forming secondary electron structures.     

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.     

Plasma heating in a variable magnetic field

The problem of particle acceleration in a periodically variable magnetic field that either takes a zero value or passes through zero is considered. It is shown that, each time the field [0]passes through zero, the particle energy increases abruptly. This process can be regarded as heating in the course of which plasma particles acquire significant energy within one field period. This mechanism of plasma heating takes place in the absence of collisions between plasma particles and is analogous to the mechanism of magnetic pumping in collisional plasma considered by Alfvén.     

Dynamics of a magnetic field in dusty plasma

The problem of the expansion of a magnetic field in a complex (e.g., dusty) plasma is considered, with a focus on the effects produced by the independent transport of charged components that does not break plasma quasineutrality. Solutions to a set of nonlinear equations are obtained for different initial and boundary conditions. In particular, it is shown that the field rapidly penetrates into the plasma when the dust is charged negatively and does not penetrate at all when the dust charge is positive.     

Influence of the pulsating electric field on the ECR heating in a nonuniform magnetic field

According to a computer simulation, the randomized pulsating electric field can strongly influence the ECR plasma heating in a nonuniform magnetic field. It has been found out that the electron energy spectrum is shifted to the high energy region. The obtained effect is intended to be used in the ECR sources for effective X-ray generation.     

Instability of a low-pressure hollow-cathode discharge in a magnetic field

Mechanisms responsible for current oscillations at the ion branch of the probe characteristic are investigated experimentally and theoretically. A comparison between experiment and theory shows that the oscillations in a hollow-cathode discharge in a longitudinal magnetic field are most likely related to the onset of helical instability.