摘要:

Ion‐beam modes have been generated in a double plasma device containing argon in which the driver and target plasmas were separated by a hemispherical screen biased negatively with respect to both plasmas. Making the driver plasma positive with respect to the target plasma resulted in the formation of an ion beam flowing from the driver plasma to the target plasma and causing the formation of a focal region in the target plasma. Both single pulses and pulse trains were excited in the target plasma by application of positive voltage pulses and/or sine waves to the hemispherical grid. Detection of these modes was accomplished using a positively biased spherical probe capable of axial and rotational movement. The wave amplitude and the phase velocity along the axis of the target plasma were measured over a distance of more than two hemispherical grid radii. Since no theoretical treatments have as yet been published for the hemispherical case, the measured values have been compared with theoretical values for a spherical separating screen.

ISSN:0899-8221    

DOI:10.1063/1.860873

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The stability of electromagnetic ballooning modes is investigated using a toroidal two‐fluid model allowing for arbitraryLn/LB(the characteristic scale lengths of density and magnetic‐field inhomogeneities). The ballooning mode equation is solved numerically and the two‐fluid and magnetohydrodynamic stability properties are discussed and compared. The perpendicular compressibility strongly reduces the growth rate and forLn/LB∼Ln/LT∼1, the mode is completely stabilized.

ISSN:0899-8221    

DOI:10.1063/1.860874

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The three‐dimensional problem of the wave propagation from a point source in a moving magnetoactive plasma is considered for the case of monochromatic waves. The magnetic Reynolds number is assumed to be small, so that the potential approximation for the electric field perturbation can be accepted. The cases of subsonic and supersonic main flows are considered. The method of integral transformations is used for the solution of the problem. The high‐frequency asymptotic expansion of the solution is obtained and analyzed. For a supersonic flow, the existence of the electric and gas‐dynamic perturbations outside the classic Mach cone is established, and the properties of these perturbations are examined.    

ISSN:0899-8221    

DOI:10.1063/1.860875

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In this article the authors describe a perturbation method used to calculate the flow of a compressible magnetized plasma around a sharp corner with a small turning angle. The main purpose for this analysis is the study of the flow in the exit region of a plasma accelerator in the regime of high magnetic Reynolds number. The physical model is based on a one‐fluid resistive magnetohydrodynamic model (continuum model). The analysis predicts the existence of an acoustic expansion fan near the sharp corner, and a magnetoacoustic expansion fan in the far field. The current lines refract across the acoustic fan. Near an anode there is a strong mass depletion downstream of the corner. Along an anode, some of the current lines reattach far downstream, creating a magnetic boundary layer along the electrode. Near a cathode there is a region of high density and high temperature ahead of the corner. Along a cathode most of the current attaches itself ahead of the corner. Finally, the current refraction predicted by this theory correlates qualitatively with the experimental measurements in the regime of high magnetic Reynolds number.

ISSN:0899-8221    

DOI:10.1063/1.860824

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A large ion Larmor radius plasma undergoes a particularly robust form of Rayleigh–Taylor instability when sub‐Alfve´nically expanding into a magnetic field. Results from an experimental study of this instability are reported and compared with theory, notably a magnetohydrodynamic (MHD) treatment that includes the Hall term, a generalized kinetic lower‐hybrid drift theory, and with computer simulations. Many theoretical predictions are confirmed while several features remain unexplained. New and unusual features appear in the development of this instability. In the linear stage there is an onset criterion insensitive to the magnetic field, initial density clumping (versus interchange), linear growth rate much higher than in the ‘‘classic’’ MHD regime, and dominant instability wavelength of order of the plasma density scale length. In the nonlinear limit free‐streaming flutes, apparent splitting (bifurcation) of flutes, curling of flutes in the electron cyclotron sense, and a highly asymmetric expansion are found. Also examined is the effect on the instability of the following: an ambient background plasma (that adds collisionality and raises the expansion speed/Alfve´n speed ratio), magnetic‐field line tying, and expansion asymmetries (that promotes plasma cross‐field jetting).

ISSN:0899-8221    

DOI:10.1063/1.860825

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The kinetic Kelvin–Helmholtz instability in a collisionless magnetoplasma is simulated numerically in cases where the ion gyroradius is comparable with or larger than the spatial scale of the cross‐field shear. The approach consists of starting the simulation from a state close to equilibrium, then observing the linear growth of instabilities and their ultimate saturation. The initial quasiequilibrium state is set up by a newly developed particle loading method; the instabilities are excited by numerical noise. The simulation is performed in two dimensions, in the plane perpendicular to the magnetic field, using an electrostatic particle code. The results for the kinetic Kelvin–Helmholtz instability are similar to those predicted by a hydromagnetic model, except that they depend slightly on the sign of the shear. Other instabilities are observed also: when the ion gyroradius is small on the scale of the shear, there is an unidentified short‐wavelength instability characterized byk &Dgr;x≥1, wherekis the wave number in the flow direction and &Dgr;xis the spatial scale of the shear; when the ion gyroradius is large, Bernstein waves, both ionic and electronic, are excited in the flow direction.

ISSN:0899-8221    

DOI:10.1063/1.860826

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Nonlinear wave–particle scattering and absorption (nonlinear Landau and cyclotron damping) of electromagnetic and electrostatic waves by high energy or relativistic electrons in a homogeneous magnetized plasma are investigated theoretically by numerical analysis of general and simplified expressions of nonlinear wave–particle coupling coefficients. It is shown that strong nonlinear absorption of the extraordinary wave can occur by nonlinear scattering into the other extraordinary waves or Bernstein waves induced by high‐energy stationary or drifting electrons, and they can be accelerated. Nonlinear absorption by Bernstein wave or ordinary wave is weaker than that by extraordinary wave. These nonlinear absorption mechanisms can compete with linear absorption in a fusion plasma, and can exceed it for a sufficiently smallk∥&rgr;b. It is verified that nonlinear scattering of extraordinary waves can induce the effective acceleration of relativistic electrons in a fusion plasma.

ISSN:0899-8221    

DOI:10.1063/1.860827

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The propagation of a high‐irradiance laser beam in a plasma whose optical index depends nonlinearly on the light intensity is investigated through both theoretical and numerical analyses. The nonlinear effects examined herein are the relativistic decrease of the plasma frequency and the ponderomotive expelling of the electrons. This paper is devoted to focusing and defocusing effects of a beam assumed to remain cylindrical and for a plasma supposed homogeneous along the propagation direction but radially inhomogeneous with a parabolic density profile. A two‐parameter perturbation expansion is used; these two parameters, assumed small with respect to unity, are the ratio of the laser wavelength to the radial electric‐field gradient length and the ratio of the plasma frequency to the laser frequency. The laser field is described in the context of a time envelope and spatial paraxial approximations. An analytical expression is provided for the critical beam power beyond which self‐focusing appears; it depends strongly on the plasma inhomogeneity and suggests the plasma density tailoring in order to lower this critical power. The beam energy radius evolution is obtained as a function of the propagation distance by numerically solving the paraxial equation given by the two‐parameter expansion. The relativistic mass variation is shown to dominate the ponderomotive effect. For strong laser fields, self‐focusing saturates due to corrections of fourth order in the electric field involved by both contributions.

ISSN:0899-8221    

DOI:10.1063/1.860828

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The theory of stochastic processes is applied to analyze the effect of magnetic‐field perturbations on the motion of classical charged particles. For arbitrary field fluctuations along the path of a particle, an approximate diffusion equation is obtained by means of the cumulant technique. The limit of white noise, in which the model is exact, and the difference between adiabatic and nonadiabatic motion are discussed. These cases relate results obtained for astrophysical plasmas and tokamak plasmas. It is shown that large longitudinal perturbations give rise to a new regime for diffusion perpendicular to the mean magnetic field.

ISSN:0899-8221    

DOI:10.1063/1.860971

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A type of eddy‐damped quasinormal Markovian (EDQNM) closure is shown to be potentiallynonrealizablein the presence of linear wave phenomena. This statistical closure results from the application of a fluctuation–dissipation (FD) ansatz to the direct‐interaction approximation (DIA); unlike in phenomenological formulations of the EDQNM, both the frequency and the damping rate are renormalized. A violation of realizability can have serious physical consequences, including the prediction of negative or even divergent energies. A new statistical approximation, the realizable Markovian closure (RMC), is proposed as a remedy. An underlying Langevin equation that makes no assumption of white‐noise statistics is exhibited. Even in the wave‐free case the RMC, which is based on a nonstationary version of the FD ansatz, provides a better representation of the true dynamics than does the EDQNM closure. The closure solutions are compared numerically against the exact ensemble dynamics of three interacting waves.  

ISSN:0899-8221    

DOI:10.1063/1.860829

出版商:AIP    

年代:1993

数据来源: AIP