摘要:

Coupled two‐dimensional wave equations are solved on a computer to model Langmuir wave turbulence excited by a weak electron beam. The model includes wave growth due to beam–plasma interaction, and dissipation by Landau damping. The inertial range is limited to a relatively small number of modes such as could occur when the ratio of masses between the negative and positive ions is larger than in a hydrogen plasma, or when there is damping in long wavelength Langmuir waves. A steady state is found consisting of quasistable, collapsed wave packets. The effects of different beam parameters and the assumed narrow inertial range are considered. The results may be relevant to plasma turbulence observed in connection with type III solar bursts.

ISSN:0031-9171    

DOI:10.1063/1.864221

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The generation of electromagnetic radiation from the unstable electrostatic waves of a beam–plasma system is investigated by means of computer simulation. A two‐dimensional model is employed in which the plasma is treated as a finite system and in which the effects of a continuous beam inflow are included. For the parameters of the simulation (nb/n0∼1%, an essentially nonrelativistic beam, and no external magnetic field), the radiation is concentrated near the plasma frequency, is emitted preferentially perpendicular to the beam, and is polarized withEparallel to the beam direction. Radiation at the second harmonic is found to be weak. The production mechanism is found to be the scattering of Langmuir waves from ion‐acoustic fluctuations, and the observed radiation level agrees well with the theoretical predictions for such a three‐wave coupling. The results of the numerical experiments are compared with the recent laboratory experiment of Whelan and Stenzel [Phys. Rev. Lett.47, 95 (1981)].

ISSN:0031-9171    

DOI:10.1063/1.864222

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Numerical solutions of the Vlasov and Poisson equations for the expansion of a plasma with O+and H+as the constituent ions are presented. The self‐similar nature of the expansion is examined by comparing the numerical results with available theories. The formation of a plateau in the expansion region, when the H+ions are initially minor, is investigated by varying the electron to ion temperature ratio. The numerical results show that self‐similarity holds in the quasineutral regions of the plasma.

ISSN:0031-9171    

DOI:10.1063/1.864223

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The motion of an intense, charge‐ and current‐neutralized ion beam propagating axially into a solenoidal magnetic field in vacuum is shown to be described by one of two models depending on whether or not the beam magnetic skin depthc/&ohgr;peexceeds half the beam radius. For a less‐intense beam having a large skin depth, a new theoretical model is presented which shows that a solenoidal field penetrates to the beam axis and acts as a lens. If a minimum beam density is exceeded, the electrons and ions are brought to a common focus as a result of quasineutrality. The collective focal length is the geometric mean of the focal lengths for the two species when traveling alone. For an intense beam having a small skin depth, the applied field is excluded from the center of the beam channel by a skin current and the resulting magnetic pressure gradient compresses the beam. This case is similar to the theta pinch and is described by a snowplow model. The two models are verified by experiments with a 150 kV, 0.5–5 A/cm2proton beam from a magnetically insulated diode. Possible applications in magnetic fusion, inertial fusion, and magnetospheric physics are discussed.

ISSN:0031-9171    

DOI:10.1063/1.864224

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The linear stability and nonlinear evolution of a regularized contour dynamical model of an ionospheric plasma cloud (or deformable dielectric) is examined. That is, the cloud is modeled by piecewise‐constant ion density regions; and the regularization is accomplished with a tangential diffusion operator that models aspects of the diffusion operator in two dimensions. A complete linear stability analysis of a circular cloud shows that a single‐mode excitation ‘‘cascades downward’’ in wavenumber as it grows in amplitude, a process that results from the symmetry‐breaking electric field. Approximate formulas are derived for the amplitude growth and cascade‐down phenomena and verified with precise numerical calculations. A simple rescaling shows that clouds with large &lgr; (=cloud‐ion density/ambient‐ion density) evolve more slowly and appear more dissipative. The regularized contour‐dynamical algorithm for computations in the nonlinear regime is validated against the linear analysis and truncation errors are assessed by using different spatial resolutions. Calculations in the nonlinear regime show the experimentally observed ‘‘backside’’ striations. Furthermore, at long times, a secondary structure arises on the sides of the primary striations. A comparison of simulations with different &lgr; shows that nonlinear effects arise sooner in normalized time (but longer in real time) if &lgr; is larger.

ISSN:0031-9171    

DOI:10.1063/1.864225

出版商:AIP    

年代:1983

数据来源: AIP

ISSN:0031-9171    

DOI:10.1063/1.864473

出版商:AIP    

年代:1983

数据来源: AIP