摘要:

Particle‐in‐cell (PIC) simulations of electromagnetic (EM) pump wave propagation into an overdense, strongly magnetized plasma with a linear density gradient are investigated. A localized three‐wave interaction is observed involving the pump decaying into upper and lower hybrid waves. The upper hybrid (UH) wave converts on pump induced density depletions into an electromagnetic wave which exits the plasma. When the pump wave frequency equals an electron cyclotron harmonic plus lower hybrid (LH) frequency, all excited waves vanish, implying a magnetic field strength remote diagnostic capability. These results are relevant to stimulated electromagnetic emissions (SEE) and also to electron heating in a strongly magnetized plasma.

ISSN:1070-664X    

DOI:10.1063/1.870680

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The effect of a resistive wall on the stability of a magnetohydrodynamic equilibrium with flow is examined. Gyroscopically stabilized negative energy modes are shown to become unstable in the presence of a resistive wall. An estimate for the growth rate is obtained. The effect of this destabilization on the limits on the safety factor and the plasma beta for a rigidly rotating plasma column are studied.

ISSN:1070-664X    

DOI:10.1063/1.870681

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The electrostatic and kinetic energies available in the longitudinal electron plasma wave created in the wake of an ultraintense laser pulse are obtained analytically. The analysis is one dimensional and assumes a square‐shaped pulse that propagates in a highly underdense plasma with the velocity of light in vacuum. The length scale for laser depletion is given as a function of the laser irradiance and the electron density.

ISSN:1070-664X    

DOI:10.1063/1.870682

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An experimental study of the rapid step‐switching of the frequency of a submillimeter wave gyrotron is described. Modulation of the energy of the electron beam in turn modulates the electron mass and hence the cyclotron resonance frequencyfcor its second harmonic 2fc, while the magnetic fieldB0remains the same. This opens up the possibility of switching from one cavity mode to another. The first experimental results for frequency switching between two fundamental modes and between a fundamental and a second harmonic mode are demonstrated.

ISSN:1070-664X    

DOI:10.1063/1.870683

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Wave breaking of electron plasma waves in an unmagnetized, cold, collisionless plasma with a parabolic density profile is considered when the plasma resonance is situated near the top of the profile. For this case, wave breaking takes place at a density lower than the resonant one and limits the growth of the plasma wave. The dynamics of wave breaking in such conditions are studied as a function of position of resonance in the profile and of amplitude of the driver field.

ISSN:1070-664X    

DOI:10.1063/1.870684

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Results of a study of the magnetic compression of one‐dimensional energetic ion layers in the presence of conductive background plasmas in cylindrical geometry are presented. For this, an updated hybrid (particle‐ion and fluid‐electron) computer model has been used. The first observation is that when the magnetic field at the wall is ramped quickly relative to the radial Alfve´n wave transit time across the cylinder then a magnetic probe on axis would measure a larger field perturbation. When the wall field is ramped slowly, the layer energy increases proportionally with the external magnetic‐field strength and, as expected, the layer self‐field on axis does not change relative to external field. A study is also made of the effects of transient fluctuations on the layer energy when the field is ramped quickly. When the ramped field strength is small compared with the ambient field (the linear case), the average layer kinetic energy gained is not substantially different from the case where ramping is slow; however, the fluctuating energy is more than twice the average change. The interesting observation is made that the thermal spread in particle energy for a layer is not necessarily related to the strength of the self‐field. This is explained in terms of the single particle motion in a self‐potential well, and is different from the expected result for a two‐dimensional ring.  

ISSN:1070-664X    

DOI:10.1063/1.870685

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Braginskii’s two‐fluid equations, with collisional effects neglected and electron fluid assumed to be isothermal along the magnetic field lines, are employed to obtain the singular mode equations. While the ordering analysis in the paper indicates that, in the intermediate frequency regime (i.e., that with the frequency of the modes higher than the ion acoustic frequency along the magnetic field lines, but lower than the electron one), the stabilizing finite Larmor radius (FLR) effect on the modes dominates over other collisionless, two‐fluid effects, the paper is devoted to the investigation of the comparable frequency regime (i.e., that with the frequency of the modes comparable to the ion acoustic frequency along the field lines). It is found that, in such a regime, the parallel electric field is excited and the plasma behaves nonadiabatically. The singular mode equations consist of two coupled second‐order partial differential equations, in which the FLR, the plasma compression, and the parallel electric field effects are present. The results are discussed, and a comparison with the ideal magnetohydrodynamic formalism is given.

ISSN:1070-664X    

DOI:10.1063/1.870633

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The long‐time relaxation of ideal two‐dimensional (2‐D) magnetohydrodynamic (MHD) turbulence subject to the conservation of two infinite families of constants of motion—the magnetic and the ‘‘cross’’ topology invariants—is examined. The analysis of the Gibbs ensemble, where all integrals of motion are respected, predicts the initial state to evolve into an equilibrium, stable coherent structure (the most probable state) and decaying Gaussian turbulence (fluctuations) with a vanishing, but always positive temperature. The nondissipative turbulence decay is accompanied by decrease in both the amplitude and the length scale of the fluctuations, so that the fluctuation energy remains finite. The coherent structure represents a set of singular magnetic islands with plasma flow whose magnetic topology is identical to that of the initial state, while the energy and the cross topology invariants are shared between the coherent structure and the Gaussian turbulence. These conservation laws suggest the variational principle of isotopological relaxation that allows one to predict the appearance of the final state from a given initial state. For a generic initial condition havingxpoints in the magnetic field, the coherent structure has universal types of singularities: current sheets terminating atYpoints. These structures, which are similar to those resulting from the 2‐D relaxation of magnetic field frozen into an ideally conducting viscous fluid, are observed in the numerical experiment of D. Biskamp and H. Welter [Phys. Fluids B1, 1964 (1989)] and are likely to form during the nonlinear stage of the kink tearing mode in tokamaks. The Gibbs ensemble method developed in this work admits extension to other Hamiltonian systems with invariants not higher than quadratic in the highest‐order‐derivative variables. The turbulence in 2‐D Euler fluid is of a different nature: there the coherent structures are also formed, but the fluctuations about these structures are non‐Gaussian.

ISSN:1070-664X    

DOI:10.1063/1.870634

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

For a particle moving in the electric field of many, randomly phase waves, numerical measurements show that the Lyapunov exponent (1) asymptotes to a value of 0.4 times the resonance broadening frequency, in contrast with previous calculations, and (2) has a peak at intermediate values of the overlap parameter. This latter result indicates that the previously observed enhancement of the diffusion coefficient is not due to coherence effects near stable regions of phase space. These results provide further evidence that the standard model is an inadequate description of the turbulent electric field. 

ISSN:1070-664X    

DOI:10.1063/1.870635

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Two‐dimensional simulations of the electron‐beam plasma instability with large system size are carried out and are compared with recent one‐dimensional simulations for plasma parameters appropriate to the electron foreshock. It is found that wave propagation and diffusion perpendicular to the beam drift are significant at all times. Because a plateau cannot be maintained in this case, the wave level decreases much more rapidly than in one‐dimensional simulations. The nonlinear wave scattering process which occurs at late times also differs in that it generates a broad secondary spectrum rather than a condensate. The two‐dimensional model, in addition, allows the investigation of the effects of increasing magnetic field strength (e.g., along auroral field lines). For intermediate magnetic fields Langmuir waves and a highly oblique spectrum belonging to the lower hybrid branch are simultaneously excited. The oblique wave spectrum for strong magnetic fields can be explained by mapping from the magnetic field direction.

ISSN:1070-664X    

DOI:10.1063/1.870636

出版商:AIP    

年代:1994

数据来源: AIP