摘要:

A plasma sheath in a strongly tilted magnetic field is considered. It is shown that, if the incoming plasma flux contains two ion populations with clearly distinguishable energies (e.g., an admixture of cold ions to the main ion distribution), vigorous beam-type instabilities may be excited in the sheath. One should emphasize that these instabilities may be present even if the ion distribution function in the bulk of the plasma does not have beam structure. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872206

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effects of negative magnetic shear on the toroidicity induced (TI) eigenmode instability are studied with a numerical WKB shooting scheme. The ion temperature gradient (ITG or&eegr;i) and the parallel velocity shear (PVS) of the ions are included. It is found that for a given set of plasma parameters, the damping mechanism from a negative magnetic shear is much stronger than it is from a positive shear of equal magnitude. The effects of PVS on TI modes are demonstrated to be destabilizing. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872212

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Effects of electron–positron pair creation on the stationary one-dimensional propagation of relativistically coupled electromagnetic and electrostatic waves are studied based on the self-consistent model presented in the previous paper for an electron–ion plasma [L. N. Tsintsadze and K. Nishikawa, Phys. Plasmas3, 511 (1996)]. The pair-created particles are treated as a cold electron–positron plasma at their creation point. In addition to the results similar to those obtained for the electron–ion plasma, a new type of instability of the finite amplitude plane wave is found which is purely growing in the wave frame near threshold. A novel solution describing an envelope shock which represents a wake-field excitation by a solitary electromagnetic pulse is obtained by taking into account the trapping of the pair-created particles in the upstream region. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872207

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

It is demonstrated using conventional fluid theory that angular momentum can be injected into a single component plasma confined in a Penning–Malmberg trap via an externally generated, oscillating, nonaxisymmetric, electric field. The torque exerted on the plasma by the electric field is a highly nonmonotonic function of the plasma angular rotation velocity. The torque vs angular velocity curve is dominated by sharp resonances at which the angular phase velocity of a particular poloidal harmonic of the external field matches the plasma angular rotation velocity. The torque exerted on the plasma by a given poloidal harmonic is negative when the field rotates faster than the plasma, and vice versa. This rather surprising behavior is shown to be entirely consistent with a standard result in hydrodynamic theory, but is generally not observed in present-day experiments. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872208

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The role of a fast electron population, produced by the strong electric field driving the axial current in a capillary discharge, can be essential in order to justify the appreciable concentration of highly charged C ions (C5+,C6+) as observed in many experiments with low electron temperature. Under the assumption that a quasistationary state is established during the discharge, the runaway electron distribution function is determined analytically forZeff>1, following the approximated method developed by Lebedev [A. N. Lebedev, Sov. Phys. JETP21, 931 (1965)]. The strong enhancement of the ionization rate, due to the presence of an electron tail in the energy range of the ionization potential, is then demonstrated. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872188

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Multipactor discharges can cause severe problems in high voltage rf systems like rf antennae or transmission lines of ion cyclotron resonance heating (ICRH) in nuclear fusion devices, where they may initiate gas breakdown. To study this eventual transition from a starting multipactor into an ordinary rf discharge detailed investigations were performed using a parallel plate geometry, 50 MHz operating frequency and up to one kilovolt rf amplitude. Measurements of electric data (rf amplitude, absorbed and reflected power, discharge current), electronic parameters (electron current density and energy distribution) and light emission were used for characterization. As a main result a new type of well defined discharge regime was identified, which seems important for the transition from multipactor into gas breakdown in rf devices. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872564

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The detailed spectral content of strong Langmuir turbulence generated by an electron beam is investigated experimentally in an unmagnetized plasma. It is found that the power spectra of the electrostatic waves follows a consistent, reproducible pattern in which the Langmuir energy is concentrated in high frequencies (410 MHz) immediately following a collapse event, transitions to lower frequencies between events, with collapse taking place at the lowest frequencies (350 MHz). Also, after a collapse event, the electrostatic wave intensity falls by about two orders of magnitude in a microsecond, but then continues to fall over the next approximately 10 &mgr;s by an additional factor of 10 before beginning to build toward the next collapse. The spectral width and electron saturation current also exhibit reproducible patterns. Measurements are also performed to determine the low-frequency, ion acoustic spectral content. It is found that the peak of the ion acoustic spectrum scales inversely with the average time between collapse. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872209

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

One- and two-dimensional simulations and supporting analysis of nonlinear ion acoustic waves as might be associated with the saturation of stimulated Brillouin backscattering (SBBS) are presented. To simulate ion wave phenomena efficiently, while retaining a fully kinetic representation of the ions, a Boltzmann fluid model is used for the electrons, and a particle-in-cell representation is used for the ions. Poisson’s equation is solved in order to retain space-charge effects. We derive a new dispersion relation describing the parametric instability of ion waves, evidence for which is observed in our simulations. One- and two-dimensional simulations of plasma with either initially cold or warm ions (and multi-species ions) exhibit a complex interplay of phenomena that influence the time evolution and relaxation of the amplitude of the excited ion wave: ion trapping, wave steepening, acceleration, heating and tail formation in the ion velocity distribution, parametric decay into longer wavelength ion waves, modulational and filamentation instabilities, and induced scattering by ions. The additional degrees of freedom in two dimensions allow for a more rapid relaxation of the primary ion wave. One-dimensional electrostatic simulations with externally driven ion waves agree qualitatively with electromagnetic simulations in one dimension in which the ponderomotive driving potential is computed self-consistently by solving a Schroedinger-like equation for the electromagnetic waves and calculating the low-frequency ponderomotive force on the electrons. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872187

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A wavenumber dependent eddy viscosity is calculated for a simple two-dimensional drift wave model from direct numerical simulations for a wide range of parameters and cutoff wavenumber. The damping rate given by this eddy viscosity is well modeled by a hyperviscosity, where the power and magnitude are parameterized as functions of the shear in the resolved advecting velocity. Tests in simulations with low resolution find that the use of this parameterized hyperviscosity yields somewhat better results than the use of hyperviscosity with fixed power and is significantly better the use of no extra damping term or a Smagorinsky type eddy viscosity. This parameterized hyperviscosity is very useful computationally, since reducing resolution requirements by even a factor of 2 reduces the computational requirements by a factor of 8 in two dimensions, or 16 in three dimensions. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872210

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Three-dimensional nonlinear simulations of collisional plasma turbulence are presented to model the behavior of the edge region of tokamak discharges. Previous work is extended by including electron temperature fluctuationsT˜e.The basic paradigm that turbulence and transport are controlled by resistive ballooning modes in low temperature plasma and nonlinearly driven drift wave turbulence in higher temperature regimes persists in the new system. Parallel thermal conduction strongly suppresses the ability of the electron temperature gradient∇Teto drive the turbulence and transport everywhere except the very low temperature edge of the resistive ballooning regime. As a consequence, over most of the resistive ballooning regime only the density gradient drives the turbulence and the temperature fluctuations are convected as a passive scalar. In the drift wave regime only the density gradient acts to drive the nonlinear instability and the temperature fluctuations have a relatively strong stabilizing influence on the turbulence due to an enhanced damping of density and potential fluctuations resulting from local electron heating. ©1997 American Institute of Physics.

ISSN:1070-664X    

DOI:10.1063/1.872185

出版商:AIP    

年代:1997

数据来源: AIP