摘要:

Injection of a neutral beam into a plasma in a magnetic field has been studied by means of numerical plasma simulations. It is found that, in the absence of a rotational transform, the convection electric field arising from the polarization charges at the edges of the beam is dissipated by turbulent plasma convection, leading to anomalous plasma diffusion across the magnetic field. The convection electric field increases with the beam density and beam energy. In the presence of a rotational transform, polarization charges can be neutralized by the electron motion along the magnetic field. Even in the presence of a rotational transform, a steady‐state convection electric field and hence anomalous plasma diffusion can develop when a neutral beam is constantly injected into a plasma. Theoretical investigations of the convection electric field are described for a plasma in the presence of a rotational transform.

ISSN:0031-9171    

DOI:10.1063/1.866945

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The possible current decay and termination of reversed‐field pinch discharges via stable, Ohmic, self‐similar states that do not require any instabilities or dynamo effect for their maintenance are investigated. The equations that describe such states are derived; numerical solutions that are functions of the form of the resistivity profile, the current decay rate, and the pinch parameter are presented. Relevant constraints on stable termination are discussed.

ISSN:0031-9171    

DOI:10.1063/1.866946

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Magnetic probe measurements carried out on the ZT‐40M reversed field pinch [Fusion Technol.8, 1571 (1985)] for low current (50 kA) discharges with and withoutF‐&THgr; pumping are described. The internal probe measurements are used to determine the evolution of the axisymmetric equilibrium profiles. Of particular interest is the profile response to the large‐amplitude, oscillatory, toroidal, and poloidal driving voltages applied duringF‐&THgr; pumping. The results are compared with a two‐field‐line stochastic magnetic field model.

ISSN:0031-9171    

DOI:10.1063/1.866947

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

If a spheromak plasma with a flux hole relaxes sufficiently rapidly to the flat current profile of a Taylor state, then the plasma current can be maintained, or increased, by varying certain geometric ratios of the plasma. The theory of this type of current drive is developed in terms of the internal and external inductances of a spheromak.

ISSN:0031-9171    

DOI:10.1063/1.866948

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

In the vicinity of a density transition region or surface, electron temperature fluctuations are known to drive low frequency electromagnetic surface waves which may be connected with the self‐generation of magnetic fields in the laser–plasma interaction. Two methods of destabilization are considered. In the first approach an accelerating plasma surface is considered and this leads to instability only when an accompanying temperature gradient scale length is at most as large as the density gradient scale length. In the second approach the introduction of current is also found to cause instability near the plasma surface provided an identical temperature gradient threshold is exceeded. In both cases the growth rates for each instability are enhanced by the square root of the ion to electron mass ratio over the associated growth rates for ion‐inertial‐type instability.

ISSN:0031-9171    

DOI:10.1063/1.867021

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A detailed simulation of the spectra produced from a bromine dot target by laser irradiation is presented. This study incorporates the results of a two‐dimensional hydrodynamic calculation and an extensive atomic rate data base to generate synthetic spectra under conditions that match those of an earlier experiment. A dynamic picture of the plasma’s evolution is given that is consistent with experimental results. Overall plasma characteristics are calculated that produce spectra largely in agreement with those observed. Also, sensitive dielectronic recombination features are reproduced corroborating the adequacy of this approach.

ISSN:0031-9171    

DOI:10.1063/1.866898

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

When a short‐pulse laser beam is focused onto a solid target in a few torr ambient gas, the expanding target plasma couples to the resulting ambient plasma. Dark‐field shadowgrams show that a thin, nearly spherical blast front is produced. These shadowgrams also show that certain regions of the blast front (called aneurisms) may project well ahead of the neighboring, spherical part. Several mechanisms for aneurisms are discussed. One class of aneurisms is consistently produced along the laser axis when the laser energy is greater than about 10 J. Two‐dimensional, hydrodynamic computer simulations of these on‐axis aneurisms show that they can be accounted for by the laser heating of an on‐axis channel.

ISSN:0031-9171    

DOI:10.1063/1.866899

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The problem of parametric instabilities in the presence of space‐time fluctuation in homogeneous or inhomogeneous plasmas can be cast into the problem of finding an effective HamiltonianHeffof the linear stochastic equation. For the Kubo–Anderson–random‐phase model (RPM) [J. Math. Phys.15, 524 (1974); Phys. Rev. Lett.29, 565 (1972); J. Phys. Soc. Jpn.9, 316 (1954)],Heffis generally found to be of the formH0+&Dgr;&ohgr;H1, whereH0represents all the coherent effects and &Dgr;&ohgr;H1represents the finite bandwidth modification. It is shown thatH1is a trivial constant matrix for all parametric processes. A method called frame transformation is introduced to treat all kinds of group velocities in a unified fashion. The theory can be used to calculate average amplitude as well as average power. Importantly, the theory is not limited to wide bandwidths.

ISSN:0031-9171    

DOI:10.1063/1.866900

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

McIntoshetal. [Phys. Rev. Lett.57, 337 (1986); Phys. Fluids29, 3451 (1986)] have carried out experiments in which electron plasma waves produced in a gas jet target by a CO2‐laser pulse were studied by picosecond‐resolution Thomson scattering of a ruby‐laser pulse. In addition, spectra of the scattered laser light and of the fast electrons were obtained. Their analysis of the data was based on the assumption that the plasma waves are created by the onset of the stimulated Raman scattering instability. This led to a number of difficulties in the interpretation of the data. It is shown here that an alternative model of the origin of these plasma waves [Phys. Rev. Lett.53, 1912 (1984)], used previously in analyzing Raman data in several other experiments, removes all these difficulties and is in agreement with the data. In this model, the plasma waves are produced by pulses of fast electrons arising at the quarter‐critical surface.

ISSN:0031-9171    

DOI:10.1063/1.866901

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A relativistic electron beam in the presence of a whistler wave excites a negative‐energy beam space charge mode and a high frequency laser radiation (&ohgr;1≫&ohgr;c). The instability is explosive for a copropagating beam and the whistler pump, resulting in the excitation of an intense laser in a short pulse. The explosion time &tgr;={(4/kv0)[(&ohgr;1−k1vb−&ohgr;c) &ggr;0&ohgr;1/(&ohgr;1−k1vb)&ohgr;’pb]1/2}. For both the counterpropagating beam and the whistler wave, the instability is nonexplosive. The laser frequency in this case is somewhat larger, whereas the exponentiation time is comparable to the explosion time of the earlier case.

ISSN:0031-9171    

DOI:10.1063/1.866902

出版商:AIP    

年代:1988

数据来源: AIP