摘要:

The interchange instability induced by gravity in a compressible plasma is investigated by numerical methods based on the equations of ideal magnetohydrodynamics. Eulerian computer codes have been developed to integrate in time both the linear and nonlinear forms of these equations. In the linear case the results are compared with a generalization of a normal mode analysis performed by Newcomb. Predictions regarding the growth rate and spatial distribution of these modes and their stabilization by sheared magnetic fields are confirmed in the computer runs. In the nonlinear case inclusion of diffusion and damping terms in the numerical code permits the unstable mode to be followed to a final stable state. The numerical results indicate that for a sheared magnetic field there is a displaced equilibrium state involving a strongly distorted plasma distribution.

ISSN:0031-9171    

DOI:10.1063/1.862401

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

When ion beams were injected into axially bounded plasmas, ion waves, whose wavelengths were determined by boundaries, were found to be very distinctly excited. The selection of the excited oscillation frequency was determined by the boundaries such that the spacing of the boundaries was approximately an integer times the wavelength of the ion waves. The observed phase velocity was in fairly good agreement with the theoretical velocity.

ISSN:0031-9171    

DOI:10.1063/1.862402

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The evolution and effects of current‐driven instabilities in isothermal, inhomogeneous plasmas are investigated in both theory and experiment. Sucessive destabilizations of four different instabilities, low‐frequency drift wave, ion‐cyclotron drift wave, high‐frequency (continuous‐spectrum ion‐cyclotron) drift wave, and high‐frequency electron wave, are observed inQ‐device plasmas with increased current, and explained by a theory based on fluid and kinetic equations. Anomalous effects resulting from wave‐particle interactions, i.e., enhanced resistivity, ion heating, and electron viscosity, are compared with predictions based on quasi‐linear calculations. Analogous to ion sound causing important anomalies in the transport coefficients for plasmas withTe/Ti≫1, high‐frequency, continuous‐spectrum drift waves determine anomalous plasma behavior in inhomogeneous plasmas withTe/Ti≃1.

ISSN:0031-9171    

DOI:10.1063/1.862403

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The Cerenkov interaction of a weak electron beam with a plasma is studied numerically. Three parts of the nonlinear evolution of the convective instability are discussed. In the first part the understanding of the saturation amplitude of the initial wave and its harmonics is extended. In the second part it is shown that a quasi‐linear‐like cascade process is responsible for the simutaneous appearance of beam particles and waves which move slower than the original beam velocity. These modes destroy the trapped particle oscillations of the initial wave. The third part of the instability is the development of fast waves whose phase velocities are greater than the original velocity of the beam. It is shown that these waves arise due to a mode coupling mechanism.

ISSN:0031-9171    

DOI:10.1063/1.862404

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Two aspects of double sheaths (nonneutral regions) in plasmas are studied via analytic models. First, the motion of the structure through the plasma is analyzed by considering the force balance among the various species. A general proof is presented which shows the net acceleration of the sheath is zero in the absence of dissipative processes. A specific example is given which shows that a pressure difference between the two sides of the sheath results in the enhancement of the beam current into the low pressure side rather than acceleration of the sheath. The effect of beam‐plasma interactions is also discussed. The second part of the analysis is the derivation of separate (though nearly identical) density criteria for sheath nucleation and sheath maintenance in equilibrium. These are compared with experimental results of Lutsenko, Sereda, and Kontsevoi and reasonable agreement is obtained.

ISSN:0031-9171    

DOI:10.1063/1.862405

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The motion of an ion in a lower hybrid wave in a tokamak type plasma is studied. For ions with &ngr;⊥≳&ohgr;/k⊥the motion is stochastic for fields satisfyingE/B0≳ (1/4)(&OHgr;i/&ohgr;)1/3(&ohgr;/k⊥). Provided that the perpendicular phase velocity, &ohgr;/k⊥, can be slowed down to a few times the ion thermal speed, this stochastic ion motion may be an important mechanism by which injected rf power near the lower hybrid frequency can directly heat the ions.

ISSN:0031-9171    

DOI:10.1063/1.862406

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The sharp‐boundary model of ideal magnetohydrodynamics is presented in a form readily applicable to axisymmetric, high‐&bgr; plasmas of arbitrary cross section. The usual vacuum region outside the plasma is replaced here by a layer of force‐free currents, and the influence of force‐free currents and cross‐sectional shaping on the critical &bgr; is studied. Specific results are given for circular, elliptical, and racetrack‐like cross sections. A new parameterq* is introduced in place of the Mercierq, and it is shown thatq*≈1 is a reasonable estimate of the toroidal current limitation due to external kink modes in high‐&bgr; plasmas with noncircular cross sections.

ISSN:0031-9171    

DOI:10.1063/1.862407

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Motions of ions in the plasma focus are investigated theoretically using a model for az‐pinch type plasma focus. Macroscopic radial motions of plasma are calculated numerically using the snowplow model, the Leontovich–Osovets’ equation, and a circuit equation. The equations of motion of ions are solved in the presence of fields induced in the pinched phase of the macroscopic motion. It is found that in the collapse phase there appears a betatron‐like acceleration process of ions with negligible drift velocity. It is also shown that a strong free‐streaming acceleration process leading to high, axial velocity of ions in the direction of current flow appears in the expansion phase.

ISSN:0031-9171    

DOI:10.1063/1.862417

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A study is made of the generation of intense relativistic electron beams in rectangular and cylindrical foil‐less diodes. The diode space charge is treated self‐consistently. The electron emission from the cathode is assumed to be space‐charge‐limited. A strong axial magnetic field is assumed to prevent the electrons from reaching the anode surface(s) directly, and to constrain the electron motion to be approximately one‐dimensional. A useful dimensionless measure of the diode potential &fgr;cis &egr;≈[mec2/(e&fgr;c)]1/2, withmeandethe electron rest mass and charge, andcthe speed of light. Properties of the diodes are first analyzed in the ultra‐relativistic limit, &egr;=0, where the condition for space‐charge‐limited emission gives rise to a linear singular integral equation. This equation is solved for rectangular diode geometry, and the solutions are studied in detail. In particular, the diode impedance is independent of &fgr;c, and the beams are, in general, hollow. The beam particle kinetic energy flux, &Ggr;p, decreases as the beam width,b, increases; forb=a, &Ggr;p=0, whereais the diode width. A treatment of the diodes is then given for small but nonzero values of &egr;. For finite &egr;, there is a nonrelativistic Child–Langmuir sheath of thickness ≈&egr;aat the cathode surface. Forb=a, &Ggr;pis shown to be proportional to &fgr;c3/2.

ISSN:0031-9171    

DOI:10.1063/1.862418

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Parylene (C8H8) and tungsten‐glass (W2O/P2O5) disks have been irradiated with 150–400 psec Nd:YAG‐glass laser pulses focused to diameters of 250–300 &mgr;m with flux levels in the 1013–1015W/cm2range. An extensive array of diagnostics was used to measure the temporal and energy distributions of the focused laser light at the target, the angular distribution of the scattered laser light, the x‐ray spatial and spectral emission characteristics, and the emitted ion and electron distributions. Analysis of the experimental results indicates that the laser‐plasma interaction was characterized by a variety of collective phenomena which appeared stronger in the tungsten‐glass experiments.

ISSN:0031-9171    

DOI:10.1063/1.862419

出版商:AIP    

年代:1978

数据来源: AIP