摘要:

A large‐amplitude whistler is taken as a model of a collisionless laminar shock and shown to be unstable for perturbations propagating at angles to the whistler. The parameter space of instability is delineated and growth lengths as short as several whistler wavelengths are shown to exist for certain classes of perturbations. The deterioration of a laminar shock structure to a turbulent one, therefore, seems highly probable.

ISSN:0031-9171    

DOI:10.1063/1.1693633

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

An expression for the effective collision frequency is derived in terms of⟨E2⟩/4&pgr;n&kgr;Te. This quantity is shown by similarity considerations to be the same in many different situations. From its value 0.07, and even from values down to(me | mi)2, it is concluded that present quasilinear theory and nonlinear wave equations cannot be used to explain the experimental results. The value 0.07 can be understood as the equipartition between the kinetic energy of the trapped particles and the energy in the electric fields.

ISSN:0031-9171    

DOI:10.1063/1.1693634

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Diffusion of plasma in two dimensions is studied in the guiding center model. It is shown that in this model diffusion always exhibits the anomalous 1/Bvariation with magnetic field. The velocity correlation function and the diffusion coefficient are calculated in detail using functional probabilities. In addition to the 1/Bfield dependence, the diffusion coefficient is unusual in that it depends weakly on the size of the system. These theoretical results are compared with those from computer experiments and their significance for real plasma is discussed.

ISSN:0031-9171    

DOI:10.1063/1.1693635

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The convergent classical kinetic equation of Kihara and Aono is used to calculate the collisional part of the ac electrical conductivity of a magnetoplasma to second order in the wave vector. Both dominant (Coulomb logarithm) and subdominant terms are evaluated exactly to first order in the plasma parameter.

ISSN:0031-9171    

DOI:10.1063/1.1693636

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

For a toroidally symmetric sharp boundary plasma, the magnetohydrodynamic stability analysis of Lu¨st, Suydam, Richtmyer, Rotenberg, and Levy is extended to equilibria with a small ratio of azimuthal to longitudinal magnetic fields and a large aspect ratio(&agr;). These equilbria exist only when&bgr;<&agr;(field ratio)2/2. In the limit of small&bgr;, the stability criteria are identical to those of a straight cylindrical plasma with length2&pgr;&agr;. This is true to leading order in1/&agr;in spite of mathematical complications arising from toroidal effects. However, as&bgr;is increased to its maximum value for field ratios of order1/&agr;, toroidal effects help stabilize long‐wavelength kink modes. The limits considered apply to tokamaks.

ISSN:0031-9171    

DOI:10.1063/1.1693637

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The linear and nonlinear response of a plasma sheath is studied by subjecting it to rf potentials at frequencies corresponding to certain natural modes of oscillation of the sheath‐plasma system. The sheath investigated is one that exists around a spherical dipole probe, immersed in an essentially collisionless uniform plasma and excited with sinusoidal signals at frequencies between 50 and 250 MHz. It is shown that the linear rf current‐voltage characteristics agree with both the simple sheath model of Mayer and Harp and with the more refined model of DeAngelis and Baldwin. The nonlinear response (which is observed as the generation of higher harmonics) is quantitatively explained as being due to an oscillation of the sheath‐plasma boundary induced by the applied rf field. This oscillation, in effect, acts as a time varying sheath capacitance.

ISSN:0031-9171    

DOI:10.1063/1.1693638

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The probabilities of trapping‐state transitions of single‐particle drift orbits are obtained from a theorem due to Kruskal that is an extension of the usual adiabatic theory. Because such transitions determine the total orbit, they are important to diffusion studies. To apply Kruskal's theorem, we use drift invariants obtained for a large‐aspect‐ratio stellarator (withl = 1andl = 3helical windings) by means of the stellarator expansion are used. In this model, toroidal and helical field modulations are comparable and the total rotational transform is finite. These three effects interact with the particle drift and, with transitions, determine the orbits in the large. When a small horizontal magnetic field is added, the bilateral symmetry of the flux surfaces—and hence the blocked and passing particles—are modified. Localized particles are affected through stochastic transitions, and particle orbits are globally ergodic.

ISSN:0031-9171    

DOI:10.1063/1.1693639

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The problem of the nonlinear time evolution of a cold beam‐plasma system, for which weak turbulence theory is well known to be inapplicable, is examined under the restrictions to one‐dimensional electrostatic oscillations and for systems where the ratio of the beam density to the background plasma density is a small parameter. In this case, it may be shown that the electrostatic field undergoes rapid growth to a state of meta‐equilibrium, followed by a slower time development. The mechanism for the nonlinear saturation of this growth is the trapping of beam electrons in the wave troughs of the electrostatic field. The existence of a unique single wave nonlinear Bernstein‐Greene‐Kruskal stationary state is established for this system and its properties (e.g., its energy content, wavelength, phase velocity) are evaluated. This Bernstein‐Greene‐Kruskal state is apparently approached closely by the system in the course of its time evolution. The predictions of the theories are compared with computer calculations and are found to be in good agreement.

ISSN:0031-9171    

DOI:10.1063/1.1693640

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The study of collisional processes in plasmas produced by neutral‐atom injection into magnetic mirror fields is described. The emphasis is on the many collisional processes which occur as the plasma density increases. Experimental and theoretical results are given. The experimental results are discussed first in terms of a simple model which assumes a Maxwellian electron distribution and a monoenergetic ion component of much higher energy. Analytical solutions may be obtained for this model. Also presented is a more complete theory employing two time‐dependent Fokker‐Planck equations to describe the behavior of the electron and ion distribution functions. Both models are in good agreement with measured values of the electron temperature and plasma potential. The equilibrium values of these two quantities are found to vary as the35power of the ratio of the plasma density to the background‐gas density.

ISSN:0031-9171    

DOI:10.1063/1.1693641

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Approximately1011electrons are trapped in a single stage 15‐kG magnetic mirror and compressed to a volume of roughly1 cm3. The trapped electron distribution function is peaked at 0.5 MeV and shows a spread of 0.45 MeV. A base pressure of5×10−8 mm Hginsures negligible neutralization of the electron cloud for about 2 msec. The trapped electron configuration has a low‐frequency (2MHz) precession motion about the magnetic axis.

ISSN:0031-9171    

DOI:10.1063/1.1693642

出版商:AIP    

年代:1971

数据来源: AIP