摘要:

Although resistive diffusion is much slower than the growth of resistive instabilities, the conventional neglect of diffusion in tearing mode calculations is incorrect. The proper criterion for neglect of diffusion in resistive instability calculations, which is not satisfied for tearing modes, is &ohgr;≫v/&dgr;, wherevis the resistive diffusion velocity and &dgr; is the resistive layer thickness. The effect of diffusion is calculated in the limit of large and small &ohgr;&dgr;/vfor the plane slab model, and new expressions for growth rate and stability boundary are obtained. The diffusion appears to have a stabilizing effect.

ISSN:0031-9171    

DOI:10.1063/1.861803

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A bounded uniform one‐dimensional collisionless plasma is considered. The velocity distribution is of the ’’bump‐on‐tail’’ type and two linear eigenmodes with closely spaced wave vectors have phase velocities located on the lowest part of the positive slope of the bump. All other eigenmodes are assumed reasonably separated in phase velocity and located on negative (stable) slope regions. The time‐asymptotic saturated nonlinear solutions are evaluated by a recently developed variation of the Bogoliubov method. In the ’’quasi‐linear’’ approximation (i.e., zero‐frequency mode coupling terms, alone, are retained) the only stable solution is one in which the initially fastest growing mode attains its ’’single‐mode’’ amplitude and the second mode has zero amplitude. In the full nonlinear calculation, the result is the same unless the two modes are above a critical distance from the bottom of the slope. Above this critical value, there are two stable solutions. One as just described. The other in which the lower mode reaches its full single‐mode amplitude and the first has zero amplitude. No stable solution exists for which both modes have finite amplitude. Similar mode survival competition may obtain in multi‐mode situations.

ISSN:0031-9171    

DOI:10.1063/1.861792

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Electrostatic lower hybrid ’’pump’’ waves are often launched into tokamak plasmas by structures (e.g., waveguides) whose dimensions are considerably smaller than characteristic plasma sizes. Such waves propagate in well‐defined resonance cones and give rise to parametric instabilities driven by electron E×B velocities. The finite size of the resonance cone region determines the threshold for both convective quasi‐mode decay instabilities and absolute instabilities. The excitation of absolute instabilities depends on whether a traveling or standing wave pump model is used; traveling wave pumps require the daughter waves to have a definite frequency shift. Altogether, parametric instabilities driven by E×B velocities occur for threshold fields significantly below the threshold for filamentation instabilities driven by pondermotive forces. Applications to tokamak heating show that nonlinear effects set in when a certain power‐per‐wave‐launching port is exceeded. For sufficiently high powers, these instabilities will occur in the low‐density edge region of a tokamak. They are characterized by a daughter wave frequency 10% below the pump wave frequency, in agreement with experimental observations.

ISSN:0031-9171    

DOI:10.1063/1.861804

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Microstability properties of the Tormac sheath are discussed with particular emphasis on the influence of strong inhomogeneities and magnetic shear on the lower‐hybrid‐drift and mirror‐drift‐cone instabilities. It is found that magnetic shear can completely stabilize the lower‐hybrid‐drift instability (Maxwellian ions), and lead to a reduction in growth rate of the mirror‐drift‐cone instability (loss‐cone ions). The persistence of the mirror‐drift‐cone instability (at a reduced growth rate) in the presence of moderate magnetic shear suggests that the addition of a cold plasma component (as in the 2X‐IIB mirror experiment) may be desirable to further reduce the instability growth rate and suppress the associated anomalous transport.

ISSN:0031-9171    

DOI:10.1063/1.861805

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The collisional friction force between two plasma species determines several quantities, such as particle diffusion rates, of central importance in tokamak transport theory. For an impure plasma in a regime of mixed collisionality, as pertains to most present tokamak experiments, electrostatic trapping leads to an unusually strong friction force, associated with slowly moving particles, which cannot be evaluated using the standard techniques of neoclassical kinetic theory. The Pfirsch–Schlu¨ter part of the friction force is computed, and shown to depend logarithmically on parameters characterizing the tokamak equilibrium.

ISSN:0031-9171    

DOI:10.1063/1.861806

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Stimulated scattering processes in a homogeneous plasma inside an optical cavity are studied theoretically. In particular, attention is focused on the Raman and Brillouin instabilities. The coupled equations for the wave amplitudes are solved subject to optical cavity boundary conditions, and it is shown that for a wide range of plasma lengths and cavity mirror reflectivities, the threshold values for instability are approximately those for the temporal problem (waves uniform in space, growth or decay in time). Sample results are calculated numerically for the Raman instability in a typical laboratory plasma. Finally, the physical significance of these results is discussed.

ISSN:0031-9171    

DOI:10.1063/1.861807

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Experimental evidence of a steepened electron density profile near critical density has been obtained by studying the light scattered by targets (10‐&mgr;m thick disks and 100‐&mgr;m diam glass spherical microshells filled with deuterium and tritium gas) illuminated by linearly polarized, 1.06‐&mgr;m light. Scale lengths on the order of 1 &mgr;m have been inferred both from the polarization of the reflected light and from the azimuthal asymmetry (asymmetry about the beam axis) of the time‐integrated scattered light with respect to the laser electric field. Azimuthally asymmetric heating of the microshells targets is indicated both by x‐ray micrographs and by the spatial distribution of the plasma blowoff.

ISSN:0031-9171    

DOI:10.1063/1.861808

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Scattering of laser radiation from ion‐acoustic waves in a plasma is investigated analytically and experimentally. The formulation predicts a coherent component of the scattered power on a largely incoherent background spectrum when the acoustic analog of Bragg’s law and Doppler shift conditions are satisfied. The experiment consists of a hybrid CO2laser system capable of either low power continuous wave or high power pulsed mode operation. A heterodyne light mixing scheme is used to detect the scattered power. The proportionality predicted by the theory is verified by scattering from externally excited acoustic and ion‐acoustic waves; continuous wave and pulsed modes in each case. Measurement of the ion‐acoustic dispersion relation by continuous wave scattering is also presented.

ISSN:0031-9171    

DOI:10.1063/1.861809

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The effect of laser light having Gaussian spatial and temporal variations upon ion‐acoustic waves is investigated analytically. At high power densities this light can interfere with coherent propagating waves in addition to exciting waves streamers which propagate away from the perturbed region at the ion sound velocity.

ISSN:0031-9171    

DOI:10.1063/1.861810

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Multiple‐bounce and snowplow models are employed to analyze the expansion dynamics of the interface separating exploding sheet and line mass plasmas from constant external pressure fields. The snowplow model is shown to be a successively poorer approximation to the multiple bounce when comparing systems of increasing total interface curvature; specifically, planar, cylindrical, and spherical systems are investigated.

ISSN:0031-9171    

DOI:10.1063/1.861811

出版商:AIP    

年代:1977

数据来源: AIP