摘要:

The theory of parametric excitation of plasma waves is applied to a plasma in which there is a static drift between electrons and ions. For appropriate values of drift speed and elevated electron‐ion temperature ratio, there exists a solution of the dispersion relation and a threshold for instability at a frequency which is close to zero in the electron frame. This instability occurs for pump frequencies which are slightly below the Bohm‐Gross frequency. It has some similarities to the oscillating two‐stream instability in a plasma in which there is no static drift. An approximate analytic solution of the dispersion relation in the neighborhood of zero frequency is obtained.

ISSN:0031-9171    

DOI:10.1063/1.1694731

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The nonlinear evolution and stabilization of absolute whistler instabilities driven by electron pressure anisotropy and propagating obliquely to the external magnetic field in finite&bgr;plasmas are investigated without introducing the assumption of random phases. Both in the firehose and the mirror instability regimes the pressure anisotropy relaxes quasilinearly toward a more stable state. The nonlinear wave equation has also been obtained. By choosing the parameters in such a way that the plasma is sufficiently close to linear marginal stability, it has been possible to show both numerically and analytically that the (firehose) instability stabilizes nonlinearly at sufficiently low levels of excitation. The fluctuation level does not asymptotically approach a constant value, but oscillates in time, the maximum amplitude being proportional to the linear growth rate. Also, close to marginal stability, finite Larmor radius effects can play a significant role even in the limitkR ≪ 1.It is found that finite Larmor radius effects are stabilizing iftan2&agr; > 4&bgr;‖/&bgr;⊥,where&agr;is the angle between the direction of wave propagation and the direction of the background magnetic field and&bgr;‖(⊥)is the ratio of parallel (perpendicular) kinetic to magnetic pressure.

ISSN:0031-9171    

DOI:10.1063/1.1694732

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A two‐dimensional study is presented, based on computer simulation, of strong electron‐ion streaming instabilities induced by counterstreaming ion beams, for initially cold electrons(ve ≪ vd).Wave growth initially agrees well with linear theory. Higher harmonics appear later, in good agreement with mode coupling theory. The wave spectrum, initially very broad perpendicular to the streaming direction, narrows as the electrons heat and the waves saturate, in accordance with quasilinear theory. Electron heating is explained by nonresonant quasilinear diffusion at early times, and by resonant turbulent diffusion at later times. In contrast to previous one‐dimensional studies, it is found that electron heating continues well after wave saturation and afterve > vd.However, the heating rate at late times is somewhat reduced by partial plateau formation in the electron velocity distribution. Coherent electron trapping is not seen, as it is in the one‐dimensional situation, but coherent ion trapping does occur and dominates the wave dynamics at late times, terminating wave growth on the time scale studied.

ISSN:0031-9171    

DOI:10.1063/1.1694733

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A toroidal, elliptic cross‐section, sharp‐boundary model of a high‐pressure tokamak with currents confined to the surface, is tested for stability against kink modes. It is shown that the optimum configuration corresponds to a vertical ellipse in which the ratio of the major to minor axes is 2.2. For this case, the maximum critical&bgr;for stability against kink modes is&bgr; = 0.37a/R.For&bgr; > 0.37a/Rthe model is unstable for all values of the safety factor above and below the Kruskal‐Shafranov limit.

ISSN:0031-9171    

DOI:10.1063/1.1694734

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The stability with respect to kink modes of a high&bgr;straight screw pinch with elliptical cross section is examined using sharp boundary theory. No restriction is placed on the ellipticity or&bgr;. It is found that the criticalqneeded for stability is no longer unity but is a strongly increasing function of ellipticity. There are indications that large ellipticity in toroidal systems is too strongly destabilizing to allow such equilibria to be useful; however, small ellipticity may be useful for achieving higher&bgr;.

ISSN:0031-9171    

DOI:10.1063/1.1694735

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Experimental observations of infrared emission(2‐10.6 &mgr;m)from a dense plasma focus have been made, along with the standard diagnostics of neutrons, x rays, and visible light. The thermal bremsstrahlung spectrum has been observed over the range2 &mgr;m ≲ &lgr; ≲ 6.5 &mgr;m. Occasional bursts of strong, nonthermal infrared occur for&lgr; ≳ 5 &mgr;mwith enhancements up to 106over thermal radiation level. At a filling pressure of 1.6 Torr D2, the enhanced radiation occurs during the dense pinch stage of the discharge; however, at lower pressure (1.1 Torr), the maximum emission occurs during and shortly after the breakup of the dense pinch.

ISSN:0031-9171    

DOI:10.1063/1.1694736

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Two propagating modes of low‐frequency perturbations are excited by applying oscillating currents to a small coil surrounding a plasma column in a single‐endedQmachine. One of them has the same phase velocity and damping factor as grid‐excited waves, in spite of quite different exciting mechanisms. The other is observed to propagate only when the exciting frequency is higher than the frequency slightly above the ion cyclotron frequency. Its phase velocity increases with an increase in the magnetic field. This mode follows a dispersion relation of electrostatic ion cyclotron waves propagating along a magnetized plasma column.

ISSN:0031-9171    

DOI:10.1063/1.1694737

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The stability of a waveguide enclosed relativistic electron beam in a rippled magnetic field has been theoretically investigated. The presence of the ripple can couple negative energy beam modes to positive energy waveguide modes to produce instability and thereby generate microwaves. In the experiment, the thickness of the beam is small compared with all other relevant lengths. Therefore, the nonlinear dynamics of an infinitesimally thin sheet beam are analyzed. The system is described by the orbit equations for the particles, Maxwell's equations, a set of nonlinear boundary conditions across the moving surface, and a continuity equation for the moving surface.

ISSN:0031-9171    

DOI:10.1063/1.1694738

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The phenomenology of thermonuclear burn in deuterium‐tritium microspheres at high densities is described, and numerical results characterizing the burn for a broad range of initial conditions are given. The fractional burnup, bootstrap‐heating, and depletion of the DT fuel, its expansive disassembly, and thermonuclear ignition by propagating burn from central hot spots in the microspheres are discussed. Extensive numerical results from a 3 T Lagrangian simulation code are presented. The yieldsY0from uniform 10, 1, and 0.1&mgr;gmicrospheres with densities&rgr; = 1 to 4 × 104 g/cm3and temperaturesTe = Ti = 1.8 to 100 keVare given. It is shown thatY0 ∼ &rgr;R, &rgr;R < 0.3(Ris the microsphere radius) or, equivalently,Y0 ∼ &rgr;2/3for spheres of fixed massm. The gain‐factorG0 ≡ Y0/mI0(I0is the internal energy) is shown to measure burn efficiency in uniform microspheres. More than a four‐fold increment in the gain factor is shown to derive from apportionment of the internal energy in a central hot spot. The limiting effects of electron degeneracy on the gain factor are outlined. As a guideline, the experimental observation of 1013neutrons/kJ of input laser energy is established as proof of good absorption; 1015/kJ will imply yields exceeding break even.

ISSN:0031-9171    

DOI:10.1063/1.1694739

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A general set of relations, useful in the analysis of the propagation and instability characteristics of small signal space charge and polarization interfacial waves, are derived for field and flow variables on the perturbed surfaces of a uniformly charged, incompressible, viscous prototype layer.

ISSN:0031-9171    

DOI:10.1063/1.1694740

出版商:AIP    

年代:1974

数据来源: AIP