摘要:

The excitation of low‐frequency parametric instabilities by a finite wavelength pump in a system consisting of a warm electron plasma traversed by a warm electron beam is investigated in a fluid dissipationless model. The dispersion relation for the three‐dimensional problem in a magnetized plasma with arbitrary directions for the waves is derived, and the one‐dimensional case is analyzed numerically. For the one‐dimensional back‐scattering decay process, it is found that when the plasma‐electron Debye length (&lgr;Dp) is larger than the beam‐electron Debye length (&lgr;Db), two low‐frequency electrostatic instability branches with different growth rates may exist simultaneously. When &lgr;Dp≃&lgr;Db, the large growth rate instability found in the analysis depends strongly on the amplitude of the pump field. For the case &lgr;Dp<&lgr;Db, only one low‐frequency instability branch is generally excited.

ISSN:0031-9171    

DOI:10.1063/1.863549

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An analysis of the stability of an arbitrary &bgr; collisionless plasma to modes with wavelengths greater than the ion gyroradius is presented. The stability of such a plasma to perturbations that grow on the hydrodynamic time scale is determined by the Kruskal–Oberman energy principle. However, a configuration which is predicted to be stable on the basis of this kinetic energy principle may still be unstable to modes that grow with a frequency comparable to the diamagnetic or curvature drift frequency. A new variational principle that gives sufficient conditions for instability of these low‐ frequency modes is derived. The new principle indicates that two types of instabilities are possible; the first corresponds to the low‐frequency electrostatic, trapped particles mode, and the second is the low‐frequency limit of magnetohydrodynamic (interchange and ballooning) modes. The kinetic modifications to the interchange (Mercier) criterion are evaluated and the effect of the kinetic terms on ballooning modes is estimated.

ISSN:0031-9171    

DOI:10.1063/1.863550

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The noise spectrum from which stimulated Brillouin scatter grows has two sources in laser fusion plasmas; a broadband source due to ion‐acoustic fluctuations, and a line source, usually much larger, which is the nonabsorbed light returning from the plasma critical surface. We give a theoretical description of stimulated Brillouin backscatter when the fluctuation source may be neglected and the scatter grows exclusively from the nonabsorbed light. Gradients of background density, velocity, and temperature are allowed. Theoretical predictions are compared to numerical simulations of scatter for parameters of recent experiments. It is found that stimulated Brillouin scatter can be greatly enhanced by the presence of a critical surface and that it can become an important part of the total energy balance.

ISSN:0031-9171    

DOI:10.1063/1.863551

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A method for solving the integral eigenmode equation for drift waves in cylindrical (or slab) geometry is presented. A leading‐order kinematic effect that has been noted in the past, but incorrectly ignored in recent integral‐equation calculations, is incorporated. The present method also allows electrons to be treated with a physical mass ratio (unlike earlier work that is restricted to artificially smallmi/meowing to resolution limitations). Results for the universal mode and for the ion‐temperature‐gradient driven mode are presented. The kinematic effect qualitatively changes the spectrum of the ion mode, and a new ’’second region of instability’’ fork⊥ &rgr;i≳1 is found.

ISSN:0031-9171    

DOI:10.1063/1.863552

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The generation of second and third harmonics at the oblique incidence of ap‐polarized wave on an inhomogeneous plasma (resonance absorption) is studied. The relevant wave equations are solved numerically. First, the dependence of the emitted harmonics on the temperature, the density scale length, and the angle of incidence is investigated for a linear density profile. The strong oscillations of the harmonic emission in a certain temperature range are attributed to the position of the point where the condition for phase matching is fulfilled. In the second part a modified density profile with a caviton is considered, and it is shown that a large enhancement of harmonic emission is possible because of the formation of electrostatic resonant structures. This effect will become important under experimental nonstationary conditions when the density profile is deformed by the ponderomotive force.

ISSN:0031-9171    

DOI:10.1063/1.863553

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The structure of the shear Alfve´n waves is investigated in a collisional plasma with shear. The fourth‐order equation obtained by combining Ampere’s law and the quasi‐neutrality condition is solved by the method of matched asymptotic expansions fork⊥ &rgr;s≫≪1 to obtain the dispersion relation. A hierarchy of damped, localized modes are found which can have either even or odd parity. The solutions basically have the structure of kinetic Alfve´n modes trapped between the two Alfve´n cutoffs on either side of the rational surface. The mode damping arises from Ohmic dissipation by electrons near the rational surface. The relation between these modes, microtearing modes, and the Alfve´n continuum is discussed.

ISSN:0031-9171    

DOI:10.1063/1.863554

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

With the inclusion of finite Larmor radius effects in the shear Alfve´n eigenmode equation, the continuous Alfve´n spectrum, which has been extensively discussed in ideal magnetohydrodnamics, is removed. Neutrally stable, discrete radial eignmodes appear in the absence of sources of free energy dand dissipation. Alpha (or energetic) particle toroidal drifts destabilize these modes, provided the particles are faster than the Alfve´n speed. Although the electron Landau resonance contributes to damping, a stability study of the parametric variation of the energy and the density scale length of the energetic particles shows that modes with low radial mode numbers remain unstable in most cases. Since the alpha particles are concentrated in the center of the plasma, this drift‐type instability suggests anomalous helium ash diffusion. Indeed, it is shown that stochasticity of alpha orbits due to the overlapping of radially neighboring Alfve´n resonances is induced at low amplitudes,ei&fgr;/Ti≳0.05, implying a diffusion coefficientDr&agr;≳4.4×103cm2/sec.

ISSN:0031-9171    

DOI:10.1063/1.863555

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Preliminary results are presented from an active wave experiment performed with a double‐dipole probe aboard the F4 rocket launched during the PORCUPINE campaign. They show that electrostatic cyclotron waves can be excited linearly around the half‐harmonics of the electron gyrofrequency. The most striking fact is that, under certain plasma conditions, the amplitude of waves excited at these peculiar frequencies can be even stronger than those observed at the characteristic resonances of the magnetoplasma. The fine structure of the signals received is also observed and interpreted. Theoretical numerical calculations of the potential created by a dipole antenna in a Maxwellian plasma are presented; good agreement is obtained with the complicated pattern of the various experimental results.

ISSN:0031-9171    

DOI:10.1063/1.863556

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The interaction of proton beams at pellet fusion power densities (∼100 TW/cm2) with initially solid targets is calculated. Radiation losses and conduction are shown to have important effects at these power densities.

ISSN:0031-9171    

DOI:10.1063/1.863567

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Conditions are determined for the stability of two identical colliding ion beams in the presence of neutralizing electrons, but no background ions. Such a situation is envisioned for the Counterstreaming Ion Torus. The ion beams are taken to be Maxwellian in their frames of reference. The approximation of decoupled electrostatic and electromagnetic modes is made. The stability of the electrostatic modes depends on the relation between the ion‐electron temperature ratio and the relative beam velocities. The stability of the electromagnetic mode depends on the relation between the ion plasma &bgr; and the relative beam velocities.

ISSN:0031-9171    

DOI:10.1063/1.863568

出版商:AIP    

年代:1981

数据来源: AIP