摘要:

The conditions under which a spectrum of lower hybrid drift waves will decay into low‐frequency drift waves (LFD) are calculated. The purpose is to help understand why lower hybrid drift waves are not seen in all field‐reversed configuration (FRC) experiments in which they are predicted. It is concluded that if there is in the plasma a LFD wave amplitude above a critical level, lower hybrid waves will decay into low‐frequency drift waves. The critical level required to stabilize TRX‐2 [Phys. Fluids30, 1497 (1987)] is calculated and found to be reasonably consistent with theoretical estimates.

ISSN:0899-8221    

DOI:10.1063/1.859036

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Narrow Raman spectra can be produced when the collisional damping rate becomes comparable to the homogeneous growth rate for stimulated Raman scattering (SRS). Landau damping limits the SRS spectrum at short wavelength, and collisional damping can limit it at long wavelength through the variation of the damping rate with plasma density. Data are shown to demonstrate this effect for the first time, from experiments in which constant‐intensity pulses of 0.35 &mgr;m light irradiated 3 &mgr;m thick CH targets at intensities up to 2×1014W/cm2. The observed spectra have a peak wavelength, spectral width, spectral shape, and shift of the peak in time that are consistent with the anticipated effects of damping. Reduction of the emission from the plasma to near thermal levels at heavily damped frequencies is also demonstrated. In addition, the application of two specific models of SRS to these data is discussed.

ISSN:0899-8221    

DOI:10.1063/1.859037

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The influence of kinematic viscosity on the properties of resistive instabilities developing in equilibrium configurations in which flows are present is examined. The cross‐field viscosity (&ngr;⊥) is retained in the linearized resistive magnetohydrodynamic equations, which are solved numerically. The magnetic and velocity equilibrium configurations are the same as those studied in a previous paper [Phys. Fluids29, 2563 (1986)] neglecting viscosity, so that its effect can be easily studied in the present work. The results show that the inclusion of the viscosity changes the stability properties considerably when &tgr;&ngr;≤&tgr;r, and where &tgr;&ngr;and &tgr;rare the viscous and magnetic time scales, respectively. Depending on the relative importance of the velocity with respect to the Alfve´n velocity and of the velocity scale length with respect to the magnetic one, the viscosity can have both a stabilizing and a destabilizing effect.

ISSN:0899-8221    

DOI:10.1063/1.859038

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Plasma flow measurements in the presheath have been performed using two types of directional electric ‘‘Mach’’ probes, in the PISCES facility at UCLA [J. Nucl. Mater.121, 277 (1984)]. A fast scanning versatile probe combination has been developed, which operates simultaneously as a ‘‘magnetized’’ Mach probe, an ‘‘unmagnetized’’ Mach probe (with characteristic probe size greater than and smaller than ion gyroradius, respectively), and an emissive probe. Presheaths have been investigated by inserting a small object at the center of the plasma column. Variations in plasma flow velocity, density, and potential along the presheath have been deduced by fluid and kinetic theories. A comparison is made between Mach numbers obtained from the magnetized probe and the unmagnetized probe. Incorporation of shear viscosity of order ∼0.5nmiD⊥in the cross‐field transport along the presheath seems best to model the results. The cross‐field diffusivity (D⊥) is found to scale approximately proportional toB−1/2, with magnitude about 4× larger than Bohm in the PISCES plasma. The effect of an electrical bias applied to the object on the presheath characteristics is discussed.

ISSN:0899-8221    

DOI:10.1063/1.859039

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A numerical study of the plasma presheath has been performed, extending earlier work to include the effects of general source functions, and allowing for collisions in the presheath. Analytic results for a collisionless presheath with particular source functions have been confirmed, and comparisons with a numerical study of a weakly collisional presheath have been made. The extrapolation of these calculations to high collision frequency, where they are compared to a simple analytic model, and the effects of the choice of source functions are presented.

ISSN:0899-8221    

DOI:10.1063/1.859040

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The time‐dependent behavior of a transversely magnetized, two‐dimensional plasma–wall sheath has been studied through particle simulations, with the aim of modeling plasma behavior in the vicinity of the limiters and walls of magnetized plasma devices. The model assumes a magnetic field perfectly parallel to the confining surfaces. The simulations have shown that the cross‐field sheath between a wall and a plasma is a self‐sustaining turbulent boundary layer, with strong potential fluctuations and anomalous particle transport. The driving mechanism for this turbulence is the Kelvin–Helmholtz instability, which arises from the sheared particle drifts created near the wall. In this paper, the transient behavior leading to the turbulent steady state is presented, and the processes of linear growth, vortex saturation, and vortex coalescence are examined. An analytic model for the boundary Kelvin–Helmholtz instability is derived and shown to correctly predict the growth rates of the long‐wavelength modes. In a companion paper, the steady‐state structure and behavior of the cross‐field sheath will be discussed in detail.

ISSN:0899-8221    

DOI:10.1063/1.859041

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The steady‐state behavior of the magnetized plasma–wall sheath has been studied through two‐dimensional particle simulations, which have shown that the sheath maintains itself in a strongly nonlinear, turbulent equilibrium, continuously driven by the edge Kelvin–Helmholtz instability. The sheath assumes a thickness of orderlx∼5&rgr;i, and maintains large, long‐lived vortices, with amplitudes &dgr;&fgr;∼2.5Ti/e, which drift parallel to the wall at half the ion thermal velocity. The sheath also maintains a large, spatially averaged potential drop from the wall to the plasma with &Dgr;&fgr;≊−2Ti/e, opposite in sign to that of the unmagnetized sheath. Accompanying the long‐wavelength vortices are shorter‐wavelength fluctuations, which induce an anomalous cross‐field transport, scaling in accordance to Bohm diffusion when &ohgr;pi≥2&ohgr;ci. At lower densities, &ohgr;pi<2&ohgr;ci, the diffusion coefficient has an additional factor, proportional to the density. These results permit the modeling of the cross‐field sheath by a simple effective boundary condition.

ISSN:0899-8221    

DOI:10.1063/1.859204

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Coherent high‐frequency oscillations near the electron plasma frequency (&ohgr;≲&ohgr;p) are generated by electrodes with positive dc bias immersed in a uniform Maxwellian afterglow plasma. The instability occurs at the sheath–plasma resonance and is driven by a negative rf sheath resistance associated with the electron inertia in the diodelike electron‐rich sheath. With increasing dc bias, i.e., electron transit time, the instability exhibits a hard threshold, downward frequency pulling, line broadening, and copious harmonics. The fundamental instability is a bounded oscillation caused by wave evanescence, but the harmonics are radiated as electromagnetic waves from the electrodes acting like antennas. Wavelength and polarization measurements confirm the emission process. Electromagnetic waves are excited by electrodes of various geometries (planes, cylinders, spheres), which excludes other radiation mechanisms such as orbitrons or beam–plasma instabilities. The line broadening mechanism has been identified as a frequency modulation via the electron transit time by dynamic ions. Ion oscillations at the sheath edge give rise to burstlike rf emissions. These laboratory observations of a new instability are important for antennas in space plasmas, generation of coherent beams with diodes, and plasma diagnostics.

ISSN:0899-8221    

DOI:10.1063/1.859042

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The purpose of this Brief Communication is to calculate the frictional force between two fully ionized plasmas that are moving with respect to one another. Such a calculation is relevant to the expansion of explosion debris into a preionized gas, and to the heating or fueling of a fusion reactor by plasma injection. When the charge densities of the two plasmas are of smilar magnitude, the frictional force is significantly different from that appropriate to the stopping of test particles or to the calculation of electrical conductivity.

ISSN:0899-8221    

DOI:10.1063/1.859043

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

It is demonstrated through computer simulations that a cyclotron autoresonance maser (CARM) amplifier with finite output pulse width operated in a high‐order waveguide mode, such as the TE01mode, is feasible. This is because the absolute instability grows from the initial beam noise fluctuation and takes a certain time interval to become disruptive to the desired amplification process. A drive wave with sufficiently high intensity can further reduce the growth rate of the absolute instability and prolong the stable amplification period.

ISSN:0899-8221    

DOI:10.1063/1.859044

出版商:AIP    

年代:1989

数据来源: AIP