摘要:

Earlier investigations of resistive instability have shown that an equilibrium flow, including a flow of the order of the natural diffusion, has a profound influence on the stability properties of the tearing mode. It has been reported that a perpendicular flow of the same order as the natural diffusion (&eegr;/a) makes a finite shift in the stability threshold as &eegr;→0, and only the sign of the velocity matters in this limit. Here the primary concern is with how a moderate or small magnetic Prandtl number changes the stability threshold of a magnetofluid in the presence of a perpendicular flow: it is found that the threshold is again continuous atv=0. The flow is still stabilizing or destabilizing according to the direction of the flow. The viscosity plays important different roles in the static and nonstatic cases. The stability of a static plasma is independent of the magnetic Prandtl number, but the growth rate is not. The stability depends on this number in the nonstatic cases. It is found that the magnitude of the shift in the stability threshold decreases with increasing magnetic Prandtl number. This effect tends to oppose the stabilizing or destabilizing effect of the flow. An increase in the viscosity may in fact destabilize modes when the flow is diffusive. The boundary in parameter space between the inertial and viscous regime is estimated. High‐beta devices are well within the viscous regime. It has been possible to demonstrate that the correction caused by viscosity is far more important than those corrections caused by a perpendicular or a shear parallel flow for modes of finite growth rate.

ISSN:0031-9171    

DOI:10.1063/1.865201

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Dissipative trapped particle modes are studied in tandem mirrors by including electron collisions and ion Landau damping. A variational approach is used to obtain a collisional response in terms of the collisionless result plus a collisional term. The collisional term is then self‐consistently solved in all collision frequency regimes. When &ngr;/&ohgr;≪1 (&ngr;≡electron collision frequency) and the dissipationless mode is stable through a positive (negative) charge uncovering mechanism, there are two stable waves with phase velocities in the ion (electron) diamagnetic direction. The higher frequency wave with &ohgr;∼&ohgr;@B|i(&ohgr;*e) is destabilized by ion (electron) dissipation while the lower frequency wave is destabilized by electron (ion) dissipation. At high collision frequency (&ngr;>&ohgr;@B|e), the only unstable trapped particle wave has &ohgr;∼&ohgr;*e, with electron collisions being destabilizing.

ISSN:0031-9171    

DOI:10.1063/1.865202

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A drift‐cyclotron instability is observed at low magnetic field strengths in a linear multiple mirror. It is characterized by a frequency which decreases with increasing magnetic field strength, propagation in the direction of the electron diamagnetic current, and an azimuthal mode number of 2. The mode is observed when the ion‐cyclotron frequency is less than the ion diamagnetic frequency. The frequency is Doppler‐shifted by anE×Bdrift. Effects of particles trapped in the mirror cells alter the characteristics of the instability. The frequency decreases slightly and the fluctuation level increases with increasing mirror ratio for mirror ratios less than 1.1, but then levels off at higher mirror ratios. The drift wave is analyzed by a linear theory which treats finite ion gyroradius, an electron temperature gradient, and trapped electrons. The theory predicts three unstable modes, only one of which is consistent with all of the above measurements. This instability is a Doppler‐shifted drift‐cyclotron instability modified by trapped particle effects.

ISSN:0031-9171    

DOI:10.1063/1.865203

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The backward branch of the electrostatic ion‐cyclotron wave has been observed for the first time. The wave, which was driven by a phased antenna structure inserted in a neon plasma, exists in the parameter ranges 2Ti/mi≪(&ohgr;/k∥)2≪2Te/me,n&OHgr;i<&ohgr;<(n+1)&OHgr;i,Te≳Ti, and &ohgr;pi>&OHgr;i. Double‐tip probe interferometry data agree with the theoretical dispersion relation. The antenna couples into the wave more readily on the side of the antenna where it has its smallest wavenumber.

ISSN:0031-9171    

DOI:10.1063/1.865204

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The ELMO Bumpy Torus (EBT) [PlasmaPhysicsandControlledNuclearFusion(IAEA, Vienna, 1975), Vol. II, p. 141] normally has an energetic electron ring in each of its 24 mirror sectors. The original intention of using this hot‐electron population was to provide an average local minimum in the magnetic field (through its diamagnetism) to stabilize the simple interchange and flute modes, which otherwise are theoretically inherent in a closed‐field‐line bumpy torus. To study the confinement properties of a bumpy torus without the influence of hot‐electron rings, a water‐cooled stainless steel limiter in each mirror sector was extended into the plasma to the ring location; this eliminated the hot‐electron ring population. These limiters were aptly named ‘‘ring killers.’’ Electron temperature, density, space potential, and plasma fluctuations have been measured during the ring killer experiment and are compared to standard EBT operation. The results of these experiments indicate that the hot‐electron rings in EBT do enhance the core plasma properties of EBT and do, in fact, reduce plasma fluctuations; however, these improvements are not large in magnitude. These measurements and recent theoretical models suggest that simple interchange/flute modes are stabilized, or fluctuation levels reduced, well before that condition is obtained for average minimum‐B stabilization. Several possible mechanisms for this stabilization are discussed.

ISSN:0031-9171    

DOI:10.1063/1.865205

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

CO2laser scattering and correlation techniques are used to study low‐frequency density fluctuations in plasmas in the Alcator C tokamak. Using crossed‐beam correlation techniques, these fluctuations are found to have largest amplitudes near the plasma edge. In discharges with mean plasma densitiesn¯≳2×1014cm−3, the normalized fluctuation amplitude isn˜/n&bartil;0.5 and peaks at the limiter radius; however, at lower plasma densities,n˜/npeaks in the plasma interior at a normalized minor radiusr/a&bartil;0.75 (whereais the limiter radius). The radial and poloidal wave vectors of the fluctuations are comparable, with mean wavenumbersk¯∼20 cm−1. At any givenk, the observed frequency spectrum is broad. For plasma densitiesn¯≲1.5×1014cm−3, the poloidal group propagation velocityv&thgr;of the fluctuations is measured to be approximately 2×105cm/sec in the electron diamagnetic drift direction, while at plasma densitiesn¯≳2×1014cm−3,v&thgr;&bartil;9×104cm/sec in the ion diamagnetic drift direction. The nature of these fluctuations is also studied in discharges exhibiting a boundary‐layer instability (i.e., ‘‘marfes’’).

ISSN:0031-9171    

DOI:10.1063/1.865206

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Microwave radiation can be produced at cyclotron harmonics as a result of the synchronous interaction between a rotatingElayer and an em wave. The derivation of a general growth rate expression is presented here for systems involving large orbit, thin, annular, relativistic electron beams. This growth rate formula is written in terms of parameters which contain the required information about the waveguide wall structure. This method extends previous results to systems that have empty waveguide modes which are hybrids. As an example of this latter case, growth rates for coaxial systems with dielectric liners are calculated via this formalism.

ISSN:0031-9171    

DOI:10.1063/1.865207

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The short wavelength limit of the magnetron instability of a relativistic sharp boundary electron sheath is examined. For short wavelengths the mode is localized to the sheath edge. This localization leads to a simple relationship (essentially a Lorentz transformation) between the relativistic and nonrelativistic growth rates. This relationship involves only equilibrium quantities evaluated at the sheath edge and explains previously noted trends that have been observed numerically.

ISSN:0031-9171    

DOI:10.1063/1.865208

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

In this paper, a comprehensive theoretical treatment is developed for backward wave oscillators composed of a relativistic electron beam guided by a strong magnetic field through a slow wave structure consisting of a cylindrical waveguide with a sinusoidally varying wall radius. This analysis, equally applicable to traveling wave tube operation, includes both a linearized theory of small‐amplitude perturbations and numerical simulations of the saturated, large‐amplitude operating regime. The variation of device operating characteristics with system parameters is examined in detail. Comparisons of the analytic and numerical results with experiments and additional calculations show excellent agreement and justify a high degree of confidence in the validity of the theory.

ISSN:0031-9171    

DOI:10.1063/1.865209

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Calculations have been made of the spectrum of light emitted at half‐integer harmonics of the laser light frequency &ohgr;0. The emitted light results from either the Raman scattering of laser light by Langmuir waves or by the direct conversion of Langmuir waves to light. The Langmuir wave amplitude is calculated from convective instability theory for two‐plasmon decay in an inhomogeneous, plane‐stratified medium. The Raman scattering process generates harmonics at (n/2) &ohgr;0wheren=1,3,5 ...; however, only forn=1 and 3 is the emission level significant. The conversion process produces (1/2) &ohgr;0light with a comparable efficiency to the Raman scattering process near the two‐plasmon instability threshold. Results of the calculation are in agreement with the general features of measured emission spectra.

ISSN:0031-9171    

DOI:10.1063/1.865210

出版商:AIP    

年代:1985

数据来源: AIP