摘要:

The ideal magnetohydrodynamic stability of cylindrical equilibria with mass flows is investigated analytically and numerically. The flows modify the local (Suydam) criterion for instability at the resonant surfaces wherek⋅B=0. Sheared flows below the propagation speed for the slow wave are found to be destabilizing for the Suydam modes. At a critical velocity, where the shear of the flow exactly balances the propagation of the slow wave along the sheared magnetic field, and thek⋅B=0 surface is at the edge of a slow wave continuum, there is instability regardless of the pressure gradient. Above the critical velocity, thek⋅B=0 surface is stable, but an infinite sequence of unstable modes still exists with frequencies accumulating toward the edge of the slow wave continuum at nonzero Doppler shifted frequency. The stability of the infinite sequences becomes a nonlocal problem whenever the accumulation frequency overlaps with a continuum at some other radial location.

ISSN:0031-9171    

DOI:10.1063/1.866151

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A model for stimulated Raman scattering (SRS) in a homogeneous plasma has been designed to account for the presence of stimulated Brillouin scattering (SBS) and the nonlinear coupling between Langmuir and ion waves described by Zakharov equations. The nonlinear evolution of electron plasma waves also includes an effective damping resulting from electron diffusion in localized Langmuir fields produced during simultaneous SRS and SBS evolutions. Numerical results based on this model show two distinct SRS behaviors. Close toncr/4 the Langmuir collapse dominates nonlinear evolution of the instability. At lower densities low level SRS is observed for a relatively long time after which SRS is terminated as a result of ion fluctuations produced by SBS. In addition, the anomalous absorption of backscattered SRS radiation by ion fluctuations produced by the collapse is proposed as a mechanism that may explain some recent experimental observations showing a gap in the SRS spectrum.

ISSN:0031-9171    

DOI:10.1063/1.866152

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Following Chu [inNon‐EquilibriumFlows, edited by P. P. Wegener (Dekker, New York, 1970), Vol. II, p. 37], the phenomena associated with the propagation of acceleration waves in nonequilibrium magnetogasdynamic flow, which is induced by the motion of a piston advancing with finite acceleration into a constant state of rest, are studied. Also studied is the characteristic path by using the characteristics of the governing quasilinear system as the reference coordinate system. A differential equation governing the growth and decay of an acceleration wave is derived. The critical time when the breakdown of the characteristic solution occurs in the neighborhood of the leading frozen characteristic is obtained; that is, when all the characteristics will pile up at the wave front to form a shock wave. It is shown that for the slow relaxation process the effect of the magnetic field is to slow down the motion of the breakdown point along the leading frozen characteristic, and thus to increase the cylindrical shock formation time. However, the effect of the magnetic field on expansion waves is to enhance the decay rate. For the quick relaxation process, however, the effect of the magnetic field on compressive waves is to cause an early shock formation, while the effect on expansion waves is to decrease the decay rate.

ISSN:0031-9171    

DOI:10.1063/1.866153

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Negative‐mass and cyclotron‐maser instabilities in electron rings or layers lead to breaking of the azimuthal symmetry. The short‐wavelength modes are generally thought to be harmless since they saturate by increasing the thickness and energy spread of the original layer. The long‐wavelength (or low‐order) modes, on the other hand, can be devastating to the integrity of the layer. Linear theory has predicted that the growth rates for negative‐mass instability modes are proportional to the azimuthal mode number, so that the low‐order modes would not be important until quite late in time. In contrast, the simulations presented here show that low‐order modes can arise quite early in time, out of nonlinear states of the high‐order modes. At low currents, azimuthal instabilities can be effectively suppressed (as predicted by linear theory) by starting out with a thicker layer, with the particle energy varying with radius. The dispersion in circulation frequency so introduced effectively damps all growing waves. Unfortunately, at high currents (field reversal factors &zgr;≳2%), making the layer thicker does not help. All growth rates are faster at the higher densities, and the high mode numbers grow to substantial amplitudes before damping is effective. Beating of nearby high‐order modes gives rise to global modes that destroy the layer within a few rotations.

ISSN:0031-9171    

DOI:10.1063/1.866154

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

A small signal theory of a free‐electron laser (FEL) with a rotating electron beam in a uniform axial magnetic field and in an azimuthal wiggler field (the ‘‘circular’’ FEL) is developed. The analysis includes the low and high gain regimes and the influence of longitudinal space‐charge forces. It is found that the circular FEL instability has two regimes: a strong pump regime and a negative mass dominated regime. The negative mass regime replaces the weak pump (Raman) regime found for the usual FEL geometry in which the electron beam propagates in the axial direction (the ‘‘linear’’ FEL). The dispersion relation is evaluated, and the resulting growth rates are compared with those of the linear FEL. For a cold beam, at fixed output frequency, the growth rate in the strong pump regime is larger, by a factor of &ggr;2/3, in the circular FEL. The negative mass instability is shown to increase the growth rate and modify the bandwidth of the circular FEL in the strong pump regime. However, the circular FEL performance is found to be more sensitive to the energy spread than the linear FEL.

ISSN:0031-9171    

DOI:10.1063/1.866155

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Three‐halves harmonic spectra measured at three different observation angles from 0.53 &mgr;m laser produced plasmas are presented. The results are compared with existing models. On plastic targets a very good fit is obtained with Barr’s model withTe&bartil;1.6 keV and a plasma velocity atnc/4 ofv&bartil;8.107cm/sec. For higherZmaterial, the electron temperature seems to increase and Landau damping may become significant.

ISSN:0031-9171    

DOI:10.1063/1.866156

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

AKa‐ band gyrotron oscillator powered by a 600 kV pulse‐line accelerator has produced approximately 100 MW at 35 GHz in a circular TE62mode. It has also demonstrated frequency tuning over the range 28 to 49 GHz by operating in a family of TEm2modes, with the azimuthal indexmranging from 4 to 10, by variation of the guide magnetic field. Operation is in general agreement with the predictions of theory.

ISSN:0031-9171    

DOI:10.1063/1.866157

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Low‐frequency electrostatic turbulence generated by the ion–ion beam instability was investigated experimentally in a double‐plasma device. Real time signals were recorded and examined by a conditional statistical analysis. Conditionally averaged potential distributions reveal the formation and propagation of structures with a relatively long lifetime. Various methods for making a conditional analysis are discussed and compared. The results are discussed with reference to ion phase space vortices and clump formation in collisionless plasmas.

ISSN:0031-9171    

DOI:10.1063/1.866158

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The contribution of inelastic atom–atom collisions to the production of electrons and excited atoms in two‐temperature (with electron temperatureTe, atomic temperatureTa, and atomic densityNa), steady‐state, nonequilibrium atomic hydrogen plasma is investigated. The results are valid for plasmas having large amounts of atomic hydrogen as one of the plasma components, so thate–H and H–H inelastic collisions and interaction of these atoms with radiation dominate the production of electrons and excited hydrogen atoms. Densities of electrons and excited atoms are calculated in low‐temperature plasma, withTeandTa≤8000 K and 1016cm−3≤Na≤1018cm−3, and with different degrees of the reabsorption of radiation. The results indicate that inelastic atom–atom collisions are important for production of electrons and excited atoms in partially ionized plasmas with medium and high atomic density and temperatures below 8000 K.

ISSN:0031-9171    

DOI:10.1063/1.866159

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Emmertetal. [Phys. Fluids23, 803 (1980)] have modeled the flow of a one‐dimensional collisionless plasma to a material wall by formulating and solving the warm‐ion plasma equation. In contrast to the result of the cold‐ion plasma equation it was found that the electric field at the plasma–sheath boundary was finite. It is first shown that Emmert’s solution satisfies the generalized Bohm criterion, and is thus a valid solution, before discussing the cause of the difference in the results of the two models in calculating the boundary electric field.

ISSN:0031-9171    

DOI:10.1063/1.866160

出版商:AIP    

年代:1987

数据来源: AIP