摘要:

Light spectrometric investigations of ac‐excited binary noble‐gas mixtures show ac cataphoretic phenomena. Time‐averaged and discrete‐time analysis techniques are used in studies of He‐Ne, Ne‐Ar, and Kr‐Xe mixtures. Enhancement of the readily ionized gas component can occur either at the discharge tube ends or at the center. Time‐dependent analyses indicate the importance of ionization waves when ac cataphoretic segregation is detected. The dependence of enhancement on various discharge parameters is reported.

ISSN:0021-8979    

DOI:10.1063/1.326030

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

New results on the plasmas produced by an e‐beam in a vacuum diode are presented. In the near‐uv range, we measure the plasma expansion speeds by the Doppler‐shift and by the time‐of‐flight methods. The maximum observed speed of 2.4×107cm/sec for the C+3ions leads us to estimate the electric field which is probably in the range of 1–6 kV/cm and may exist for tens of nsec. Some aspects of the spectral characteristics, the collective field effect, and the partition of the discharge energy are discussed in connection with recent literature.

ISSN:0021-8979    

DOI:10.1063/1.326031

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The cyclotron maser instability for a solid relativistic electron beam propagating parallel to a uniform axial magnetic fieldB0?zis investigated. The stability analysis is carried out within the framework of the linearized Vlasov‐Maxwell equations. It is assumed that &ngr;/?≪1, where &ngr; is Budker’s parameter and ?mc2is the electron energy. Stability properties are investigated for the choice of equilibrium distribution function in which all electrons have the same value of total perpendicular energy, the same value of axial velocity, and a step‐function distribution in canonical angular momentum. The instability growth rate is calculated including a determination of the optimum value of the beam radiusR0for maximum growth. It is found that the maximum growth rate for a solid beam is comparable to the maximum growth rate for a hollow beam.

ISSN:0021-8979    

DOI:10.1063/1.326032

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The electromagnetic surface modes at the interface between two plasma media are studied and the excitation of these surface waves in a plasma by a relativistic electron beam is discussed. Under the resonance condition &ohgr;?kzvb, it is shown that an electron beam crossing a plasma can excite a surface electromagnetic wave at the plasma frequency &ohgr;=&ohgr;p.

ISSN:0021-8979    

DOI:10.1063/1.326033

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Temperatures and densities were determined from the plasma formed by implosion at the center of symmetrical exploded‐wire arrays. Temperatures of 500–725 eV were found from line ratios in Al and Si spectra using various plasma models. The recombination temperatures for these elements were 500–550 eV. Higher implosion plasma temperatures of 1.5–2 keV were found in Ti and Fe data. The densities were ∼1×1020electrons/cm3for the spatially integrated implosion region. Intense emitting regions of ∼500 &mgr;m to ∼1.5 mm in size were observed in densitometer contours of Al‐plasma pinhole camera x‐ray images.

ISSN:0021-8979    

DOI:10.1063/1.326034

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The effective‐field concept is used to derive a breakdown criterion for a model gas subjected to strong laser and microwave fields. This classical model includes diffusion, collisions, and the finite pulse length. The pressure dependence of this criterion is explored, and it is found that, under the proper conditions, it is possible to produce one or two narrow pressure regions in which breakdown will occur, or two broad breakdown regions separated by an extremely narrow nonbreakdown region. The criterion is extended to account for time‐dependent field intensities and frequencies.

ISSN:0021-8979    

DOI:10.1063/1.326035

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The calibration of electrostatic triple‐probe voltage and current response in a supersonic continuum plasma has been carried out using a low‐pressure shock tube. The electron temperatureTeand charge number densityNeare compared to probe voltage and current response, respectively, for a useful range of plasma parameters (1≲Rp/&lgr;D≲10, 2≲M≲5, and 7<&fgr;13<35, whereRpis the probe radius, &lgr;Dis the Debye length,Mis the shock Mach number, and &fgr;13is the nondimensional probe voltage). The dependence of probe response on Debye ratioRp/&lgr;Dand various flow parameters was investigated. The probe voltage response was found to depend significantly on the Debye ratio. Expressions for electron temperature determination by triple probe are presented.

ISSN:0021-8979    

DOI:10.1063/1.326036

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A multiplier‐assisted discharge is a new type of electron source obtained by operating a high‐gain electron multiplier in the presence of a feedback mechanism. At sufficient multiplier gain such a closed feedback loop results in self‐sustaining currents. In one such feedback configuration—called ion feedback—the multiplier is operated in a background gas pressure of 10−6–10−4Torr. At this pressure, and with multiplier gains of ?105, positive ions that are created by electron‐gas collisions are accelerated to the multiplier input and produce sufficient secondary electron emission to sustain exponential current buildup. The operation of a nonlinear mechanism, most notably electron space charge, serves to ’’saturate’’ the current at a stable dc level. A second feedback mechanism, optical feedback, is obtained by operating a multiplier with a photocathode at the input that is spectrally matched to a phosphor at the output. The feedback mechanism in this case consists of that part of the phosphor emission that reaches the photocathode and causes photoelectrons that enter the multiplier. Again, space charge acts to saturate the currents to a stable dc level. Both theoretical expressions and experimental measurements were obtained for the operating parameters and the dynamic characteristics of feedback cells. For ion feedback, the relation that was developed between the multiplier gain and the pressure required for sustained emission was found to agree with the experimental values over the range of rare gases from helium to xenon. Current rise times were measured for this range of gases and agreed with predictions; exponential rise times as short as 26 nsec are available in cells using helium as background gas. Space‐charge effects, in the form of both cell saturation due to electrons and neutralization due to the positive currents, were observed; the latter effect showed the dependence on pressure and gas species that is expected on the basis of simple theory. There was also measured the production and decay rates of metastable atoms, which are created along with positive ions as a result of electron‐gas collisions. A similar correlation of theory with experiment was achieved for optical feedback. Particularly noteworthy in the optical feedback case is the very fast—1.5 nsec—rise time available with a short‐time‐constant phosphor such as cerium‐doped LaPO4.

ISSN:0021-8979    

DOI:10.1063/1.326037

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

This paper presents an investigation of the excitation of an electron plasma wave in a hot collisionless magnetoplasma by a right‐handed Gaussian EM beam (pump wave) when the plasma wave and the pump wave are propagating along the static magnetic field. On account of the Gaussian intensity distribution of the pump wave, pondermotive force becomes finite and the electrons are redistributed. This redistribution is highly dependent on whether &ohgr;c≳2&ohgr;0or &ohgr;c<2&ohgr;0, where &ohgr;cis the electron cyclotron frequency and &ohgr;0is the pump‐wave frequency. The modified background electron density leads to coupling between the plasma wave and the pump wave. When the initial power of the pump wave is greater than the critical power for self‐focusing, oscillatory self‐focusing of the pump wave occurs and the coupling of the two waves are modified. Moreover, the effect of changing the intensity of the magnetic field affects the self‐focusing of the pump wave, and the plasma‐wave excitation is accordingly affected.

ISSN:0021-8979    

DOI:10.1063/1.326038

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A Gaussian electromagnetic beam propagating in a plasma in the direction of the static magnetic field gives rise to (i) a ponderomotive force transverse to the magnetic field and (ii) nonuniform heating of electrons. These effects eventually modify the local magnetic field and thus produce a nonlinearity relevant to self‐focusing. The critical power for self‐focusing on a time scale shorter than the diffusion time (&tgr;D≳t) isvth/ctimes shorter than that on the longer time scale (t≳&tgr;D), wherevthis the thermal speed of electrons.

ISSN:0021-8979    

DOI:10.1063/1.326039

出版商:AIP    

年代:1979

数据来源: AIP