摘要:

Electrical contacts of W&sngbnd;Ga and Re&sngbnd;Ga were studied using a field ion microscope in which the emitter tips of the microscope were contacted with liquid gallium in vacuum. Threshold values of the closing voltage and opening current needed to create an amorphous state of the surface atoms of the emitter were obtained for W&sngbnd;Ga contacts. Similar characteristics were noted for the Re&sngbnd;Ga contact although a clear determination of the threshold values was overshadowed by a Re&sngbnd;Ga reaction. The observed threshold closing voltages are significantly lower than known values for short arcs. The cause of amorphism on a tip surface at these low closing voltages, therefore, is explained to be the result of the field emission from the Ga cathode in the case of positive tip and of the bombardment by the field‐ionized gallium ions in the case of the negative tip. The threshold values for closing and opening contacts were dependent on tip radius.

ISSN:0021-8979    

DOI:10.1063/1.1663301

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A Fokker‐Planck (F‐P) formulation is used to describe electron avalanche breakdown in a high‐frequency field. It is shown that the dispersion term dominates the ``average electron'' term. The effect of the statistical fluctuations is to reduce the breakdown field and increase the rate of buildup of the electron population compared to the average electron theory. The F‐P coefficients, which are derived for an energy‐dependent collision frequency with an arbitrary ratio of laser frequency to collision frequency, are also useful for obtaining energy distributions in the theory of gaseous discharges. Numerical results are obtained for the e‐folding time in sapphire based on coefficients which were previously given by Zverevet al.in the context of average electron theory.

ISSN:0021-8979    

DOI:10.1063/1.1663302

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Antenna voltage breakdown arising from the transmission of high‐power microwave radiation may be suppressed on a reentry vehicle by injecting electrophilic chemicals into the flow field. This paper describes numerical procedures for calculating the amount of high‐power microwave radiation that can be transmitted through a plasma chemically seeded with an electron‐attaching substance. The power incident on a reentry plasma slab may be sufficient to induce electron temperature changes, resulting in variations in time of the electron density and effective collision frequency. The plasma properties are determined as a function of time by numerically integrating the two continuity equations for the electron and positive‐ion densities. The processes of electron impact ionization, electron attachment, detachment, and recombination are taken into consideration. The diffusion of the electrons and positive ions in the ambipolar limit is influenced by the presence of negative ions. Runge‐Kutta numerical integration of Maxwell's equations together with the above set of plasma transport equations yield the time‐dependent plane‐wave transmission and reflection coefficients as a function of incident power level for a very wide range of reentry plasma conditions. For one particular set of reentry conditions, graphs are presented of the electron density profile, reflection coefficient, and transmission coefficient as functions of time.

ISSN:0021-8979    

DOI:10.1063/1.1663303

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A Bernstein wave propagating obliquely to the magnetic field is excited by a coaxial antenna in a streaming plasma and detected by an interferometer system, which shows that the dispersion relation for the wave is consistent with the theoretical result. Another wavelike signal with a shorter ``wavelength'' is observed downstream only of the exciting antenna, and its ``phase velocity'' is approximately equal to the velocity of the plasma stream calculated from the Doppler shifts of Bernstein waves in both upstream and downstream regions, which may show that the signal does not propagate as the wave but floats down on the plasma stream as the modulation of plasma particles.

ISSN:0021-8979    

DOI:10.1063/1.1663304

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A new approach to the problem of electromagnetic wave propagation through a vacuum‐plasma interface is formulated and used to solve two simple antenna problems. First, the impedance of a spherical body covered by a vacuum sheath and immersed in a plasma is calculated. The result is found to coincide with the empirical formula commonly used so far. Second, a cylindrical antenna also isolated from the plasma by a sheath is studied. The antenna can support guided modes whose dispersion relation is derived. An experiment has been performed in order to verify this dispersion relation. Experimental results are found to be in favor of the present approach.

ISSN:0021-8979    

DOI:10.1063/1.1663305

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A simple theoretical calculation is given in which the energy distribution of low‐energy secondary electrons emitted from metals is derived. The main feature of the calculation is an energy‐dependent electron mean free path. The theory predicts that the peak of the energy distribution occurs at a value of the secondary electron energy equal to &phgr;/3, where &phgr; is the metal work function.

ISSN:0021-8979    

DOI:10.1063/1.1663306

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Electrostatic probe measurements taken under continuum conditions in a flowing ionized gas can overestimate the charged‐particle density by several orders of magnitude if the convection of charged particles into the plasma sheath is not included in the analysis. A simple physical model which includes both diffusion and convection currents is described, together with an experiment in which charged‐particle densities inferred from spherical and disk probes were compared to microwave electron density measurements. The experiment was performed in a room‐temperature flowing afterglow plasma over a Reynolds number range 0.4–70 under thick‐plasma‐sheath conditions. The agreement between the microwave and the spherical probes' ion current measurements was within 30%, and the ion current‐voltage characteristic was described to better than 10% at both a high and low Reynolds numbers. The interpretation of electron current data is also discussed.

ISSN:0021-8979    

DOI:10.1063/1.1663307

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A derivation of a density‐matrix equation of motion exact to all orders in the driving field is presented for a system of dynamically independent electrons interacting with a set of randomly distributed static scattering centers. Nonlinear ``driving'', ``collision'', and ``interference'' terms are exhibited. An expansion of the equation of motion through second order in the field is readily obtained from the exact nonlinear equation.

ISSN:0021-8979    

DOI:10.1063/1.1663308

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A zero‐current electrometer method is presented. It allows static charge measurements with a precision comparable to that of classical electrometers and current measurements without any current flow in the external circuit. The advantages of this technique are pointed out. The apparatus is used in the investigation of a new charge‐decay process due to the interaction of the electret with molecules of a solvent. The observed effects are interpreted in terms of hopping conduction stimulated by chain motion.

ISSN:0021-8979    

DOI:10.1063/1.1663309

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The growth of a sinusodial ``perturbation'' in the surface of a flat plate smoothly accelerated by expanding detonation products has been observed using the Los Alamos high‐energy x‐ray facility. Through modeling with a two‐dimensional elastic‐plastic numerical hydrodynamics code we have determined that amplitude growth is apparently largely governed by the dynamic yield strength of the material.

ISSN:0021-8979    

DOI:10.1063/1.1663310

出版商:AIP    

年代:1974

数据来源: AIP