摘要:

If the source of a field that satisfies Poisson’s equation can be written as the divergence of a vectors, then a scalar multipole expansion of the source can be evaluated in terms ofs, which is a dipole density. A multipole expansion in terms of the dipole density can be computed about different origins. This allows us to evaluate the expansion of a dipole displaced from the origin and find a method of approximating some multipole expansions by displaced dipoles. In many physical applications it is known that the source is a displaced dipole, and we can find its location from a multipole expansion at some convenient location. It is possible to derive pictures in terms of dipole densities that in the proper limit become the individual multipoles. There are, however, ambiguities in that for some multipoles more than one picture gives the proper field.

ISSN:0021-8979    

DOI:10.1063/1.334589

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Computer models developed so far on electron‐beam‐excited KrF(B–X, 248 nm) lasers that include the vibrational relaxation process in the upper lasingBlevel at the finite rate could not predict the high intrinsic laser efficiency which was experimentally reported. This is attributed to the reduction of the laser extraction efficiency. We have developed a four‐level KrF laser model that includes the vibrational relaxation process and also the collisional mixing of the KrF*(B) and the KrF*(C) levels. The collisional quenching rates for KrF*(B,C) that we used and the vibrational relaxation rate were carefully estimated by using the effective spontaneous lifetimes for KrF*(B,C). As a result, the model prediction was in quite good agreement with many experimental results for a saturation behavior of KrF*(B–X) fluorescence, for small‐signal gains, for small‐signal absorptions, and for intrinsic efficiencies. Estimated rate constants in this model for the vibrational relaxation and the KrF*(B,C) mixing are 4×10−11and 5×10−10cm3/s, respectively, for a two‐body collision rate with argon gas.

ISSN:0021-8979    

DOI:10.1063/1.334590

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Shock‐wave initiation of solid explosives depends on localized regions of high temperature (hot spots) created by heterogeneous deformation in the vicinity of various defects. Current mathematical models of shock initiation tend to fall into two broad categories: (1) thermodynamic‐state‐dependent reaction‐rate models, and (2) the continuum theory of multiphase mixtures. The level of generality possessed by (1) appears to be insufficient for representation of observed initiation phenomena, while that of (2) may exceed necessary requirements based on present measurement capabilities. As a means of bridging the gap between these two models, we present an internal‐state‐variable theory based on elementary physical principles, relying on specific limiting cases for the determination of functional forms. The appropriate minimum set of internal‐state variables are the mass fraction of hot spots  &mgr;, their degree of reaction  f, and their average creation temperature &thgr;. The overall reaction rate &lgr;˙, then depends on  &mgr;,  f, and &thgr; in addition to the usual macroscopic thermodynamic variables (current state as well as their history). Two specific forms of this set of equations are applied to time‐resolved shock‐initiation data on PBX‐9404. Numerical solution is achieved by the method of characteristics for rate‐dependent chemical reaction. Additional questions such as the effect of thermal equilibrium between phases (solid reactants and gaseous products) on the theoretical results are discussed quantitatively.

ISSN:0021-8979    

DOI:10.1063/1.334591

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A plane transverse electromagnetic wave propagating antiparallel to a cold relativistic electron beam extending over all space is backscattered from a negative energy space‐charge wave into a frequency up‐converted plane transverse electromagnetic wave. Parametric and nonlinear three‐wave interactions are analytically examined in a systematic manner taking account of all the nonlinearities. With the inclusion of appropriate phenomenological collision terms, and frequency and wave‐number mismatches, the amplitude threshold, the growth rate, and the saturation values are determined. A heuristic estimation reveals wave amplification to be limited by the beam velocity. An expression for maximum amplification is obtained. Representative numerical results are presented to display the characteristics of the nonlinear three‐wave interaction.

ISSN:0021-8979    

DOI:10.1063/1.334592

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The effect of plasma potential profile and collisions with neutrals and charged particles upon positive and negative ion energies has been studied in a low‐pressure (2–13.6 mTorr) hybrid magnetic multicusp hydrogen discharge. When the plasma density is lower than 1011cm−3, the negative ion temperature is always close to the gas temperature, while the positive ion temperature goes down from 1 to 0.1 eV when the pressure goes up. The observed reduction of the relative negative ion density when the pressure is increased is shown to be related to the reduction of positive ion velocities and to the associated increase in the mutual neutralization rate coefficient.

ISSN:0021-8979    

DOI:10.1063/1.334593

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Using detailed atomic and radiative transfer calculations, we consider theK‐shell radiative properties of optically thick plasmas composed of two different elements with nearly the same atomic number. A sodium‐neon mixture is chosen for specificity, and the plasma conditions considered are similar to those achieved in puffed‐gas or exploding‐wireZpinches. The emissivity of an individualK‐shell line is parameterized in terms of an ‘‘ultimate’’ photon escape probability which is a function of both the single‐flight photon escape probability and the collisional quenching probability. Holding the total ion density (sodium plus neon) constant, we have evaluated the totalK‐shell emissivity as a function of sodium‐to‐neon ratio. In contrast to the optically thin case the emittedK‐shell power peaks at values of this ratio intermediate between pure neon and pure sodium. The opacity effects responsible for this phenomenon are analyzed using the escape parameterization.

ISSN:0021-8979    

DOI:10.1063/1.334594

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Thermal transport, mass ablation rates, and preheat have been studied in spherical irradiation at &lgr;=351 nm, using six of the 24 beams of the OMEGA spherical irradiation laser system at the Laboratory for Laser Energetics. Mass ablation rates are higher at 351‐nm than at 1054‐nm irradiation, even when compared at the same absorbed irradiance. Similar to the case of 1054‐nm irradiation, very deep burnthrough was found at 351 nm. However, the shallow‐gradient temperature profile at the heat front, characteristic of the experiments at 1054 nm, was not observed here, nor was the large difference between uniform and tight focus irradiation of spherical targets. Ablation pressures derived from charge‐collector data rise from 10 to 100 Mbar for absorbed irradiance in the range of 4×1013to 9×1014W/cm2.

ISSN:0021-8979    

DOI:10.1063/1.334595

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The attachment of electrons to H2O in Ar, N2or CH4is investigated using a parallel‐plate drift‐tube apparatus. Electrons are produced either by irradiation of the cathode with ArF laser photons or by two‐photon‐ionization of a trace of trimethylamine in a buffer gas. The transient voltage pulses induced by the electron motion between the electrodes are observed. The electron attachment rate of H2O is determined from the ratio of transient voltage with and without H2O added to the buffer gas. The measured electron attachment rate constants of H2O in Ar increase withE/Nfrom 2 to 15 Td. Electron attachment due to the formation of ‘‘temporary’’ negative ions in the H2O–N2and H2O–CH4mixture were observed. The lifetime of the negative ion was determined to be about 200 ns, whose nature is discussed. The ‘‘apparent’’ electron attachment rate constants for the formation of ‘‘temporary’’ negative ions in the H2O–CH4gas mixture are measured forE/Nfrom 1 to 20 Td. The electron drift velocities for the gas mixtures of H2O in various buffer gases are measured.

ISSN:0021-8979    

DOI:10.1063/1.334596

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A set of effective electron collision cross sections, including momentum transfer, vibration, and electron excitation collisions has been determined for CCl2F2gas: the momentum transfer is approximately 2×10−15cm2with the highest maximum about 4×10−15at 6 eV, two vibrational maxima with thresholds of 0.11 and 0.14 eV are about 9 and 4 (10−16cm2) at 0.2 and 0.25 eV and two excitation maxima with thresholds of 7.0 and 9.8 eV are 2.8 and 5.1 (10−16cm2) at 11 and 18 eV, respectively. The calculated transport coefficients are in satisfactory agreement with measured values, except for the attachment coefficient, which is too high at low (below 90 V cm−1 Torr−1) and high (above 180 V cm−1 Torr−1) reduced fields. The possibility of comparison in the case of N2‐CCl2F2mixtures is limited and agreement is less satisfactory than in the case of pure CCl2F2. Deviations for medium fields are within about 30% for theD/&mgr; ratio and ionization coefficients. The attachment coefficients agree well at high partial density of CCl2F2(above approximately 50%) but the experimental values are much higher than predicted at low CCl2F2content. Calculated values of the limit fields agree well with the values obtained from breakdown potential measurements available in the literature.

ISSN:0021-8979    

DOI:10.1063/1.334597

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The relationship between the pressure‐dependent electron attachment rate constants which have been observed in 1‐C3F6and in several perfluoroalkanes and the uniform field breakdown strengths (E/N)limin these gases is discussed. Also discussed are the various types of synergistic behavior in (E/N)limwhich have been observed in binary dielectric gas mixtures. For the latter, a new mechanism is outlined which explains the synergism observed in several gas mixtures where the (E/N)limvalues of the mixtures are greater than those of the individual gas constituents, which we callpositivesynergism. Model calculations are presented which support this mechanism and can be used to explain the pressure‐dependent synergistic effects which have been reported in 1‐C3F6/SF6and other gas mixtures. Experimentally observed ion–molecule reaction processes for several gases are discussed which support the proposed mechanism. Based on this mechanism, we outline several conditions which must be fulfilled in order to observepositivesynergistic behavior in dielectric gas mixtures.

ISSN:0021-8979    

DOI:10.1063/1.334598

出版商:AIP    

年代:1985

数据来源: AIP