摘要:

Recent experiments and calculations have indicated that mutual collisions between excited 6 3P0, 6 3P1, and 6 3P2atoms may be important for the ion formation in low‐pressure Hg–noble‐gas discharges. We calculate various rates for electron‐impact and for excited‐atom‐collisional ionization for a range of conditions relevant to the Hg‐Ar fluorescent lamp (pAR=3 Torr,R=1.8 cm,Twall=20–80 °C, andI=0.2–0.8 A). In the analysis use is made of experimentally determined values for particle densities and electron temperatures. Deviations from a Maxwellian electron‐energy distribution, which substantially reduce the electron‐impact ionization rates, are taken into account. We find that 6 3P–6 3Pcollisions become even more important for the ion formation than electron impact at the higher Hg vapor pressures. Both Hg+and Hg+2will be formed. The 6 3P–6 3Pcollisions are indicated to be mainly responsible for the well‐known decrease in uv efficiency of the Hg–noble‐gas discharge at temperatures above 40 °C.

ISSN:0021-8979    

DOI:10.1063/1.325384

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Model calculations on the positive column of low‐pressure metal‐vapor–noble‐gas and noble‐gas discharges usually account for direct and two‐step electron‐impact ionization. The poor agreement between experiment and model calculations for certain discharge conditions has led us to a detailed analysis of the ion formation in a metal‐vapor–noble‐gas (Na‐Ne) and a noble‐gas (Ne) positive column. Multistep electron‐impact ionization via many higher excited states is found to become a dominant process already at much lower pressures and current densities than considered so far. The multistage ionization will have the effect of lowering the electric field strength. Its highly nonlinear dependence on discharge current will further strongly favor the contraction observed, e.g., in noble‐gas discharges.

ISSN:0021-8979    

DOI:10.1063/1.325385

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Combined electrical, optical, and spectroscopic measurements have been used to determine the total flux, ion flux, particle flux, and neutral atom flux emitted from the cathode spot region of a copper vacuum arc of 80 A total current and 2‐sec duration. It is found that the cathode erosion products consist predominantly of ions and particles, the emission of neutral atomic flux from the cathode being less than 1&percent; of the total flux. The total and particle flux distributions are peaked in the direction of the cathode plane, whereas the ion‐flux distribution is forward peaked. However, both ions and particles are detected in the cathode shadow, a result which is contrary to the hypothesis of purely collisionless transport of cathode erosion products. The neutral atom density measurements are consistent with a model assuming the source of the vapor to be evaporation in flight from the hot particles emitted from the cathode spot region. The size distribution of the particles has a maximum for particles of diameter in the range 0–1 &mgr;.

ISSN:0021-8979    

DOI:10.1063/1.325386

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Q‐switched 20‐nsec ruby‐laser pulses (∼0.5 GW) were focused to about 1012W/cm2on targets containing elements in the range 6⩽Z⩽68. Time‐ and space‐integrated x‐ray emission from plasmas 200 &mgr;m in diameter with lifetimes near 12 nsec was measured with four active detectors at energies between 0.1 and 5 keV. Maxima in x‐ray intensity measured by each detector occurred when binding energies of core electrons in target atoms were 20–30 times the electron plasma temperature. Calculated detector response functions together with coronal‐model plasma emission computations were used to analyze the atomic‐number dependence of the detector signals. A plasma electron temperature of 70 eV and an electron density of about 4×1020cm−3gave the best fit to the ratios of signals. Computed spectra for this temperature showed that the peaks in the low‐energy detector (an x‐ray diode most sensitive near 0.15 keV) were due in part to line radiation. Peaks in the three detectors sensitive above 1 keV (two silicon PIN diodes and an ionization chamber) were primarily due to recombination radiation. The results of this work augment an earlier similar, but limited, study at 1012W/cm2.

ISSN:0021-8979    

DOI:10.1063/1.325387

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Single‐particle trajectory calculations have been used to study the effectiveness of beam concentration near a target pellet in two relativistic electron‐beam‐driven inertial fusion concepts: magnetic cusps and multiple current‐carrying channels in a wagon‐wheel configuration. Angular momentum constraints favor the multichannel transport scheme. The role of angular momentum nonconserving reflections from plugged‐cusp loss cones is discussed. Minimum radii are calculated for various injection conditions in a 40‐channel configuration. The resulting superposition of beams may provide an order‐of‐magnitude improvement in beam concentration over single‐beam results.

ISSN:0021-8979    

DOI:10.1063/1.325388

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

X‐ray experiments were conducted giving data on radiation and electron transport from plasmas produced by the interaction of 10.6‐&mgr;m laser light focused on layered targets. The targets employed were evaporated aluminum on SiO2and copper substrates. The penetration depth of the plasma was measured from the thickness of aluminum required to produce a 1/eattenuation of the highly ionized silicon lines (1s2p‐1s2) and (1s3p‐1s2). The penetration of the plasma into the target surface was 95 A˚ for incident 10.6‐&mgr;m laser irradiances of 3×1013W/cm2. Estimates of electron energies produced in the plasma directed toward the target were made from measurements of the attenuation of theKline of the substrate as a function of aluminum thickness. End‐point electron energies were calculated from the best fit of experimental data with targets of SiO2and copper substrates for laser irradiances from 3×1013to 4×1014W/cm2. When the laser irradiance (IL) is increased, there is an increased dependence of electron energies absorbed by the target. For irradiances greater than 1014W/cm2the effective end‐point energy is proportional toI1/2L.

ISSN:0021-8979    

DOI:10.1063/1.325389

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

Rate coefficients, as a function of the electron temperature, have been determined from the integral cross sections for excitation of the 19 singlet and triplet electronic states of N2within 14.2 eV of the ground state. For electron temperatures less than 10 eV, the rates for excitation of theA 3&Sgr;+u,B 3&Pgr;g,W 3&Dgr;u, anda 1&Pgr;gare all greater than that for excitation of theC 3&Pgr;ustate. The differential cross sections for excitation of these same electronic states were also used to calculate the inelastic momentum transfer associated with the excitation of these states. The total momentum transfer cross section for electrons in N2, as a function of the electron energy, was obtained by adding the inelastic momentum transfer to that associated with elastic scattering. Inelastic momentum transfer accounts for about 25&percent; of the total momentum transfer above 15 eV. Over the energy range 20–60 eV, inelastic scattering contributes 20–25&percent; to the total N2electron scattering cross section. The total scattering cross section obtained here is in good agreement with a recent direct measurement of this cross section.

ISSN:0021-8979    

DOI:10.1063/1.325390

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

A theory has been proposed which seeks to explain the cathode mechanism in low‐current vacuum arcs. It is shown that the high current density at the cathode surface has been successfully discussed by relating the energy‐balance equation, space‐charge equation, electron‐emission equation, current equation, MHD equation of motion, and the minimum principle of joule heating. When four equations and two limiting conditions combine the five dependent variables, which are temperature of the arc root, electric field at the cathode, total current density, current density carried by electrons, and the radius of the arc root, the current density and temperature at the arc root are obtained as a function of the arc current with the cathode‐fall voltage as a parameter. It is also shown that the relationship between the arc current and the evaporation rate from the arc cathode root can be calculated. The theoretical results are in good qualitative agreement with the experimental data of the current density and the erosion rate.

ISSN:0021-8979    

DOI:10.1063/1.325391

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

The purpose of this paper is to report the results of a transmission‐electron‐microscopy (TEM) characterization of structural defects observed in a Czochralski‐grown silicon crystal after a two‐step anneal, i.e., 3 h at 700 °C and 1 h at 1000 °C. Six types of defects were identified, i.e., extrinsic stacking faults, microprecipitates on or near the plane of the stacking fault, particles, generator‐dislocation loops, small strain centers, and a ’’new’’ precipitate structure, whose characterization is emphasized in this study. It was concluded that the new defect structure places the surrounding matrix in a compressive condition, i.e., it is of interstitial type. The possibility of carbon or oxygen being responsible for this precipitate structure was discussed. It was suggested that future electron energy‐loss spectroscopy work should, in principle, distinguish between an oxide and a carbide.

ISSN:0021-8979    

DOI:10.1063/1.325392

出版商:AIP    

年代:1978

数据来源: AIP

摘要:

An analysis is given of the x‐ray diffraction line broadening caused by concentration variations occurring within coherently diffracting crystallites. The sine Fourier coefficients of the line profile are employed. The method is applied to sintered and unsintered AuPt alloy catalysts. In the interior of the crystallites of the sintered specimen, the concentration maintained a constant value which agreed very well with the average composition. The surface region was enriched by gold, in agreement with experimental results from other methods and with theoretical predictions. Concentration variations in the unsintered specimen were much larger. The crystallites consisted of a platinum‐rich nucleus surrounded by a gold‐rich shell as may be expected from the preparation method (chemical reduction of platinum and gold ions in solution). In addition, a method for correction of the ’’hook’’ effect in line profile analysis is suggested.

ISSN:0021-8979    

DOI:10.1063/1.325393

出版商:AIP    

年代:1978

数据来源: AIP