摘要:

Experiments have been performed to study the propagation of an intense relativistic electron beam [input (&ngr;/&ggr;)≃2.5] in a drift tube filled with either neutral gas or preexisting plasma. Beam front velocity, magnetic neutralization, and energy transport are measured. Results for propagation into neutral gas confirm previous work. Propagation into a preionized background is characterized by nearly complete magnetic neutralization. However, the beam front velocity never exceeds 60% of its potential value, and a two‐stream instability is suggested as a possible explanation. The efficiency of beam energy propagation through the plasma is found to depend on the neutral background density, and a possible cause of this effect is suggested. Hollow beam profiles are observed under suitable conditions.

ISSN:0021-8979    

DOI:10.1063/1.1660459

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The 3‐tube symmetrical electrostatic unipotential lens was analyzed in great detail. Starting from a mathematical expression for the axial potential distribution, the cardinal points and coefficients of spherical and chromatic aberration were obtained with the help of a digital computer. Values forS/R, the separation between the tubes, were taken equal to 0.25, 0.375, 0.5, 0.75, and 1.00 and the length of the central electrode,L/R, was varied from 0.25 to 2.00 in steps of 0.25. It was shown that the accelerating lens has a lower coefficient of spherical aberration than the decelerating lens. A comparison was made between the data obtained using the mathematical model and the relaxation technique. The agreement was excellent.

ISSN:0021-8979    

DOI:10.1063/1.1660460

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Tungsten wires, 20–80 mm in length and 0.05–0.20 mm in diameter, were exploded in vacuum (1×10−5Torr) by means of a 0.4‐&mgr;F capacitor charged to 30 kV. A pronounced dip and peak in thedI/dtand voltage oscillograms, respectively, corresponded in time to each other and to an ultrafast x‐ray pulse. Calculations showed that the wire temperature reached melting point at the same moment. It was found that for a constant wire length, the magnitude of the voltage peak attained a weak maximum at a diameter of about 0.10 mm. In addition, for a constant wire diameter, this magnitude increased with increasing wire length. The x‐ray output was measured by a scintillator‐photomultiplier (PM) combination and an x‐ray film. The PM pulses (half‐width 20 nsec) depended only slightly on the wire parameters. Their peak values plotted against wire length and diameter showed the same feature as the peak voltage diagram did. Furthermore, studies of the film blackening indicated that various wire dimensions gave rise to various rates of blackening. It was also found that the higher the voltage peak (or PM pulse) the more dense the blackening on the film. A model, based on Ohmic heating of the wire with accompanying thermionic electron emission, which qualitatively explains the generation of the ultrafast x‐ray pulse, is presented. In addition, it was found that the electron currents which gave rise to the x‐ray burst were up to 10 times higher than those predicted by Richardson's formula.

ISSN:0021-8979    

DOI:10.1063/1.1660461

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Long electrical spark discharges have been directed through air along predetermined paths defined by a concentrated laser beam. At apparent optical power densities of 50–100 giga W/cm2, 35‐nsec half‐width laser pulses at 10 600 Å have been effective in channeling streamer discharges from a 350‐kV positively charged electrode for distances up to 28 cm in a total streamer length of 71 cm. The averageEfield required to obtain a discharge between electrodes was reduced from 7.3 to 5.5 kV/cm with the laser powers employed.

ISSN:0021-8979    

DOI:10.1063/1.1660462

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The high‐frequency shot noise of Pt&sngbnd;GaAs Schottky barrier diodes has been measured at 60 MHz for both guard‐ring and non‐guard‐ring structures. The measured equivalent noise temperature agrees well with the shot‐noise theory of semiconductor junctions at relatively high forward currents where the metal‐semiconductor interface effects can be neglected. At low levels of forward bias the measured noise is greater than that predicted by conventional theory. A measured excess‐leakage conductance showing full thermal noise is added to the model and accounts for part of the difference between theory and experiment. The requirements for low‐noise diodes are outlined.

ISSN:0021-8979    

DOI:10.1063/1.1660463

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The annealing at atmospheric pressure of polymethyl methacrylate (PMMA) densified in various pressure‐temperature‐time cycles has been examined in the temperature range 25°–90°C. PMMA densified in the glassy state was found to be less stable than that densified by cooling through the glass transition under pressure, in agreement with previous results on inorganic glasses. Moreover, the relative stability of densified PMMA depended on the sample residence time in the glass transition region at pressure. In the one case where an energy spectrum for annealing could be calculated, the data indicated a multiplicity of molecular processes with activation energies in the range 14–19 kcal/mole.

ISSN:0021-8979    

DOI:10.1063/1.1660464

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Shock wave experiments with inclined prisms have been used to determine the compression for shocks propagated along three axes of single‐crystal Si. At stresses less than about 200 kbar, the shocks along the⟨110⟩and⟨1111⟩axes exhibited four wave structures. The first three waves were taken to be related to the Hugoniot Elastic Limit and to two succeeding polymorphic phase transitions. Scatter in the magnitude of the HEL along the⟨100⟩axis may indicate mixing between the elastic wave and the first phase transition wave.

ISSN:0021-8979    

DOI:10.1063/1.1660465

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The Weizsa¨cker‐Williams method is used to calculate the gain due to the induced emission of radiation into a single electromagnetic mode parallel to the motion of a relativistic electron through a periodic transverse dc magnetic field. Finite gain is available from the far‐infrared through the visible region raising the possibility of continuously tunable amplifiers and oscillators at these frequencies with the further possibility of partially coherent radiation sources in the ultraviolet and x‐ray regions to beyond 10 keV. Several numerical examples are considered.

ISSN:0021-8979    

DOI:10.1063/1.1660466

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Spectroscopic measurements have indicated that laser lines obtained with a xenon ``pinch'' laser arise from transitions in triply ionized species. Measurements of the time‐dependent electron density and of the plasma dynamics indicate that this is indeed the case.

ISSN:0021-8979    

DOI:10.1063/1.1660467

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

This paper describes the results of an investigation of the interrelated physical parameters that determine laser threshold for three organic dyes: rhodamine B, rhodamine 6G, and fluorescein. Not only are these dyes (rhodamine 6G in particular) among the most widely used and important laser dyes, but insights into the threshold condition derived from our studies of these particular dyes should be generally applicable to future dye laser research. Our detailed characterization of threshold for these dyes and our novel insights result from the use of triplet state spectral data in a thorough and quantitative way for the first time, coupled with the physically realistic approximation that the triplet population is proportional to the singlet excited state population. For the case of self‐tuning of the laser emission wavelength, solutions to the threshold equations are presented that establish relations between all the parameters affecting lasing: critical inversion, emission wavelength, extrinsic losses, dye concentration, length of active medium, and triplet to excited singlet population ratio. For reasonably high extrinsic cavity losses, the critical inversion is a simple power law function of extrinsic loss, and the emission wavelength is self‐tuned in such a manner that the ratio of extrinsic losses to intrinsic singlet absorption losses is substantially constant. There exists a region of low, but still physically realizable, cavity losses, where laser action is determined exclusively by intrinsic characteristics of the dyes (triplet absorption, singlet emission, and absorption). Asymptotic limits on the maximum wavelength of emission and on the minimum critical inversion appear in this region, beyond which there is no point in reducing extrinsic cavity losses. The fraction of triplet state molecules also determines long wavelength cutoffs for an externally tuned laser. Methods are outlined for measuring the ratio of triplet state to excited singlet state populations for dyes with known triplet absorption spectra and for determining semiquantitatively the triplet effects in dyes where this information is not known. A final interesting conclusion is that molecular modifications of rhodamine 6G cannot be expected to improve its threshold characteristics as a laser dye by much more than a factor of 4.

ISSN:0021-8979    

DOI:10.1063/1.1660468

出版商:AIP    

年代:1971

数据来源: AIP