摘要:

Treated is the steady state of a beam interacting with the plasma which it produces in a low‐density gas and in which magnetic self‐focusing must be considered. Self‐focusing is assumed to exist when the second time derivative of the moment of inertia of the beam electrons is negative and when that of the ions is neglected. Predictions are related to experiments with 10–15‐keV beam electron energies, beam currents of several hundred milliamps, and system pressures of 10−4Torr. Simultaneous pinching is observed with approximately equal radial forces on the beam electrons due to overneutralization and due to magnetic self‐focusing.

ISSN:0021-8979    

DOI:10.1063/1.1662800

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The results of an experimental study of the dynamic response of aluminum exposed to a high‐fluence low‐energy pulsed electron beam are presented and compared with calculations based on two equations‐of‐state (EOS) models for metals in the melt‐dominated regime. The first model, the widely used classical Mie‐Gru¨neisen EOS, is a simplified scheme in which no thermodynamic distinction is made between solid and liquid phases. The second model ‐ the so‐called GRAY EOS recently developed at Lawrence Livermore Laboratories ‐ is a three‐phase EOS which provides a more detailed and thermodynamically complete description of metals in the melt region. The experimental studies consisted of measuring the dynamic response of 6061 aluminum to a pulsed 185‐keV electron beam for fluences ranging from 15 to 50 cal/cm2, resulting in peak energy depositions of 450–1500 cal/g. The experimental condition produced a mass blow‐off which was dominated by mass in the melt state. The response was characterized by simultaneous (for each electron pulse) measurements of impulse, transmitted stress, blow‐off velocities, and mass loss. The experimental results were compared with the predicted response to the rapid heating calculated with a hydrodynamic computer code (RIP) in conjunction with the two EOS models. The essential result of the comparison was that after the onset of melting the simple Mie‐Gru¨neisen model fails to give reasonable agreement with experiment, whereas the GRAY EOS provides acceptable agreement with the experimental results. In particular, the prediction of GRAY that the thermal pressure in the melt regime can be described by an effective ``average'' temperature‐dependent Gru¨neisen parameter, which at the completion of melting is approximately 40% higher than the solid Gru¨neisen value, is clearly confirmed by the experiments. The results demonstrate the necessity of adequately taking melting into account in modeling material response to pulsed energy deposition.

ISSN:0021-8979    

DOI:10.1063/1.1662801

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Fission fragment tracks produced on prepared surfaces of uranium dioxide were observed by replica electron microscopy. Several statistical analyses and measurements were performed on the observed tracks to be compared with theoretical studies of a fission fragment passing through a stopping medium. A temperature distribution around the fragment is evaluated by applying the thermal diffusion model to the particle energy losses. Since the temperatures attained at the surface could not explain any track formation, a shock wave is shown to be a possible cause by considering processes due to thermal pressures engendered by the very hot core surrounding the fragment path.

ISSN:0021-8979    

DOI:10.1063/1.1662802

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A general formulation of the displacements induced by plastic distortion, &bgr;*ji, with small intertia forces in viscoelastic materials is derived. The elastic fields of a twist disclination in viscoelastic materials are then obtained from this formulation. Finally, the fading property of the strength of the disclination is determined for the disclination in the standard linear viscoelastic materials, and the variations of the elastic fields of the disclination are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1662803

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The hypersonic elastic constants and the ratio of the elasto‐optic coefficients for two lead silicate optical‐grade glasses and for a zinc silicate optical‐grade glass have been obtained by observing the Brillouin scattering of 6328‐Å laser light over the temperature range 24–107 °C. Brillouin scattering at 90° from both the transverse and longitudinal acoustic modes has been observed. The values of the hypersonic Young's modulus and shear modulus agree with low‐frequency values and show a slight temperature dependence. Poisson's ratio and the ratio of the elasto‐optic coefficients are found to be independent of temperature over the temperature range used. There is no evidence of dispersion from audio to hypersonic frequencies.

ISSN:0021-8979    

DOI:10.1063/1.1662804

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

For cylindrical conductors made of thin copper and tungsten and heated by the electrical pulse from a high‐voltage condenser bank which had inductance and resistance much lower than those of the conductors, the following items were studied as functions of the diameter and the applied voltage: (i) the variations in time of the temperature and resistance of the conductors up to the time they reach their melting points; (ii) the dips on the recordings of the current derivative and the voltage across the conductors which appear near their melting points; (iii) the first current‐pulse crest and the time to the current‐pulse crest, as well as the influence of the ambient pressure on the current crest and on the time to the current crest; and (iv) the events occurring at the tail of the first current pulse and the beginning of the dwell time, which may have a decisive effect on the type of restrike mechanism which will follow the conductor explosion.

ISSN:0021-8979    

DOI:10.1063/1.1662805

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The article is concerned with determination of the type of modes in wide injection lasers and in particular with identification of the normal modes in the profile of the beam in the plane of the junction. In a cavity 70 &mgr;m wide × 120 &mgr;m long, with cleaved facets and sawed sides, the modes are found to be those of a rectangular parallelopiped of the same geometry. The smooth lateral profile of the radiated beam is broken into a series of sharp lobes by inserting a low‐resolution high‐intensity spectrometer between angle scanner and detector. The spectrometer resolution of a fraction of the separation of successive longitudinal mode groups rejects most of the off‐axis modes. Longitudinal mode numbers are unambiguously assigned to the lobes in the beam profile, out to 70° from the facet normal, and the dependence of lobe position on longitudinal number identifies the geometry of the modes. Consideration of the lobe widths and irregularities reveals that the coherence width of the laser modes is essentially its geometric width. The experimental value of the index of refraction of the propagating modes is found to be 3.54±0.07 and its normalized dispersion is 0.30±0.06.

ISSN:0021-8979    

DOI:10.1063/1.1662806

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The effect of space charge upon unipolar drift currents in uniformly doped semiconductor regions is studied. The geometry is assumed one dimensional and diffusion is neglected. Both majority‐ and minority‐carrier injection is considered. First, the carrier mobility is taken to be constant. The current and voltage are expressed as functions of the injection level and the normalized carrier transit time, not regarding the actual boundary condition (at the injecting electrode). A space‐charge factor which indicates the nonuniformity of the electric field is also defined and computed. Then, various physical situations, such as the space‐charge‐limited current diode, the shallow‐barrier contact diode, the Schottky emission diode, and the emitter‐current‐limited injection can be studied. Universal diagrams for majority‐ and minority‐carrier injection, respectively, allow evaluation of the space‐charge effects and calculation of theJ‐Vdependence by taking into account the proper boundary condition. Finally, an arbitrary velocity‐field dependence and the possibility of a negative conductance on the static characteristic is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1662807

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A simple Rogowski electrode configuration with capacitively coupled arcs on the sides of the discharge is described. With this configuration the authors have succeeded in dumping up to 500 J/1 into a gas mixture with as little as 41% He. The resulting small‐signal gain characteristics and discharge current waveform are described. A 3‐m oscillator has yielded output power > 1 GW and output energy of 125 J.

ISSN:0021-8979    

DOI:10.1063/1.1662808

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The room‐temperature surface mobility of free carriers in metal‐oxide‐semiconductor systems has not been satisfactorily explained by treating phonons as the only important scattering mechanism. The present work shows that, in addition to phonon scattering, both Coulombic and surface‐roughness scattering must be taken into account. Experimental results are presented showing that, contrary to the prediction of phonon scattering, surface mobility is process dependent, and that a strong correlation is observed between the detailed mobility behavior and the surface‐state density and the degree of surface roughness. A theoretical calculation of surface mobility, based on the combined effect of scattering due to phonons, charged centers, and surface roughness, is also presented.

ISSN:0021-8979    

DOI:10.1063/1.1662809

出版商:AIP    

年代:1973

数据来源: AIP