摘要:

The spatiotemporal development of electron and positive‐ion densities, electric field, and luminosity are calculated for electron‐pulse experiments in parallel‐plane gaps by numerically solving continuity equations together with Poisson's equation. Experimental coefficients for electron transport, cathode photoemission, and gas photoionization are used. The results show that anode‐ and cathode‐directed ionizing waves, or streamers, develop from the electron‐pulse‐initiated avalanche when it reaches midgap, and produce a weakly ionized plasma. Both filamentary plasmas and uniform plasmas that fill the interelectrode volume have been experimentally observed. Calculated and experimental streak photographs agree well in both cases. In filamentary discharges, a return ionizing wave also propagates from the anode when the anode‐directed wave arrives. The results show that electrons must be present ahead of the wave for cathode‐directed wave propagation. These electrons are supplied by photoionization, photoemission, or both depending on the experimental conditions.

ISSN:0021-8979    

DOI:10.1063/1.1663543

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The forces acting on a current‐carrying wire moving above a ground plane with discontinuities are investigated. Two types of discontinuities are considered: (i) a gap, and (ii) a step. In both cases, the forces on the wire are calculated and graphed for different wire locations. The relevance of these calculations to guideway discontinuities in magnetically levitated systems is pointed out.

ISSN:0021-8979    

DOI:10.1063/1.1663544

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The electrification of a system consisting of a metal and Teflon (polytetrafluoroethylene) has been investigated in vacuum under relative motion. A plane strip of Teflon was moved in relation to a metal electrode connected to an electrometer, which permitted the electrification charge to be continuously recorded. The electrification current was found to be closely proportional to the velocity, and it was practically independent of the radius of curvature of the tip of the electrode. The normal force was also varied, and the linear charge was found to be proportional to the square root of this force. Electrodes of Al, Au, Cu, Dural, Mg, Ni, and Pt were used, and the charge deposited was found to be linearly dependent on the metal work function, except for a large anomaly in the case of Cu. For a given metal the charge was shown to be closely proportional to the area swept out by the electrode on the insulator surface. The linear dependence of charge on metal work function and the short charging time (0.06 s) observed indicate that electrons from the metal fill empty surface states in the insulator up to a level corresponding to the Fermi level of the metal. The surface‐state density of Teflon was found to be 7×1010eV−1cm−2.

ISSN:0021-8979    

DOI:10.1063/1.1663545

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Further experimental results are presented concerning high conduction and switching in Al&sngbnd;SiO2&sngbnd;Si sandwich structures. The high‐conduction state is filamentary and is governed mainly by the voltage across the sandwich. It is relatively insensitive to variations in device geometry. Periodic current oscillations are observed for certain combinations of series load resistance and applied voltage. Once the high‐conduction state is achieved, it can be maintained at applied voltages below the one required to cause the transition but larger than a minimum voltage, which may be related to the minimum ionization energy in SiO2. Fluctuations are observed that depend on external circuit configuration, i.e., the load line, and on the ambient temperature. It is shown that a two‐branched current‐voltage characteristic can account for the observed behavior. A physical model is presented in an attempt to provide a qualitative explanation of the origin of this form of characteristic. Local damage is evident whenever the high‐conduction state is seen (even with a single 50‐nsec pulse). Electron microscopic analysis of a damaged region is presented.

ISSN:0021-8979    

DOI:10.1063/1.1663546

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The pressure on the molten surface of a vacuum‐arc cathode due to the recoil from outward‐directed ion jets is calculated for copper arcs. The calculation uses current density data and electrode force per unit current data from experiment. In addition, the force per unit current measured by Tanberg is shown to be consistent with a calculation of the same quantity based on the energy of outgoing ions measured by Davis and Miller and the so‐called saturated positive‐ion current measured by Kimblin. The ion recoil pressure is shown to be sufficient to remove molten metal from a cathode‐spot crater in time of the order of 25–250 nsec with a velocity of 2×103−2×104cm sec−1. It is shown that motion of molten metal in the cathode‐spot crater must be considered as a first‐order effect in rigorous calculations of surface temperature and heat flow in the metal in contact with the cathode‐spot plasma. It is suggested that the rapid removal of metal by the plasma pressure causes molten droplets to be ejected, as has been observed experimentally, and causes a preference for the cathode to operate, after the liquid is ejected, on the rim of the crater or nearby on the surface where hotter metal may exist due to liquid‐metal overflow and spatter.

ISSN:0021-8979    

DOI:10.1063/1.1663547

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The instability of an electromagnetic wave propagating obliquely to a counterstreaming plasma is investigated for the case in which the magnetostatic field is normal to the plane containing the wave vector and the stream direction. The stream direction normal to the wave vector is a singular case for which, if the magnetostatic field is less than a critical value, the instability occurs for wave numbers exceeding a minimum value, the maximum growth rate being attained for very large wave numbers. An increase in the magnetostatic field increases the minimum wave number and reduces the growth rate. For all other stream directions unstable waves exist between two limiting wave numbers for all values of the magnetostatic field. The maximum growth rate is found to be practically the same for all stream directions other than normal to the wave vector. An increase in the magnetostatic field reduces the range of wave numbers for which there is instability, causes the region of instability to shift to shorter wavelengths, and decreases the maximum growth rate. The dependence of the growth rate on the stream direction with respect to the wave vector and the validity of the quasistatic approximation in predicting the nature of this instability are also examined.

ISSN:0021-8979    

DOI:10.1063/1.1663548

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The hydrodynamic equations describing the average behavior of a one‐dimensionally expanding plasma heated by laser radiation are solved analytically. For the small parameter approximation, the hydrodynamic expansion depends on the parameter(16P/9Ml02)1/3&tgr;L, wherePis the laser power,Mis the plasma mass,l0is the initial plasma length, and &tgr;Lis the laser pulse duration. With &tgr;L=&tgr;ei, the electron ion relaxation time, it is possible to compute&phgr;max (≈5.6×10−6n04l03miT−9/2, wheremiis the plasma ion mass andTis the temperature), the maximum flux density below which the hydrodynamic expansion may be neglected. We find that a 5‐kJ 75‐nsneodymiumglass laser is capable of acheiving break‐even conditions for a 50% mixture of deuterium‐tritium gas.

ISSN:0021-8979    

DOI:10.1063/1.1663549

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

According to Cranberg's hypothesis, tiny aggregates of metallic matter called microparticles, emanating from the electrode surfaces, are responsible for the breakdown of a vacuum gap under high dc stress. Experiments are carried out using a scanning electron microscope to see whether such particles appear within the interelectrode gap prior to breakdown. It was observed that micron‐ and submicron‐sized particles are released in abundance well below the breakdown voltage. Investigations were continued to identify the microparticles with the electrode materials using an electron microprobe analyzer. It was found that the microparticles were composed of the electrode materials. Direct examination of the electrode surfaces, after application of the voltage, under the scanning electron microscope also showed features suggesting that the microparticles are released from the electrode surfaces.

ISSN:0021-8979    

DOI:10.1063/1.1663550

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The downstream energy and impurity content of the plasma produced by a pulsed crossed‐field source is investigated. With a positive center electrode and an optimum neutral gas loading, the source ionizes and accelerates 7×1017protons containing 140 J of energy to the end of a 2‐m‐long axial magnetic guide field. This energy is distributed equally between axial motion and gyromotion. The dominant acceleration method is the−&mgr;·∇Bforce resulting from a gradient in the axial magnetic field within the source. Impurities, in the form of C2+and O2+ions, account for about 10% of the plasma. Reversing the source polarity results in a lower energy efficiency and the appearance of Fe2+ions in the downstream plasma.

ISSN:0021-8979    

DOI:10.1063/1.1663551

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Secondary electron yields &dgr;PEdue to primary electrons of kilo‐electron‐volts are estimated for various metals by measuring total electron yields &dgr;Tin an UHV system and using a Monte Carlo calculation to derive &dgr;PEfrom &dgr;T. The dependences of &dgr;T/&dgr;PEas well as &dgr;PEon both the atomic number Z and primary energy are also discussed from a theoretical viewpoint based on the Monte Carlo technique for primary energies higher than a few keV and on a simple theory for lower primary energies.

ISSN:0021-8979    

DOI:10.1063/1.1663552

出版商:AIP    

年代:1974

数据来源: AIP