[en] The PBFA II vacuum insulator was originally designed for optimum coupling to a proton ion diode with minimum inductance. In July 1983 it was decided that lithium ions at 30 MeV would be the baseline for PBFA II. This requires the use of Plasma Opening Switches (POS) and vacuum inductor to reach 30 MV. To achieve this, the vacuum magnetically insulated transmission lines had to be redesigned as an inductive energy store. To gain optimum coupling to this vacuum inductor, the output impedance of the water section was increased by the use of a water-dielectric transformer. The calculations leading to the final design will be discussed

[en] Re-examination of surface flashover results of many authors, and some recent work of the author, has lead to the conclusion that insulator flashover, from vacuum (10^-6 torr) to atmospheric pressure, is a local high pressure phenomenon. Voltages ranged from dc to microsecond pulses. Desorption of adsorbed atoms and molecules occurs prior to flashover and these species make up the ambient in which the gaseous breakdown forms. Examination of the insulator surface after flashover shows evidence of the initial electron 'hop' from the triple point (insulator-electrode-gas/vacuum). Field strengths of various insulators appear to lie in a small range from 18-80 kV/cm for various gap spacings from about 7 x 10^-2- 1 cm. Velocities of propagation of the ionization wave fronts, whether on an insulator or in gas, range from about 10⁵ to 10⁷ m/sec

[en] Desorbed gases from insulator surfaces may be much more important in flashover initiation than properties of the dielectric surface. Ambient neutral densities, immediately above the insulator surface, in the initial stages of flashover have been calculated to lie in the range from 2 x 10¹⁷/cm³ to 2 x 10²¹/cm³ with a mean of about 1 x 10¹⁹/cm³, whether flashover occurred on surfaces in air at atmospheric pressure or in vacuum as low as 10^-6 Torr. The evidence presented indicates that, for dc and microsecond pulse voltages, surface flashover is a local, relatively high-pressure phenomenon. Comparison of field strengths of surface flashover for various insulating materials perpendicular to the electrodes, operating pressures, and voltage waveforms appears to indicate that flashover values range well within an order of magnitude (approx.18--80kV/cm). A filamentary nature of the flashover discharge is shown and is consistent with gas breakdown phenomena. Ionization wave front velocities are examined and these compare favorably with those obtained from gaseous breakdown studies

No abstract available

[en] There has been considerable interest over the last few years in high-voltage vacuum devices for pulsed power applications in inertial confinement fusion and weapons related areas. This paper describes both the microparticles created in vacuum diodes and the impact craters that are formed when these microparticles collide at high and hypervelocities with the diode components. Particles formed from molten electrodes can have field enhancement factors greatly above that for a spherical particle on the electrode surface. This enhancement of the local electric field on the particles can lead to the high velocities encountered. Velocities of the particles are calculated to be in the range 10⁴-10⁶ m/s, and these compare favorably with those found in the literature. The presence of microparticles in high-voltage diodes can lead to lower breakdown voltages and premature degradation of insulator and other diode components

[en] Some aspects of plasma opening switch flashboards are examined experimentally. Ion velocities ≅ 8 x 10/sup 6/ cm/s are observed with flux measurements in the range 10-60 A/cm/sup 2/. The effect of a 80% transparent screen placed between the flashboard and the Magnetically-Insulated Transmission Line (MITL) on the ion velocity and flux is examined. The velocity was unaffected, whereas the flux was reduced considerably. Ion velocity and flux were also examined in terms of the flashboard driving potential in the range 25-40 kilovolts. The driving force behind fast ion velocities (≅10/sup 7/ cm/s) from flashboards for plasma opening switches is examined

[en] The author's discuss the effects of high and hypervelocity microparticles in magnetically-insulated transmission lines (MITLs) and how they may be a possible source for ion production near the anode in early stages of the voltage pulse, and current carriers during and after the power pulse, resulting in power flow losses. Early losses in the voltage pulse, due to microparticles, are estimated to be approximately 0.3 mA/cm/sup 2/. Blistering of the electrode surface, thought to be due to H/sup -/ bombardment, was also observed and appears to be consistent with losses due to negative ions previously reported by one of the authors

[en] We discuss the effects of high-velocity and hypervelocity microparticles in the magnetically insulated transmission lines of multiterawatt accelerators used for particle beam fusion and radiation effects simulation. These microparticles may be a possible source for plasma production near the anode and cathode in early stages of the voltage pulse, and current carriers during and after the power pulse, resulting in power flow losses. Losses in the current pulse, due to microparticles, are estimated to be approximately 12 mA/cm² (0.3 kA) as a lower limit, and --0.3 A/cm² (7.2 kA) for microparticle initiated, anode plasma positive ion transport. We have calculated the velocities reached by these microparticles and the effects on them of Van der Waals forces. Field emission from the particles and their effects on cathode and anode plasma formation have been examined. Particle collision with the electrodes is also examined in terms of plasma production, as in the electron deposition in the particles in transit across the anode-cathode gap. Blistering of the electrode surface, thought to be due to H^- bombardment was also observed and appears to be consistent with losses due to negative ions previously reported by J. P. VanDevender, R. W. Stinnett, and R. J. Anderson [App. Phys. Lett. 38, 229 (1981)]

[en] The POS system on PBFA II is unique because of its size and the voltage at which it operates. The PBFA II magnetically-insulated vacuum power feed is a two-sided conical structure which delivers one-half of its current through each side. The entire POS system (four flashboards and four drivers) on PBFA II is contained inside the anode, under vacuum. Currents of 4-5 MA have been conducted for over 50 ns by the PBFA II POS system. The most recent results from the PBFA II POS system tests are presented

[en] Hydrogen has been an integral atmospheric component of sealed electrical contacts for decades because of its effect on reliability. It is well known that hydrogen is a needed component to prevent high contact resistance due to carbon-spot formation. This hydrogen benefit has been attributed to hydrogenation of hydrocarbons (HC), enhanced volatilization of previously deposited carbon, enhanced catalytic activity, and thermal cooling. By a variety of physical and chemical laboratory techniques, this paper shows that the hydrogen is not itself directly active. However, some of the hydrogen is converted to water during the glass sealing process. It is this water which plays the active role in the process of quenching carbon formation from hydrocarbon impurities and also acts as a diluent of HC's adsorbed on the contact surface and in the arcing volume. This limits the carbon accumulation on the contacts and increases lifetime reliability