Search

We present an improved numerical model for anode initiated electron avalanche breakdown of a dielectric-vacuum interface. An initiating event occurs in the high field region near the anode triple junction to start the first avalanche. The first avalanche grows and then stops, and a new one starts closer to the cathode. The breakdown process is modeled as a sequence of avalanches progressing from the anode to the cathode.

We propose a modification of a vacuum-insulator-electrode geometry that is commonly found in particle accelerators to reduce the electric field near the anode triple junction and reduce dielectric flashover.

We have recently initiated an investigation of electron emission from ferroelectric cathodes. Our experimental apparatus consisted of an electron diode and a 250 kV, 12 Ω, 70 ns pulsed high voltage power source. A planar triode modulator driven by a synthesized waveform generator initiates the polarization inversion and allows inversion pulse tailoring. The pulsed high voltage power source is capable of delivering two high voltage pulses within 50 μs of each other and is capable of operating at a sustained repetition rate of 5 Hz. Our initial measurements indicate that emission current densities above the Child-Langmuir space charge limit, JCL, are possible. We explain this effect to be based on a non-zero initial energy of the emitted electrons. We also determined that this effect is strongly coupled to relative timing between the inversion pulse and application of the main anode-cathode pulse. We also have initiated brightness measurements of the emitted beam and estimate a preliminary lower bound to be on the order of 109 A/m2rad2. As in our previous measurements at this Laboratory, we performed the measurement using a pepper pot technique. Beamlet profiles are recorded with a fast phosphor and gated cameras. We describe our apparatus and preliminary measurements.

High voltage insulation is one of the main areas of pulsed power research and development since the surface of an insulator exposed to vacuum can fail electrically at an applied field more than an order or magnitude below the bulk dielectric strength of the insulator. This is troublesome for applications where high voltage conditioning of the insulator and electrodes is not practical and where relatively long pulses, on the order of several microseconds, are required. Here we give a summary of our approach to modeling and simulation efforts and experimental investigations for understanding flashover mechanism. The computational work is comprised of both filed and particle-in-cell modeling with state-of-the-art commercial codes. Experiments were performed in using an available 100-kV, 10-μs pulse generator and vacuum chamber. The initial experiments were done with polyethylene insulator material in the shape of a truncated cone cut at +45° angle between flat electrodes with a gap of 1.0 cm. The insulator was sized so there were no flashovers or breakdowns under nominal operating conditions. Insulator flashover or gap closure was induced by introducing a plasma source, a tuft of velvet, in proximity to the insulator or electrode.

High voltage insulation is one of the main areas of pulsed power research and development, and dielectric breakdown is usually the limiting factor in attaining the highest possible performance in pulsed power devices. For many applications the delivery of pulsed power into a vacuum region is the most critical aspect of operation. The surface of an insulator exposed to vacuum can fail electrically at an applied field more than an order or magnitude below the bulk dielectric strength of the insulator. This mode of breakdown, called surface flashover, imposes serious limitations on the power flow into a vacuum region. This is especially troublesome for applications where high voltage conditioning of the insulator and electrodes is not practical and for applications where relatively long pulses, on the order of several microseconds, are required. The goal of this project is to establish a sound fundamental understanding of the mechanisms that lead to surface flashover, and then evaluate the most promising techniques to improve vacuum insulators and enable high voltage operation at stress levels near the intrinsic bulk breakdown limits of the material. The approach we proposed and followed was to develop this understanding through a combination of theoretical and computation methods coupled with experiments to validate and quantify expected behaviors. In this report we summarize our modeling and simulation efforts, theoretical studies, and experimental investigations. The computational work began by exploring the limits of commercially available codes and demonstrating methods to examine field enhancements and defect mechanisms at microscopic levels. Plasma simulations with particle codes used in conjunction with circuit models of the experimental apparatus enabled comparisons with experimental measurements. The large scale plasma (LSP) particle-in-cell (PIC) code was run on multiprocessor platforms and used to simulate expanding plasma conditions in vacuum gap regions. Algorithms were incorporated into LSP to handle secondary electron emission from dielectric materials to enable detailed simulations of flashover phenomenon. Theoretical studies were focused on explaining a possible mechanism for anode initiated surface flashover that involves an electron avalanche process starting near the anode, not a mechanism involving bulk dielectric breakdown. Experiments were performed in Engineering's Pulsed Power Lab using an available 100-kV, 10-{micro}s pulse generator and vacuum chamber. The initial experiments were done with polyethylene insulator material in the shape of a truncated cone cut at +45{sup o} angle between flat electrodes with a gap of 1.0 cm. The insulator was sized so there were no flashovers or breakdowns under nominal operating conditions. Insulator flashover or gap closure was induced by introducing a plasma source, a tuft of velvet, in proximity to the insulator or electrode.

We have recently initiated an investigation of electron emission from ferroelectric cathodes. Our experimental apparatus consisted of an electron diode and a 250 kV, 12 ohm, 70 ns pulsed high voltage power source. A planar triode modulator driven by a synthesized waveform generator initiates the polarization inversion and allows inversion pulse tailoring. our initial measurements indicate that emission current densities above the Child-Langmuir Space Charge Limit, JcL, are possible. We explain this effect to be based on a non-zero initial energy of the emitted electrons. We also determined that this effect is strongly coupled to relative timing between the inversion pulse and application of the main anode-cathode pulse. We also have initiated brightness measurements of the emitted beam and estimate a preliminar lower bound to be on the order of 109 A/m2-rad2 for currents close to JcL and factor of two less at currents over 4JcL. We describe our apparatus and preliminary measurements.

Insulators composed of finely spaced alternating layers of dielectric and metal are thought to minimize secondary emission avalanche (SEA) growth. Most data to date was taken with small samples (order 10 cm/sup 2/ area) in the absence of an ion or electron beam. We have begun long pulse (>1 /spl mu/s) high voltage testing of small hard seal samples. Further, we have performed short pulse (20 ns) high voltage testing of moderate scale bonded samples (order 100 cm/sup 2/ area) in the presence of a 1 kA electron beam. Results thus far indicate a 1.0 to 4.0 increase in the breakdown electric field stress is possible with this technology.

As part of the Dual Axis Radiography Hydrotest Facility, Phase II (DARHT II) Multipulse Bremsstrahlung Target effort, we have been performing an investigation of (1) the possible adverse effects of backstreaming ion emission from the bremsstrahlung converter target and (2) the hydrodynamic behavior of the target after the electron beam interaction. Theory predictions show that the first effect would primarily be manifested in the static focusing system as a rapidly varying X-ray spot. From experiments performed on ETA-II, we have shown that the first effect is not strongly present when the beam initially interacts with the target. Electron beam pulses delivered to the target after formation of a plasma are strongly affected, however. Secondly, we have performed measurements of the time varying target density after disassembly was initiated by the electron beam. The measurements presented show that the target density as a function of time compares favorably with our LASNEX models.

The DARHT accelerator will deliver several intense relativistic electron beam pulses to an x-ray conversion target during a few microseconds. Plasma from the target can cause a partial neutralization of the vacuum self-Er field resulting in an unacceptably large beam radius at the target. The Livermore group has been developing barrier foils to block the plasma from moving upstream. Positive ions accelerated upstream from the foil in the self-Ez field during a single pulse could defocus the beam. In May, 2001 LANL used a sensitive ''two foil'' experiment to search for such effects. They measured significant time dependent effects using conducting foils (1). In January, 2002, the Livermore group repeated the experiment using the ETA II accelerator. We expected to see similar effects and planned to collect data that we could model. We saw no significant effect from conducting foils unless the beam radius was small enough to damage the foil. The reason for the different results has not been explained and is still being investigated. Possibilities have to do with the longer pulse length at LANL, (60 ns compared to 40) or with the higher energy at LANL, (20 Mev compared to 5.7) We also did some tests on more » dielectric targets where there is a strong effect to test our techniques. « less

Ions extracted from a solid surface or plasma by impact of an high intensity and high current electron beam can partially neutralize the beam space charge and change the focusing system. We have investigated ion emission computationally and experimentally. By matching PIC simulation results with available experimental data, our finding suggests that if a mix of ion species is available at the emitting surface, protons dominate the backstreaming ion effects, and that, unless there is surface flashover, ion emission is source limited. We have also investigated mitigation, such as e‐beam cleaning, laser cleaning and ion trapping with a foil barrier. The temporal behavior of beam spot size with a foil barrier and a focusing scheme to improve foil barrier performance are discussed.

Desorption and subsequent ionization of the monolayers from the vacuum wall of an accelerator system can have a detrimental effect on the performance of the beam transport system. Ions extracted from the resultant plasma neutralize the spacecharge and dynamically perturb the net focusing forces within the beam. To study the effect, a transparent first foil, presumably with contaminants on the surface, intercepts the beam. Placing an imaging foil tens of centimeters downstream from the first foil allows observation of minor fluxuations in the envelope. Using conducting foil targets, we see no effect unless the beam radius is small enough to damage the foil. Non-conducting foils produce a strong effect.

We have investigated the properties of a magnetically delayed, low-pressure gas discharge switch. We performed measurements of the closure and recovery properties of the switch; performed quantitative erosion measurements; and observed the onset of x-ray production in order to compare switch properties with and without delay. Further, we performed qualitative optical measurements of transition line spectra to correlate our electrical recovery measurements with plasma deionization.

A new and relatively simple method has been developed to focus and guide electron beams without the use of a magnetic field. The scheme relies on the electrostatic charging of a highly resistive wire in the presence of a beam. The beam is then strongly guided and focused by the oppositely charged wire. In addition, the highly anharmonic nature of the wire potential leads to rapid phase-mix damping of transverse beam displacements and radial pulsations.

BS>The injection scheme described is of the type in which trapping is accomplished within a steady confining magnetic field configuration by means of an increase in the e/m of the particles. A beam of neutral deuterium atoms is sent into the mirror machine normal to the field line and in the symmetry plane. A fraction of the atoms are trapped in thee field as a result of being ionized by collisions with either cold neutral atoms or trapped hot ion. (M.H.R.)

Positrons emitted isotropically from a uniform volume source were contained in a static magnetic-mirror machine for a mean containment time greater than 10 sec, which corresponds to more than 10/sup 10/ cyclotron periods. This is of practical importance in various research fields such as studies of controlled fusion and studies of the Van Allen radiation belt. A description is given of the apparatus which consists of a magnetic-mirror machine having coaxial 60-in.-mean-diameter coils with 64 in. between coil centers, which are located adjacent to the cyclotron used to produce Ne/sup 13/. The present results agree with the fast filling'' experiment. They appear to agree with, and to extend, the conclusions of an experiment in which ionization currents created by 18-kev electrons in a mirror machine were measured. (B.O.G.)

We describe a simple electrostatic focusing method which efficiently damps transverse motion of a high intensity (both high current and high particle energy) electron beam. The principle is to create an electrostatic central force anharmonic potential field which phase mix damps transverse beam motion into increased beam emittance. The non-linear electrostatic field is created by a very resistive wire (L/ R time ~2 ns) strung axially down the accelerator vacuum pipe. Positive image charge equal to ~1/2 the negative beam charge is induced onto the wire. Since the beam self fields cancel to within 1/?2, the resultant wire electric field dominates. Experimental results showing the guiding, focusing and damping of transverse motion will be presented.

Analytic solutions are given for the pair of differential equations obtained from classical theory which express the time rate of change of the angle between the momentum vector and the magnetic field vector and the time rate of change of the energy of the electron. (auth)

The idea of using saturable reactors as the basis of high power pulse generators is not a new concept, but there have been few recent applications of this technology. Here the principle of magnetic pulse generation is briefly described and some of the basic guidelines used to design these circuits are discussed. A demonstration of the principles by a small scale pulse amplifier is presented, and finally there is an extrapolation to a large scale system.

The operational characteristics of the 10 kA, 60 ns, 2.5 MeV ATA injector are presented. Studies of beam emittance, beam profile, rf spectrum and other aspects of beam dynamics were performed, as was a detailed study of the operation and interaction of the plasma cathode and the extraction grid.

This report describes experiments in beam propagation with the ETA beam during 1982 following accelerator cavity modifications which allowed a maximum beam current of up to 8 kA at the entrance to the propagation tank. A prominent new feature of the propagation in high pressure gas was an enhancement of the net current as the beam propagated. In some cases this enhanced current was nearly double the injected beam current. The strong current enhancement was associated with strong transverse hose motion of the beam. The absence of microwave emissions in the range from 6.6 GHz to 31 GHz indicates that this current enhancement is not due to a two-stream instability.

Measurements of the radial expansion from gas scattering of a low ν/γ relativistic electron beam are presented. The measured current density profiles approach a Bennett shape, as predicted theoretically, and the rate of expansion of the beam radius with distance is in good agreement with the predictions of an envelope equation. locus has been determined. It terminates at zero temperature at