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.

High-gradient-copper linac accelerator structures capable of surface field gradients in excess of 100 MV1m exist, but present klystrons can not provide sufficient peak power to obtain these field gradients. In earlier work, it has been suggested that the microwave energy-compression technique could be applied to this problem. In addition, it offers an approach for trading duty cycle for beam energy at a fixed total input energy. Here we discuss further some aspects of this technique as it applies to the accelerator problem.

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.

The model assumes that an “initiating event” results in positive ions on the surface near the anode and reverses the direction of the normal component of electric field so that electrons in vacuum are attracted to the dielectric locally. A sequence of surface electron avalanches progresses in steps from the anode to the cathode. For 200kV across 1cm, the spacing of avalanches is predicted to be about 13μm. The time for avalanches to step from the anode to the cathode is predicted to be about a nanosecond.

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.

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.

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.

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.

An experiment is underway to measure the increase in emittance of a 13.2 KeV, 50{mu}A, Kr{sup 84} beam and a 8.5 KeV, 20{mu}A, Xe{sup 131} beam in passing through the ATA 30{degree} bend beam director.'' The ratio of the non-linear term divided by the linear term in Kelvin Neil's equation of motion is calculated for the full scale rings of the proposed recirculator and for the model experiment. 1 tab.

The Advanced Test Accelerator (ATA) has solenoidal magnets to focus and guide the electron beam. The transverse position of the center of mass of the beam is measured at many positions along the accelerator using the wall current monitors. With the steering coils off, the beam drifts off axis a few cm in the length of the machine. The drift is independent of several parameters: field strength, beam current, and beam energy. Further measurements to locate the source of the misalignment will be discussed. 1 ref., 4 figs.

The electron beam size has been determined on the Advanced Test Accelerator (ATA) by intercepting the beam with a target and measuring the resulting x-ray intensity as a function of time as the target is moved through the beam. Several types of targets have been used. One is a tantalum rod which extends completely across the drift chamber. Another is a tungsten powder filled carbon crucible. Both of these probes are moved from shot to shot so that the x-ray signal intensity varies with probe position. A third is a larger tantalum disk which is inserted on beam axis to allow determining beam size on a one shot basis. The x-ray signals are detected with an MCP photomultiplier tube located at 90/sup 0/ to the beamline. It is sufficiently shielded to reject background x-rays and neutrons. The signals were digitized, recorded and later unfolded to produce plots of x-ray intensity versus probe position for several times during the pulse. The presumption that the x-ray intensity is proportional to beam current density is checked computationally. Details of the probe construction and PMT shielding, as well as sample measurements are given.

The first gas propagation experiment on ATA is planned to be conducted in a 1-foot diameter tank of up to 10 m length. The primary objectives are to measure beam parameters at injection to determine whether the desired beam conditioning is achieved, and to observe how such conditioned beams propagate in air and neon.

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.

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.