The Mini-G explosive pulsed power system is a two-stage helical-coaxial FCG that is geometrically a half-scale version of LLNL's FFT device. The generator is capable of delivering 60 MA currents and 10 MJ of energy to suitable inductive loads. The Mini-G is presently used in high-energy-density physics experiments that require efficient current delivery through a vacuum power flow region to the load. As with the FFT device, the Mini-G system requires a compact, high-voltage gas-to-vacuum insulator and low-inductance vacuum power flow channel to achieve high performance and maximum energy delivery. In designing the Mini-G system, we followed the successful approach used in developing the FFT device. This included shaping the electrodes and insulators to manage electric field enhancements, applying coatings to cathode surfaces to suppress electron field emission, introducing baffles to the power flow channel to block UV, and applying coatings to electrode surfaces to absorb UV. This paper describes the design of the Mini-G vacuum interface and power flow region, and results of modeling and simulations that were done to evaluate and optimize performance. Appropriate codes were used to examine electric field enhancements, magnetic insulation, flashover inhibition and UV ray tracing in the channel. In this paper, we also present results of laboratory testing on HV vacuum insulator materials and shapes, UV induced insulator flashover, along with measurements of HV thresholds for electron emission. We also report on UV reflectance data for some of the coatings considered. To date, there have been eight experiments performed using the Mini-G system. For the first two tests, the power flow channel had an extremely low vacuum inductance of 0.9 nH. On the second Mini-G test it appeared that a partial shorting occurred in the power flow channel, limiting full energy delivery to the load. The design was modified to reduce electrical stress, improve UV attenuation, and incorporate additional diagnostics. This increased the inductance of the power flow channel to 1.5 nH. On the third Mini-G test the partial shorting reoccurred and the new diagnostics (inner Bdot probe) helped to identify the location at the vacuum insulator surface - about 10% of total current of 41 MA was diverted into the short. Further design modifications were incorporated to decrease electrical stress across the insulator and reduce UV illumination of the insulator surface. This increased the inductance of the power flow channel to 1.9 nH. On subsequent Mini- G experiments full current delivery to the load has been achieved with no occurrence of shorting.

Summary form only given. Vacuum insulators are necessary components in many pulsed power systems. They provide mechanical support as well as separate vacuum and non-vacuum regions. Unfortunately, the surface of the insulator is often the weakest part of a system. Surface flashover can occur for electric field values much lower than bulk breakdown through the material. It is important to develop models that can be used to optimally design insulators under different operating conditions and reliably predict when flashover will occur. Although many theories and a lot of empirical data can be found in the literature, there are few, if any, good models available. In this presentation we will discuss results of FDTD-PIC simulations for initiating vacuum insulator flashover.Using the VORPAL code on Linux clusters at LLNL we have previously investigated many phenomena important to the initiation of insulator flashover. The physics investigated includes field distortion due to the dielectric, field emission, low energy secondary emission, insulator charging, and magnetic fields. We have recently included the effects of a thin gas layer next to the surface of the insulator. The electrons can ionize the gas if they have the right energy. Including these different effects in a self-consistent simulation leads to a better understanding of vacuum insulator flashover and closer to a predictive model.

The vacuum/dielectric interface of insulators is often the weakest part in high voltage and pulsed power systems. Surface flashover can occur for electric field values much lower than that of bulk breakdown through the material. Although much empirical data and many theories can be found in the literature, there are no models that can be used to optimally design insulators and reliably predict when flashover will occur. In this presentation we will discuss the results of a FDTD-PIC code that is being used to model physics phenomena common to many flashover theories. In order to simulate the initiation of vacuum insulator flashover, VORPAL is being used on the Linux clusters at LLNL. In we presented the results for implementing physics modules that included the effects of field distortion due to the dielectric, Fowler-Nordheim field emission, low energy secondary emission, insulator charging, and magnetic fields. We have extended our previous work to include a thin gas layer near the surface of the insulator. Electrons may cause ionization depending on their energies and the collision cross section of the gas. The inclusion of these physics effects leads to a more complete model and better understanding of vacuum insulator flashover.

The surface of an insulator under vacuum and under electrical charge will flashover when illuminated by a critical dose of ultra-violet (UV) radiation - depending on the insulator size and material, insulator cone angle, the applied voltage and insulator shot-history. A testbed comprised of an excimer laser (KrF, 248 nm, ~16 MW, 30 ns FWHM,), a vacuum chamber, and a negative polarity dc high voltage power supply (? -60 kV) were assembled to test 1.0 cm thick angled insulators for surface-flashover. Several candidate insulator materials, e.g. High Density Polyethylene (HDPE), RexoliteR 1400, Macor? and Mycalex, of varying cone angles were tested against UV illumination. Commercial energy meters were used to measure the UV fluence of the pulsed laser beam. In-house designed and fabricated capacitive probes (D-dots, ?12 GHz bandwidth) were embedded in the anode electrode underneath the insulator to determine the time of UV arrival and time of flashover. Of the tested insulators, the +45 degree Rexolite insulator showed more resistance to UV for surface flashover; at UV fluence level of less than13 mJ/cm2, it was not possible to induce a flashover for up to -60 kV of DC potential across the insulator's surface. The probes also permitted the electrical charge on the insulator before and after flashover to be inferred. Photon to electron conversion efficiency for the surface of Rexolite insulator was determined from charge-balance equation. In order to understand the physical mechanism leading to flashover, we further experimented with the +45 degree Rexolite insulator by masking portions of the UV beam to illuminate only a section of the insulator surface; (1) the half nearest the cathode and subsequently, (2) the half nearest the anode. The critical UV fluence and time to flashover were measured and the results in each case were then compared with the base case of full-beam illumination. It was discovered that the time for the insulator- to flash was earlier in time for the cathode-half beam illumination case than the anode-half illumination case which led us to believe that the flashover mechanism for the UV illumination is initiated from the cathode side of the insulator. Qualitatively stated, the testing revealed that the shielding of the cathode triple point against UV is more important than the anode triple junction in the design of vacuum insulators and electrodes.

Vacuum insulators are critical components in many pulsed power systems. The insulators separate the vacuum and non-vacuum regions, often under great stress due to high electric fields. The insulators will often flashover at the dielectric vacuum interface for electric field values much lower than for the bulk breakdown through the material. Better predictive models and computational tools are needed to enable insulator designs in a timely and inexpensive manner for advanced pulsed power systems. In this article we will discuss physics models that have been implemented in a PIC code to better understand the initiation of flashover.The PIC code VORPAL [1] has been ran on the Linux cluster Hera at LLNL. Some of the important physics modules that have been implemented to this point will be discussed for simple angled insulators. These physics modules include field distortion due to the dielectric, field emission, secondary electron emission, insulator charging, and the effects of magnetic fields. In the future we will incorporate physics modules to investigate the effects of photoemission, electron stimulated desorption, and gas ionization. This work will lead to an improved understanding of flashover initiation and better computational tools for advanced insulator design.

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.

Summary form only given. A critical component to a high energy pulsed power system is the insulator. The insulator's function is to standoff high voltages between two electrodes of opposite polarity. Although the intrinsic dielectric strength of the insulator is rather high, relatively low electric fields -several times less-is withstood by insulator surfaces that is exposed to vacuum. To investigate surface flashover for the widely used truncated conical insulator configuration, a vacuum chamber was fabricated where we installed a positive 45 degree conventional HD polyethylene insulator in the electrode spacing (nominal 1.0 cm). With applied 100 kV pulses of 5 microsecond duration the insulator held off the applied voltage -but experienced flashover if a source of plasma/electrons in the form of a small piece of velvet (1.0 mm dia.) was introduced in the vicinity of the insulator on either of the electrode's surfaces. Plasma expansion velocities of 1.4 to 2.7 cm/microseconds are inferred from voltage collapse and current drawn signals. It appears that the actual flashover occurs when the plasma that is launched from the cathode reaches either the insulator or the anode electrode. The breakdown mechanism and improvements in the insulator design strictly speaking can be made independently with the more advanced computational means. We also report on the progress that has been made with our PIC code modeling and its agreement with our experimental observation.

We are studying the flashover of vacuum insulators for applications where high voltage conditioning of the insulator and electrodes is not practical and for pulse lengths on the order of several microseconds. The study is centered about experiments performed with a 100-kV, 10-ms pulsed power system and supported by a combination of theoretical and computational modeling. The base line geometry is a cylindrically symmetric, +45{sup o} insulator between flat electrodes. In the experiments, flashovers or breakdowns are localized by operating at field stresses slightly below the level needed for explosive emissions with the base line geometry. The electrodes and/or insulator are then seeded with an emission source, e.g. a tuft of velvet, or a known mechanical defect. Various standard techniques are employed to suppress cathode-originating flashovers/breakdowns. We present the results of our experiments and discuss the capabilities of modeling insulator flashover.

Lawrence Livermore National Laboratory (LLNL) is evaluating design alternatives to improve the voltage regulation in our Flash X-Ray (FXR) accelerator cell and pulse-power system. The goal is to create a more mono-energetic electron beam. When an electron beam crosses the energized gap of an accelerator cell, the electron energy is increased. However, the beam with the associated electromagnetic wave also looses a small amount of energy because of the increased impedance seen across the gap. The beam-induced voltage at the gap is time varying. This creates beam energy variations that we need to understand and control. A high-fidelity computer simulation of the beam and cell interaction has been completed to quantify the time varying induced voltage at the gap. The cell and pulse-power system was characterized using a Time-domain Reflectometry (TDR) measurement technique with a coaxial air-line to drive the cell gap. The beam-induced cell voltage is computed by convoluting the cell impedance with measured beam current. The voltage was checked against other measurements to validate the accuracy.cell features. The injector voltage has added complexity because of the reflections in the cathode and anode stalks. The third term is defined as the beam-induced gap voltage that launches an electromagnetic (EM) wave into the cell and pulse-power system. A portion is reflected back from the different cell components and appears in the gap again. This is related to beam loading, but the impedance mismatches in the cell and pulse-power system creates a much more dynamic process than the name "loading" implies. This report focuses on the beam-induced energy variation.

Lawrence Livermore National Laboratory has designed and constructed a test stand to improve the voltage regulation in our flash X-Ray (FXR) accelerator cell. The goal is to create a more mono-energetic electron beam that creates an X-ray source with a smaller spot size. Studying the interaction of the beam and pulse-power system with the accelerator cell improves the design of high-current accelerators at Livermore and elsewhere. On the test stand, a standard FXR cell is driven by a flexible pulse-power system and the beam current is simulated with a switched center conductor. The test stand is fully instrumented with high-speed digitizers to document the effect of impedance mis-matches when the cell is operated under various full-voltage conditions. A time-domain reflectometry technique was also developed to characterize the beam and cell interactions by measuring the impedance of the accelerator and pulse-power component. Computer models are being developed in parallel with the testing program to validate the measurements and evaluate different design changes. Both 3D transient electromagnetic and circuit models are being used.

The Office of Heavy Vehicle Technologies supports research to enable high-efficiency diesel engines to meet future emissions regulations, thus clearing the way for their use in light trucks as well as continuing as the most efficient powerplant for freight-haulers. Compliance with Tier 2 rules and expected heavy duty engine standards will require effective exhaust emission controls (after-treatment) for diesels in these applications. DOE laboratories are working with industry to improve emission control technologies in projects ranging from application of new diagnostics for elucidating key mechanisms, to development and tests of prototype devices. This paper provides an overview of these R and D efforts, with examples of key findings and developments.