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Unabated, worldwide trends in CO2 production project growth to > 43-BMT per year over the next two decades. Efficient power electronics are crucial to fully realizing the CO2 mitigating benefits of a worldwide smart grid (~ 18% reduction for the United States alone). Even state-of-the-art SiC high voltage junction devices are inefficient because of slow transition times (~ 0.5-μs) and limited switching rates at high voltage (~ 20-kHz at ≥ 15-kV) resulting from the intrinsically limited charge carrier drift speed (7-cm-s−1). Slow transition times and limited switch rates waste energy through transition loss and hysteresis loss in external magnetic components. Bulk conduction devices, where carriers are generated and controlled nearly simultaneously throughout the device volume, minimize this loss. Such devices are possible using below bandgap excitation of semi-insulating (SI) SiC single crystals. We explored carrier dynamics with a 75-fs single wavelength pump/supercontinuum probe and a modified transient spectroscopy technique and also demonstrated a new class of efficient, high-speed, high-gain, bi-directional, optically-controlled transistor-like power device. At a performance level six times that of existing devices, for the first time we demonstrated prototype operation at multi-10s of kW and 20-kV, 125-kHz in a bulk conduction transistor-like device using direct photon-carrier excitation with below bandgap light.

High gain photoconductive switching using Si and GaAs was studied previously for pulsed high voltage switching. A laser is used to generate charge carriers within the material to render the bulk conductive. We have begun the study of photoconductive switching using wide bandgap materials. These materials appear to operate in a non-high gain mode and the on resistance can be directly controlled with the laser intensity over many decades. It is presently believed that the conduction mechanism may be due to (a) excitation of deep states or (b) multi-photon pumping of carriers from the valance band. We present the study of the physics processes and development of a device operating at >20-kV.

The second axis of the Dual Axis Radiography of Hydrodynamic Testing (DARHT) facility produces up to four radiographs within an interval of 1.6 microseconds. It accomplishes this by slicing four micro-pulses out of a long 1.8-kA, 16.5-MeV electron beam pulse and focusing them onto a bremsstrahlung converter target. The long beam pulse is created by a dispenser cathode diode and accelerated by the unique DARHT Axis-II linear induction accelerator (LIA). Beam motion in the accelerator would be a problem for radiography. High frequency motion, such as from beam breakup instability, would blur the individual spots. Low frequency motion, such as produced by pulsed power variation, would produce spot to spot differences. In this article, we describe these sources of beam motion, and the measures we have taken to minimize it.

The Beam Research Program at Lawrence Livermore National Laboratory is continuing development of the dielectric wall accelerator (DWA), a type of accelerator which uses stacked pulse-forming lines (PFLs) to apply an accelerating field directly to the beam through a nonconducting vacuum boundary. Here, we report operation of a DWA as an electron diode using a surface flashover plasma cathode. Peak perveances in excess of 6 times 10-6A/V3/2 were measured, with current extraction and pulse train format depending on flashover source timing and PFL switching speed.

Dielectric wall accelerators, a class of induction accelerators, employ a novel insulating beam tube to impress a longitudinal electric field on a bunch of charged particles. The surface flashover characteristics of this tube may permit the attainment of accelerating gradients on the order of 100 MV/m for accelerating pulses on the order of a nanosecond in duration. A virtual traveling wave of excitation along the tube is produced at any desired speed by controlling the timing of pulse-generating modules that supply a tangential electric field to the tube wall. Because of the ability to control the speed of this virtual wave, the accelerator is capable of handling any charge-to-mass-ratio particle; hence it can be used for electrons, protons and any ion. The accelerator architectures, key technologies and development challenges will be described.

We report on a technology development to address explosive detector system throughout with increased detection probability. The system we proposed and are studying consists of a pixelized X-ray based pre-screener and a pulsed neutron source quantitative post verifier. Both technologies are derived from our compact accelerator development program for the Department of Energy Radiography Mission that enables gradients >10 MV/m. For the pixelized X-ray source panel technology, we have performed initial integration and testing. For the accelerator, we are presently integrating and testing cell modules. For the verifier, we performed MCNP calculations that show good detectability of military and multi-part liquid threat systems. We detail the progress of our overall effort, including research and modeling to date, recent high voltage test results and concept integration.

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.

The Dielectric Wall Accelerator (DWA) offers the potential to produce a high gradient linear accelerator for proton therapy and other applications. The current status of the DWA for proton therapy will be reviewed. Recent progress in SiC photoconductive switch development will be presented. There are serious beam transport challenges in the DWA arising from short pulse excitation of the wall. Solutions to these transport difficulties will be discussed.

A 2 MV, 800 mA, K+ injector for heavy-ion fusion studies is under construction. This new injector is a one-beam version of the proposed 4-beam ILSE injector. A new 36-module MARX is being built to achieve a 5 μs flat top. The high-voltage generator is stiff (

Electron induction linacs have been used for over four decades for a variety of applications. As discussed in Chap. 8, these include basic studies in magnetically confined fusion, transport of intense electron beams in various gases, the generation of electromagnetic radiation from free electron lasers, radiation processing of materials and food, and flash X-ray radiography sources.

A novel compact CT-guided intensity modulated proton radiotherapy (IMPT) system is described. The system is being designed to deliver fast IMPT so that larger target volumes and motion management can be accomplished. The system will be ideal for large and complex target volumes in young patients. The basis of the design is the dielectric wall accelerator (DWA) system being developed at the Lawrence Livermore National Laboratory (LLNL). The DWA uses fast switched high voltage transmission lines to generate pulsed electric fields on the inside of a high gradient insulating (HGI) acceleration tube. High electric field gradients are achieved by the use of alternating insulators and conductors and short pulse times. The system will produce individual pulses that can be varied in intensity, energy and spot width. The IMPT planning system will optimize delivery characteristics. The system will be capable of being sited in a conventional linac vault and provide intensity modulated rotational therapy. Feasibility tests of an optimization system for selecting the position, energy, intensity and spot size for a collection of spots comprising the treatment are underway. A prototype is being designed and concept designs of the envelope and environmental needs of the unit are beginning. The status of the developmental new technologies that make the compact system possible will be reviewed. These include, high gradient vacuum insulators, solid dielectric materials, SiC photoconductive switches and compact proton sources.

Summary form given only. The dielectric wall accelerator (DWA) is a compact pulsed power device where the pulse forming lines, switching, and vacuum wall is integrated into a single compact geometry. We are presently in the process of fabricating an accelerator cell for test on ETA-II (a 5.5 MeV, 2 kA, 70 ns pulsewidth electron beam accelerator). For this effort, we embarked on an extensive compact pulsed power development program and have pursued the study of switching (gas, oil, laser induced surface flashover and photoconductive), dielectrics (ceramics and nanoparticle composites), pulseforming line topologies (asymetric and symetric Blumleins and zero integral pulse forming lines) and multilayered vacuum insulator (HGI) technology. We review our past results and report on the progress of accelerator cell testing.

The Dielectric Wall Accelerator (DWA) is an approach for manufacturing particle accelerators that can be an order of magnitude more compact than conventional accelerators. To realize the associated high level of electric field gradient it is necessary to develop and characterize materials that can maintain electric fields in excess of 100 MV/m (=1MV/cm). This high value for the breakdown strength must be maintained for many thousands of pulses under conditions of voltage reversal. The present work reports the performance of a candidate composite material composed of high dielectric constant nanoparticles dispersed in an epoxy matrix loaded to achieve a dielectric constant = 10. The effect of peak voltage on number of pulses that can be sustained is given. In addition the effects of electrode gap spacing and charging rate are explored.

We describe the design and present initial experimental results of a novel, high-gradient, compact pulsed power module. Our application focus is linear accelerators but our technology is easily applicable to a wide range of pulse-power applications. Our design incorporates and combines for the first time a number of our recently developed, enabling technologies including: a novel, bipolar pulse-forming line allowing module stacking without ferrites, very compact and fast oil-filled switches, novel high-dielectric constant insulator/energy storage material, and a novel method for reducing edge enhancements in the pulse forming structure. The combination of these technologies enables us to design a very compact stackable module that will deliver high-gradient (5–10 MV/m) voltage at 5–10kA to arbitrary loads. Our prototype is comprised of four stages. Each stage is designed to operate at 300kV producing 1.2-MV into 120 Ohms. The pulse length is 25-ns and the pulse-shape is rectangular. We present initial experimental results up to 75 kV per stage with the switches operating in self-break mode.

A compact proton accelerator for medical applications is being developed at Lawrence Livermore National Laboratory. The accelerator architecture is based on the dielectric wall accelerator (DWA) concept. One critical area to consider is the switch region. Electric field simulations and thermal calculations of the switch area were performed to help determine the operating limits of silicon carbide (SiC) switches. Different geometries were considered for the field simulation including the shape of the thin indium solder meniscus between the electrodes and SiC. Electric field simulations were also utilized to demonstrate how the field stress could be reduced. Thermal simulations results were analyzed to find the average power capability of the switches.

As part of our ongoing development of the dielectric wall accelerator, we are studying the performance of multilayer high-gradient insulators. These vacuum insulating structures are composed of thin, alternating layers of metal and dielectric, and have been shown to withstand higher gradients than conventional vacuum insulator materials. This paper describes these structures and presents some of our recent results.

The DARHT-II accelerator will produce a 2-kA, 17-MeV beam in a 1600-ns pulse when completed mid-2007. After exiting the accelerator, the pulse is sliced into four short pulses by a kicker and quadrupole septum and then transported for several meters to a tantalum target for conversion to X-rays for radiography. We describe tests of the kicker, septum, transport, and multi-pulse converter target using a short accelerator assembled from the first available refurbished cells. This scaled accelerator was operated at ~8 MeV and ~1 kA, providing a beam with approximately the same v/gamma as the final 18-MeV, 2-kA beam, and therefore the same beam dynamics in the downstream transport. The results of beam measurements made during the commissioning of this scaled accelerator downstream transport are described.

We are developing compact pulsed power systems for various defense missions. Although the system is primarily intended for pulsed radiography, its modularity makes it well suited for compact neutron sources for explosives detection and as an HPM driver for various DOD missions. To date, we have performed extensive research at the component level and are now pursuing the integration of the technology into a single accelerator cell. Cost is 10J/cm3), specialized transmission line designs, HGI insulator technology, multi-gap low jitter (

We are developing oil-switched, planar-Blumlein pulse-generators for use in high-gradient dielectric-wall accelerators (DWA). This technology is applicable to a variety of applications including both proton and electron accelerators. In our designs, simple spark-gap switches are very tightly integrated to the ends of solid-insulated planar-Blumlein pulse - forming lines. We can operate the switches with either pressurized gas or liquid (oil). By using oil as the switching medium, we have eliminated the requirement for a pressure interface around the switch. We immerse the entire assembly including the switch region in oil. In gas switches, switch-gap and pressure are the free parameters to set the operating voltage. With oil switches, the adjustment parameters are gap and charging time. We have built a single-stage prototype with Mylar/Oil insulation that generates a 5-ns pulse with

High voltage systems operated in vacuum require insulating materials to maintain spacing between conductors held at different potentials. Traditional vacuum insulators consist of a single material, often machined with a 45deg angle to suppress surface flashover. However, insulating structures composed of alternating layers of dielectric and metal can also be built, and have been experimentally shown to have higher breakdown voltages than conventional insulators. These "high-gradient insulators" allow closer spacing of components in vacuum, and therefore; have application to a wide range of high-voltage vacuum systems where compact size is important. This paper describes ongoing simulations and experimental work on these structures, as well as the theoretical understanding driving this research

In 1994 a Fusion Technology journal publication by Logan, Moir and Hoffman described how exploiting unusually-strong economy-of-scale for large (8 GWe-scale) multi-unit HIF plants sharing a driver and target factory among several low cost molten salt fusion chambers {at} 100MWe net power DEMO. This scoping study, at a very preliminary conceptual level, attempts to identify how we might meet the last two great challenges taking advantage of several recent ideas and advances which motivate reconsideration of modular HIF drivers: >60X longitudinal compression of neutralized ion beams using a variable waveform induction module in NDCX down to 2 nanosecond bunches, the proof-of-principle demonstration of fast optical-gated solid state SiC switches by George Caporaso's group at LLNL (see George's RPIA06 paper), and recent work by Ed Lee, John Barnard and Hong Qin on methods for time-dependent correction of chromatic focusing errors in neutralized beams with up to 10 % {Delta}v/v velocity tilt, allowing 5 or more bunches, and shorter bunches, and possibly 40 that would need higher peak beam intensities in order to reduce total driver energy below 1 MJ. In principle, both PLIA and induction accelerators might benefit from multiple short bunches (see June 24, 2005 talk by Logan on multi-pulsing in PLIA accelerators for IFE), although the PLIA approach, because of fixed circuit wave velocities at any z, requires imaginative work-arounds to handle the different bunch velocities required. George's RPIA06 paper also describes a different type of radial line induction linac that might be considered, but its unclear how the required pulse-to-pulse variable waveforms can be obtained with such pulselines. This initial MathCad analysis explores multi-pulsing in modular solenoid induction linacs (concept shown in Figure 1) considering high-q ECR sources, basic induction acceleration limits assuming affordable agile waveforms, transverse and longitudinal bunch confinement constraints, models to optimize bunch lengths, solenoid fields, core radial builds and switching. Figure 2 below illustrates one linac module for a driver example (not yet optimized) consisting of 40 linacs (20 at each end). Necessarily, this first look invokes many new ideas, but could they potentially meet the above challenges?

We analyze the sheath helix model of the pulseline accelerator [1]. We find the dispersion relation for a shielded helix with a dielectric material between the shield and the helix and compare it against the results from 3-D electromagnetic simulations. Expressions for the fields near the beam axis are obtained. A scheme to taper the properties of the helix to maintain synchronism with the accelerated ions is described. An approximate circuit model of the system that includes beam loading is derived.

Using dielectric wall accelerator technology, we are developing a compact induction accelerator system primarily intended for pulsed radiography. The accelerator would provide a 2-kA beam with an energy of 8 MeV, for a 20-30 ns flat-top. The design goal is to generate a 2-mm diameter, 10-rad x-ray source. We have a physics design of the system from injector to the x-ray converter. We present the results of injector modeling and PIC simulations of beam transport. We also discuss the predicted spot size and the on-axis x-ray dose.

Helix structures have been proposed [1] for accelerating low energy ion beams using MV/m fields in order to increase the coupling efficiency of the pulsed power system and to tailor the electromagnetic wave propagation speed with the particle beam speed as the beam gains energy. Calculations presented here show the electromagnetic field as it propagates along the helix structure, field stresses around the helix structure (for voltage breakdown determination), optimizations to the helix and driving pulsed power waveform, and simulations showing test particles interacting with the simulated time varying fields.

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.

In order to correct transverse beam motion during a 50 ns duration beam pulse, a set of coils has been designed with drive power in the 3 MW range to operate at a bandwidth of 50 MHz. A description of the pulser and a particular application is given along with pertinent test data.

We have developed a novel insulator concept that involves the use of alternating layers of conductors and insulators with periods less than 1 mm. We have demonstrated that these structures perform 2 to 5 times better than conventional insulators in long pulse, short pulse, and alternating polarity applications. We present new testing results showing exceptional behavior at DC, with gradients in excess of 110 kV/cm in vacuum.

The Dual Axis Radiographic Hydrodynamic Test Facility (DARHT) was constructed at Los Alamos National Laboratory as a radiographic facility to provide two-axis multi-time radiography to support the US Stockpile Stewardship Program. The DARHT 1st Axis has been operational since 1999 and has been providing excellent radiographs. The DARHT 2nd Axis construction project was completed in early 2003. However, during the subsequent commissioning efforts to bring it to the full specifications of 18 MeV, 2 kA and 2 micro-second pulse length, high voltage breakdown was observed in several of the 78 induction accelerator cells. In January 2004, the DARHT 2nd Axis Refurbishment and Commissioning Project was launched. It is a Los Alamos National Laboratory effort in collaboration with the Lawrence Livermore and Lawrence Berkeley National Laboratories. Its purpose is to first address the HV breakdown problems with the 2nd Axis accelerator cells. A redesign of the cell is currently undergoing acceptance testing and will be discussed in companion papers presented at this conference. After the cells are refurbished and reinstalled, commissioning of the injector, accelerator and downstream transport will commence. Work is also continuing on beam transport and conversion target physics to reduce overall risk and ensure that the DARHT 2nd Axis can achieve its original design goal. The DARHT 2nd Axis is scheduled for completion in early 2008, at which point the DARHT facility will be ready to support two-axis, multi-pulse radiography in support of the Stockpile Stewardship Program. In this paper, we present the overall project status, commissioning strategy and schedule.

We are developing a compact accelerator system primarily intended for pulsed radiography. Design characteristics are an 8 MeV endpoint energy, 2 kA beam current, and a cell gradient of approximately 3 MV/m: overall accelerator length is 2-3 m. Such designs have been made possible with the development of high specific energy dielectrics (> 10J/cm3), specialized transmission line designs and multi-gap laser triggered low jitter (< 1 ns) gas switches. In this geometry, the pulse forming lines, switches, and insulator/beam pipe are fully integrated within each cell to form a compact, stand-alone, stackable unit. We detail our research and modeling to date, recent high voltage test results, and the integration concept of the cells into a radiographic system.

The second axis of the dual axial radiography hydrodynamic test (DARHT-II) facility at LANL is currently in the commissioning phase. The beam parameters for the DARHT-II machine will be nominally 17 MeV, 2 kA and 1.6 mus. This makes the DARHT-II downstream system the first system ever designed to transport a high current, high energy and long pulse beam [2]. We will test these physics issues of the downstream transport system on a scaled DARHT-II accelerator with a 7.8-MeV and 950-A beam at LANL before commissioning the machine at its full energy and current. The scaling laws for various physics concerns and the beam parameters selection are discussed in this paper.

We are investigating a novel insulator concept that involves the use of alternating layers of conductors and insulators with periods less than 1 mm. These structures perform 1.5 to 4 times better than conventional insulators in long pulse, short pulse, and alternating polarity applications. We have surveyed our ongoing studies by investigating the performance under long pulse electron beam, short pulse, and full reversing conditions

The maximum vacuum insulator surface dielectric strength determines the acceleration electric field gradient possible in a short pulse accelerator. Previous work has indicated that higher electric field strengths along the insulator-vacuum interface might be obtained as the pulse duration is decreased S. Sampayan et al. (1995). In this work, a 250 kV, single ns wide impulse source was applied to small diameter, segmented insulators samples in a vacuum to evaluate the multilayer surface dielectric strength of the sample construction. Resonances in the low inductance test geometry were used to obtain unipolar, pulsed electric fields in excess of 100 MV/m on the insulator surface. The sample construction, experimental arrangement and experimental results are presented for the initial data in this work. Modeling of the multilayer structure is discussed and methods of improving insulator surface dielectric strength in a vacuum are proposed.

Triggered, multi-channel, surface discharges or surface flashover switching have been investigated as a low inductance, low pulse rate switch for conducting large currents. This paper discusses the investigation of UV (355 nm) laser triggered, single channel, low inductance, ns closure and sub-ns jitter switches for applications in switching high dielectric constant, compact pulse forming lines into accelerator loads. The experimental arrangement for evaluating the switch performance and for measuring the high field dielectric constant of the pulse forming lines is presented. Experimental results of delay and jitter measurements versus optical energy on the flashover surface and DC electric field charge.

Summary form only given. Modern flash X-ray systems employing electron linear induction accelerators require focal spot sizes of order 1 mm diameter incident on high-Z Bremsstrahlung targets. Typical beam parameters are in the range of several kiloamperes, tens of MeV with pulse lengths of order 50 ns. A single pulse with these parameters is so intense that it converts the target material into a hot plasma with velocities on the order of several centimeters per microsecond. Intense axial electric fields will exist on the target surface which may lead to the extraction of light contaminant ions. These ions can travel upstream where they will act as an electrostatic lens which can cause a time varying disruption of the focal spot. Experimental and theoretical work on this backstreaming ion mechanism will be presented along with issues for multiple pulse operation.

These kickers will be used to make fast dipoles and quadrupoles which are driven by sharp risetime pulsers to provide precision beam manipulation for high current kA electron beams. This technology will be used on the 2/sup nd/ axis of the DARHT linac at LANL. It will be used to provide 4 micropulses of pulse width up to 120 nsec. Selected from a 2 /spl mu/sec., 2 kA, 20 MeV macropulse. The fast pulsers will have amplitude modulation capability to compensate for beam-induced steering effects and other slow beam centroid motion to within the bandwidth of the kicker system. Scaling laws derived from theory will be presented along with extensive experimental data obtained on the test bed ETA-II.

A dipole "septum" magnet without a material septum has been designed and tested as part of a fast beam kicker system for use in intense, electron-beam induction accelerators. This septum magnet is a simple, iron-based electromagnet with two static, oppositely oriented dipole field regions used to provide further separation of beam centroids given a small angle kick by a fast beam kicker. The magnet geometry includes removable pole pieces to allow experimental flexibility. Field errors experienced by the beam depend crucially on the magnitude of the initial kick that provides displacement of the beam centroids from the transition region between the two dipole field regions. Results of simulations are reported.

A fast stripline beam kicker is used to dynamically switch a high current electron beam between two beamlines. The transverse dipole impedance of a stripline beam kicker has been previously determined from a simple transmission line model of the structure. This model did not include effects due to the long axial slots along the structure as well as the cavities and coaxial feed transition sections at the ends of the structure. 3-D time domain simulations show that the simple transmission line model underestimates the low frequency dipole beam coupling impedance by about 20% for our structure. In addition, the end cavities and transition sections can exhibit dipole impedances not included in the transmission line model. For high current beams, these additional dipole coupling terms can provide additional beam-induced steering effects not included in the transmission line model of the structure.

A flexible linear induction accelerator model (LIAM) was developed to predict both beam centroid position and the beam envelope. LIAM requires on-axis magnetic profiles and is designed to easily handle overlapping fields from multiple elements. Currently, LIAM includes solenoids, dipole steering magnets, and accelerating gaps. Other magnetic elements can be easily incorporated into LIAM due to its object-oriented design. LIAM is written in the C programming language and computes fast enough on current workstations to be used in the control room as a tuning and diagnostic aid. Combined with a non-linear least squares package, LIAM has been used to estimate beam energy at various locations within the ETA-II accelerator. >

The transport of a high current relativistic electron beam in a stripline beam kicker is strongly dependent on the wake properties of the structure. The effect of the beam-induced fields on the steering of the beam must be determined for a prescribed trajectory within the structure. A 3-D time domain electromagnetic code is used to determine the wake fields and the resultant Lorentz force on the beam both for an ultra-relativistic electron beam moving parallel to the beamline axis as well as a beam that follows a curved trajectory through the structure. Usually in determining the wake properties of the structure, a wake impedance is found for a beam that is moving parallel to the beamline axis. However, we extend this concept to curved trajectories by calculating beam induced forces along the curved trajectory. Comparisons are made with simple transmission line models of the structure. The wake properties are used in models to transport the beam self-consistently through the structure.

A new, high gradient insulator technology has been developed for accelerator systems. The concept involves the use of alternating layers of conductors and insulators with periods of order 1 mm or less. These structures perform many times better (about 1.5 to 4 times higher breakdown electric field) than conventional insulators in long pulse, short pulse, and alternating polarity applications. We describe our ongoing studies investigating the degradation of the breakdown electric field resulting from alternate fabrication techniques, the effect of gas pressure, the effect of the insulator-to-electrode interface gap spacing, and the performance of the insulator structure under bi-polar stress.

Recent advances in induction accelerator technology make it possible to envision a single accelerator that can serve as an intense, precision multiple pulse x-ray source for advanced radiography. Through the use of solid-state modulator technology repetition rates on the order of 1 MHz can be achieved with beam pulse lengths ranging from 200 ns to 2 /spl mu/secs. By using fast kickers, these pulses may be sectioned into pieces which are directed to different beamlines so as to interrogate the object under study from multiple lines of sight. The ultimate aim is to do a time dependent tomographic reconstruction of a dynamic object. The technology to accomplish these objectives along with a brief discussion of the experimental plans to verify it will be presented.

Recent high-voltage breakdown experiments of periodic metallic-dielectric insulating structures have suggested several interesting high-gradient applications. One such area is the employment of high-gradient insulators in high-current, electron-beam, accelerating induction modules. For this application, the understanding of the RF characteristics of the insulator plays an important role in estimating beam-cavity interactions. In this paper, we examine the RF properties of the insulator comparing simulation results with experiment. Different insulator designs are examined to determine their RF transmission properties in gap geometries.

The high gradient insulator has been demonstrated to operate at levels comparable or better than special geometry or coated insulators. Some patented insulator configurations allow for sophisticated accelerator structures, high power flow interfaces, and microwave applications not previously possible. Sophisticated manufacturing techniques available at AlliedSignal FM&T made this development possible. Bipolar and high power flow applications are specially suited for present insulator designs. The insulator shows a beneficial effect when used under RF fields or RF structures. These insulators can be designed, to a first approximation, from simple electron flight path equations. With a model of surface flashover physics we completed a set of design calculations that include effects such as layer density and dielectric/metal thickness. Experimental data, obtained in the last few years of development, is presented and reviewed. Several insulator fabrication characteristics, indicating critical design parameters, are also presented.

In the usual treatment of impedances of beamline structures the electromagnetic response is computed under the assumption that the source charge trajectory is parallel to the propagation axis and is unaffected by the wake of the structure. For high energy beams of relatively low current this is generally a valid assumption. Under certain conditions the assumption of a parallel source charge trajectory is no longer valid and the effects of the changing trajectory must be included in the analysis. Here the usual transmission line analysis that has been applied to BPM type transverse kickers is extended to include the self-consistent motion of the beam in the structure.

Potential designs for a high power superconducting delay line of approximately 10 ms duration are described. The transmitted signal should have low dispersion and little attenuation to recapture the original signal. Such demands cannot be met using conventional metal conductors. This paper outlines a proposal for a new transmission line design using low temperature superconducting material which meets system specifications. The 25 W line is designed to carry pulsed signals with an approximate rise time of 8 nsec and a maximum voltage magnitude of 25 kV. Predicted electrical design and performance of the line are presented.

Double pulse transport experiments were conducted on the front end of the ATA accelerator to obtain data on the capability of a velvet cloth cathode to produce two successive pulses. Pulses of approximately 3 kA were extracted from the cathode with interpulse spacings varying from 150 ns to 2.8 /spl mu/s using an anode-cathode voltage of about 1 MV. Analysis of the current and voltage waveform data from the injector indicate that the effects of cathode plasma on the second pulse of a two-pulse burst is minimal.

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.

Fast, low jitter command triggered switching is key to the successful implementation of the dielectric wall accelerator (DWA). We are studying a UV induced vacuum surface flashover switch for this purpose. We present our initial data using a Nd:YAG (/spl lambda/=0.06 nm) laser incident onto a high gradient insulator surface at 1/spl omega/, 2/spl omega/, 3/spl omega/, and 4/spl omega/. Best 1/spl sigma/ jitter was

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.

Summary form only given. An all solid-state prototype kicker pulser for a proton radiography system has been designed and fabricated at Lawrence Livermore National Laboratory. Multiple solid state modulators stacked in an inductive-adder configuration are utilized in this kicker pulser design. Each modulator is comprised of multiple metal-oxide-semiconductor field-effect transistors (MOSFETs) which quickly switch the energy storage capacitors across magnetic induction cores. Voltage from each modulator is inductively added by a voltage summing stalk. To minimize losses, Metglas is used as the core material. A lumped element circuit model of the inductive adder has been developed to predict the performance of the pulser. The modeling results will be compared with experimental data.