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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.

Recent studies indicate better efficacy and healthy tissue sparing with high dose-rate FLASH radiotherapy (FLASH-RT) cancer treatment. This technique delivers a prompt high radiation dose rather than fractional doses over time. While some suggest thresholds of > 40 Gy s−1 with a maximal effect at > 100 Gy s−1, accumulated evidence shows that instantaneous dose-rate and irradiation time are critical. Mechanisms are still debated, but toxicity is minimized while inducing apoptosis in malignant tissue. Delivery technologies to date show that a capability gap exists with clinic scale, broad area, deep penetrating, high dose rate systems. Based on these trends, if FLASH-RT is adopted, it may become a dominant approach except in the least technologically advanced countries. The linear induction accelerator (LIA) developed for high instantaneous and high average dose-rate, species independent charged particle acceleration, has yet to be considered for this application. We review the status of LIA technology, explore the physics of bremsstrahlung-converter-target interactions and our work on stabilizing the electron beam. While the gradient of the LIA is low, we present our preliminary work to improve the gradient by an order of magnitude, presenting a point design for a multibeam FLASH-RT system using a single accelerator for application to conformal FLASH-RT.

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.

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.

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.

A new type of compact induction accelerator is under development at the Lawrence Livermore National Laboratory that promises to increase the average accelerating gradient by at least an order of magnitude over that of existing induction machines. The machine is based on the use of high gradient vacuum insulators, advanced dielectric materials and switches and is being developed as a compact flash X-ray radiography source. Research describing an extreme variant of this technology aimed at proton therapy for cancer will be presented.

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.

High voltage systems operated in vacuum require insulating materials to maintain spacing between conductors held at different potentials, and may be used to maintain a nonconductive vacuum boundary. Traditional vacuum insulators generally consist of a single material, but insulating structures composed of alternating layers of dielectric and metal can also be built. These "high-gradient insulators" have been experimentally shown to withstand higher voltage gradients than comparable conventional insulators. As a result, they have application to a wide range of high-voltage vacuum systems where compact size is important. This paper describes ongoing research on these structures, as well as the current theoretical understanding driving this work.

Critical issues and ion beam requirements are explored for fast ignition using ion beams to provide fuel compression using indirect drive and to provide separate short-pulse ignition heating using direct drive. Several ion species with different hohlraum geometries are considered for both accelerator-produced and laser-produced ion ignition beams. Ion-driven fast ignition targets are projected to have modestly higher gains than with conventional heavy ion fusion and may offer some other advantages for target fabrication and for use of advanced fuels. However, much more analysis and additional experiments are needed before conclusions can be drawn regarding the feasibility for meeting the ion beam transverse and longitudinal emittances, focal spots, pulse lengths, and target standoff distances required for ion-driven fast ignition.

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.

Dense Plasma Focus (DPF) Z-pinches are copious sources of radiation including neutrons, x-rays, and MeV level electron and ion beams. Energetic protons and deuterons up to 10 MeV have been observed from ~cm long pinches indicating average acceleration gradients up to 1 GV/m. Corresponding electron beams with lower particle energy are also emitted. These beams contribute significantly to the neutron and x-ray output of the device. However, the mechanisms behind these gradients are not completely understood and hence a true predictive capability required for optimization or application is not currently available. At LLNL we are assembling a DPF experiment with a unique 4 MV ion probe beam designed to measure these gradients directly and to examine the possibility of using the DPF as a high-gradient acceleration stage. These unique data along with fully kinetic simulation of the DPF z-pinch will form an integrated simulation and experimental approach to understanding the DPF. In this poster we will review the design, construction, and initial operations of a 4 kJ modular DPF. We will also discuss how the probe beam will be used to measure the acceleration gradients in the plasma.

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 model-based feedback control system has been developed to correct beam displacement errors in the LLNL Advanced Test Accelerator (ATA) electron beam accelerator. The feedback control system drives an X / Y dipole steering system that has a 40 MHz bandwidth and can produce ±300 Gcm dipole fields. A simulator was used to develop the control algorithm and to quantify the expected performance in the presence of beam-position measurement noise and accelerator timing jitter. The major problem to date has been the protection of the amplifiers from the voltage that is inductively coupled to the steering bars by the beam.

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.

The Dual Axis Radiographic Hydrodynamic Test Facility (DARHT) was constructed at Los Alamos National Laboratory as a radiographic facility to provide dual-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 17 MeV, 2 kA and 2 microsecond 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, address the remaining physics of the multi-pulse electromagnetic kicker and X-ray converter target and then commission the full energy 2nd Axis accelerator. The redesign of the cell as well as long pulse beam stability tests were completed in 2005. Testing of the multi-pulse electromagnetic kicker and X-ray converter target was completed in February of this year using an 8 MeV scaled accelerator that used twenty-six refurbished accelerator cells. Commissioning of the 2nd Axis 2.5 MV, 2.0 kA electron injector and the full-energy accelerator beamline is currently underway. Commissioning of the downstream transport section and multi-pulse kicker will begin in early October of 2007 after accelerator commissioning is completed. Target commissioning is scheduled to begin in November 2007. 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.

Proton accelerator structures for medical applications using Dielectric Wall Accelerator (DWA) technology allow for the utilization of high electric field gradients on the order of 100 MV/m to accelerate the proton bunch. Medical applications involving cancer therapy treatment usually desire short bunch lengths on the order of hundreds of picoseconds in order to limit the extent of the energy deposited in the tumor site (in 3D space, time, and deposited proton charge). Electromagnetic simulations of the DWA structure, in combination with injections of proton bunches have been performed using 3D finite difference codes in combination with particle pushing codes. Electromagnetic simulations of DWA structures includes these effects and also include the details of the switch configuration and how that switch time affects the electric field pulse which accelerates the particle beam.

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.

A new type of compact induction accelerator is under development at the Lawrence Livermore National Laboratory that promises to increase the average accelerating gradient by at least an order of magnitude over that of existing induction machines. The machine is based on the use of high gradient vacuum insulators, advanced dielectric materials and switches and is stimulated by the desire for compact flash X-ray radiography sources. Research describing an extreme variant of this technology aimed at proton therapy for cancer will be described. Progress in applying this technology to several applications will be reviewed.

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.

This work describes a comparison of two candidate materials for pulse forming line fabrication with respect to bulk dielectric breakdown, frequency response of relative permittivity, and dielectric loss. One material is a commercially available microwave substrate material that can be procured in sheet form without a high voltage specification, while the other is a newly developed material that also comes in sheet form that can also be cast between the electrodes.

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.

The DARHT II accelerator at LANL is preparing a series of preliminary tests at the reduced voltage of 7.8 Me V. The transport hardware between the end of the accelerator and the final target magnet was shipped to LLNL and installed on ETA II. Using the ETA II beam at 5.2 MeV we completed a set of experiments designed reduce start up time on the DARHT II experiments and run the equipment in a configuration adapted to the reduced energy. Results of the beam transport using a reduced energy beam, including the kicker and kicker pulser system will be presented.

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.