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Advances in solid-state modulators now permit the design of a new class of high current accelerators. These new accelerators will be able to operate in burst mode at frequencies of several MHz with unprecedented flexibility and precision in pulse format. These new modulators can drive accelerators to high average powers that far exceed those of any other technology and can be used to enable precision beam manipulations. New insulator technology combined with novel pulse forming lines and switching may enable the construction of a new type of high gradient, high current accelerator. The latest developments in these areas are reviewed.

The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility will employ two perpendicular electron linear induction accelerators to produce intense, bremsstrahlung X-ray pulses for flash radiography. We intend to produce measurements containing three-dimensional information with sub-millimeter spatial resolution of the interior features of very dense, explosively driven objects. The facility will be completed in two phases with the first phase having become operational in July 1999 utilizing a single-pulse, 20 MeV, 2 kA, 60 ns accelerator, a high-resolution electrooptical X-ray imaging system, and other hydrodynamics testing systems. We describe this machine and discuss its current operating status. The first electron beams will be generated in the second phase of DARHT this year. The second DARHT accelerator consists of a 18.4 MeV, 2 kA, 2-microsecond pulse-width accelerator. Four short electron micropulses of variable pulse-width and spacing will be chopped out of the original, long accelerator pulse for producing time-resolved X-ray images. The second phase also features an extended, high-resolution electro-optical X-ray system with a framing speed of about 2 MHz. We discuss this accelerator by summarizing the overall design of the long-pulse injector and accelerator as well as some component test results. We also discuss the fast kicker used to separate the long-pulse beam into short bursts suitable for radiography.

A lumped element circuit model is derived which accounts for both electric and magnetic coupling of the beam to the electrodes and drive circuitry of an electrostatic column. A modified one dimensional Poisson equation which incorporates two dimensional effects is discussed. An effective capacitance between the beam and the column electrodes which affects the electric coupling is estimated. Simple analytic cases which treat electric and magnetic coupling separately are solved and compared against a numerical simulation. Scaling laws are given for the magnitude of the beam loading. >

Pre-breakdown current traces obtained during high pressure gas breakdown and vacuum surface flashover show similar signatures. The initial pre-breakdown current spike, a flat constant current phase, and the breakdown phase with voltage collapse and current surge differ mostly in magnitude. Given these similarities, a model, consisting of the initial current spike corresponding to a fast precursor streamer (ionization wave led by a photoionizing front), the flat current stage as the heating or glow phase, and the terminal avalanche and gap closure, is applied to vacuum surface flashover. A simple analytical approximation based on the resistivity changes induced in the vacuum and dielectric surface is presented. The approximation yields an excellent fit to pre-breakdown time delay versus applied field for previously published experimental data. A detailed kinetics model that includes surface and gas contributions is being developed based on the initial approximation.

The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility will employ two perpendicular electron linear induction accelerators to produce intense, bremsstrahlung X-ray pulses for flash radiography. We intend to produce measurements containing three-dimensional information with sub-millimeter spatial resolution of the interior features of very dense, explosively-driven objects. The facility will be completed in two phases with the first phase having become operational in July 1999 utilizing a single-pulse, 20-MeV, 2 -kA, 60-ns accelerator, a high-resolution electrooptical X-ray imaging system, and other hydrodynamics testing systems. The second phase will be operational in 2004 and features the addition of a 20-MeV, 2-kA, 2-microsecond accelerator. Four short electron micropulses of variable pulse-width and spacing will be chopped out of the original, long accelerator pulse for producing time-resolved X-ray images. The second phase also features an extended, high-resolution electro-optical X-ray system with a framing speed of 1.6-MHz. Production of the first beam from the Phase 2 injector will occur this year. In this paper we will present the overall design of the Phase 2 long-pulse injector and accelerator as well as some component test results. We will also discuss the downstream transport section that contains the fast kicker used to separate the long-pulse beam into short bursts suitable for radiography as well as the X-ray conversion target assembly. Selected experimental results from this area of the project will also be included. Finally, we will discuss our plans for initial operations.

The Dual-Axis Radiographic Hydrodynamics Test (DARHT) facility will use two perpendicular electron linear induction accelerators to produce intense, bremsstrahlung X-ray pulses for flash radiography. We intend to produce measurements containing 3D information with sub-millimeter spatial of the interior features of very dense explosively driven objects. The facility will be completed in two phases with the first operational by June 1999 utilizing a single-pulse, 19.8-MeV, 2 to 4-kA, 60-ns accelerator (activated in March 1999), a high-resolution electro-optical X-ray imaging system, and other hydrodynamics testing systems. The second phase will be operational by Sept. 2002 and features the addition of a 20-MeV, 2 to 4-kA, 2-microsecond accelerator. Four short electron micropulses of variable pulse-width and spacing will be chopped out of the original, long accelerator pulse for producing time-resolved X-ray images. The second phase also features an extended, high-resolution electro-optical X-ray system with a framing speed of about 2-MHz. In this paper we present a Figure-Of-Merit for a X-ray based flash radiography system to motivate the selection of accelerator parameters. We will then present sub-system performance measurements from Phase 1, the physics of the interaction of our high-intensity beams with the X-ray conversion target, initial Phase 1 accelerator measurements (if available), and plans for operation. We will also discuss designs and prototype testing results for the 2-microsecond Phase 2 accelerator, including prototype induction cells and pulsed power, prototype kicker magnet performance to chop the beam, and design considerations for a multipulse X-ray conversion target.

The beam breakup mode has been observed and measured on ETA. Comparison between the measurements and the results of a computer code indicate that the beam breakup instability will be the most important limitation on current transport thru ATA. ETA Experiments that will enable a more accurate determination of the magnitude of the instability on ATA are discussed.

A series of drawings illustrate the beam breakup instability, laser guiding, quadrupole channels, phase mix damping, and cavity model. Model equations are solved for a continuous beam breakup model. (LEW)

We describe a variable-emittance filter designed for the Electron Laser Facility (ELF). The filter attenuates the 4.5-MeV, 6-kA Experimental Test Accelerator (ETA) beam, permitting electrons only which fall within a given region of phase space (r, dr/dz) to pass. Initial experimental results are presented.

The principal undertaking of the Beam Research Program over the past decade has been the investigation of propagating intense self-focused beams. Recently, the major activity of the program has shifted toward the investigation of converting high quality electron beams directly to laser radiation. During the early years of the program, accelerator development was directed toward the generation of very high current (>10 kA), high energy beams (>50 MeV). In its new mission, the program has shifted the emphasis toward the production of lower current beams (>3 kA) with high brightness (>10/sup 6/ A/(rad-cm)/sup 2/) at very high average power levels. In efforts to produce these intense beams, the state of the art of linear induction accelerators (LIA) has been advanced to the point of satisfying not only the current requirements but also future national needs.

The Advanced Test Accelerator (ATA) is a linear induction electron accelerator whose design parameters are 50 MeV, 10 kA and 70 ns pulse duration. The key physics issues affecting performance of the accelerator involve beam dynamics during transport through the accelerator structure. In this report, we present experimental results describing the initial operating phases of ATA. These results illustrate the complexity of the beam transport phenomena but also indicate the means to stabilize beam dynamics. Improvements in beam transport, which result from deploying various stabilization techniques, are also presented.

Attempts to measure high electron beam brightness at low values of beam energy are plagued by the effects of space charge forces. These forces can substantially lower the phase space acceptance of various brightness measuring devices. This report considers several models for the effects of space charge upon the acceptance of both the field free, double aperture system and the magnetic ''emittance selector'' and compares them for some recent experiments on ATA and the High Brightness Test Stand. Reasonably conservative correction factors for the acceptances of these devices are derived.

A fast-running time-dependent one-dimensional particle code has been developed to simulate transients in both electrostatic quadrupole and electrostatic column heavy-ion injectors. Two-dimensional effects are incorporated through the use of an approximation to the transverse part of the Laplacian operator. Longitudinal electric fields are solved on a mesh. An external circuit is coupled to the column, and the effect of the beam on the circuit is modeled. Transients such as initial current spikes, space-charge debunching, and beam loading of the circuit, are simulated. Future directions for the code include introduction of envelope and centroid equations to provide beam radius and displacement information and the modeling of secondary electron currents arising from beam-spill. >