D. Frayer

Initial electron-beam results from the DARHT-II linear induction accelerator

The DARHT-II linear-induction accelerator has been successfully operated at 1.2-1.3 kA and 12.5-12.7 MeV to demonstrate the production and acceleration of an electron beam. Beam pulse lengths for these experiments were varied from 0.5 /spl mu/s to 1.2 /spl mu/s full-width half-maximum. A low-frequency inductance-capacitance (LC) oscillation of diode voltage and current resulted in an oscillation of the beam position through interaction with an accidental (static) magnetic dipole in the diode region. There was no growth in the amplitude of this oscillation after propagating more than 44 m through the accelerator, and there was no loss of beam current that could be measured. The results of these initial experiments are presented in this paper.

Ejected particle size measurement using Mie scattering in high explosive driven shockwave experiments

We report on the development of a diagnostic to provide constraints on the size of particles ejected from shocked metallic surfaces. The diagnostic is based on measurements of the intensity of laser light transmitted through a cloud of ejected particles as well as the angular distribution of scattered light, and the analysis of the resulting data is done using the Mie solution. We describe static experiments to test our experimental apparatus and present initial results of dynamic experiments on Sn targets. Improvements for future experiments are briefly discussed.

Long-pulse beam stability experiments on the DARHT-II linear induction accelerator

When completed, the DARHT-II linear induction accelerator (LIA) will produce a 2-kA, 17-MeV electron beam in a 1600-ns flat-top pulse. In initial tests, DARHT-II accelerated beams with current pulse lengths from 500 to 1200 ns full-width at half-maximum (FWHM) with more than 1.2-kA, 12.5-MeV peak current and energy. Experiments have now been done with a /spl sim/1600-ns pulse length. These pulse lengths are all significantly longer than any other multimegaelectronvolt LIA, and they define a novel regime for high-current beam dynamics, especially with regard to beam stability. Although the initial tests demonstrated insignificant beam-breakup instability (BBU), the pulse length was too short to determine whether ion-hose instability would be present toward the end of a long, 1600-ns pulse. The 1600-ns pulse experiments reported here resolved these issues for the long-pulse DARHT-II LIA.

Quasianamorphic optical imaging system with tomographic reconstruction for electron beam imaging

We have developed a quasianamorphic optical tomography system coupled to a streak camera to provide continuous recording of the electron beam profile of an intense, long-pulse induction accelerator. A tomographic reconstruction method based on a maximum-entropy algorithm is used to reconstruct the images. The system has simplified the calculation of beam moments, eliminated ambiguity due to beam motion, and contributed to accelerator tuning.

Commissioning the darht-II scaled accelerator

When completed, the DARHT-II accelerator will produce a 2-kA, 17-MeV beam in a 1600-ns pulse. After exiting the accelerator, the long pulse will be sliced into four short pulses by a kicker and quadrupole septum and then transported for several meters to a tantalum target for conversion to bremsstrahlung for radiography. In order to provide early tests of the kicker, septum, transport, and multi-pulse converter target we assembled a short accelerator from the first available refurbished cells, which are now capable of operating of operating at over 200 kV. This scaled accelerator was operated at ~8 MeV and ~1 kA, which provides a beam with approximately the same beam dynamics in the downstream transport as the final 17-MeV, 2-kA beam.

Commissioning the DARHT-II scaled accelerator downstream transport

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.

Fidelity of a Time-Resolved Imaging Diagnostic for Electron Beam Profiles

An optical tomographic diagnostic instrument has been fielded at the Dual-Axis Radiographic Hydrodynamic Test Facility at Los Alamos National Laboratory. Four optical lines of sight create projections of an image of an electron beam on a Cerenkov target, which are relayed via optical fiber to streak cameras. From these projections, a reconstruction algorithm creates time histories of the beams cross section. The instrument was fielded during and after facility commissioning, and tomographic reconstructions reported beam parameters. Results from reconstructions and analysis are noted.

Emittance Growth in the DARHT-II Linear Induction Accelerator

The Dual-Axis Radiographic Hydrotest (DARHT) facility uses bremsstrahlung radiation source spots produced by the focused electron beams from two linear induction accelerators (LIAs) to radiograph large hydrodynamic experiments driven by high explosives. Radiographic resolution is determined by the size of the source spot, and beam emittance is the ultimate limitation to spot size. On the DARHT Axis-II LIA we measure an emittance higher than predicted by theoretical simulations, and even though this axis produces sub-millimeter source spots, we are exploring ways to improve the emittance. Some of the possible causes for the discrepancy have been investigated using particle-in-cell (PIC) codes, although most of these are discounted based on beam measurements. The most likely source of emittance growth is a mismatch of the beam to the magnetic transport, which can cause beam halo.

Compressed streak imaging beam diagnostic

We are planning a new beam diagnostic based on Compressed Ultrafast Photography (CUP)1. A foil inserted in the beam path is used to generate a continuous optical image of the beam, which is the same technique used on the existing DARHT II beam imaging diagnostic. This existing diagnostic compresses the beam image with anamorphic lenses into four views that are streaked simultaneously on two streak cameras. In our new beam diagnostic design the anamorphic lenses are replaced with regular lenses and the beam image is stamped with a pseudo-random mask pattern. The full image is imaged onto a streak camera with its entrance slit expanded. Modifying the CUP technique, which uses a single streaked image, our design splits the image into four rotated copies and puts all four images on two streak cameras. A data cube of multiple image frames is reconstructed from the streak camera data through the use of the TwiST algorithm2 combined with total variation denoising3. The additional rotated images improve spatial resolution and reduce noise and image artifacts compared to using a single streaked image. In reconstructions of simulated data, fine detail in the beam profile can be seen and there is a remarkable absence of image artifacts compared to the existing DARHT II beam imaging diagnostic. Performance was evaluated using a pseudo-MTF derived from a simulated wave pattern test object.

An optical scattering experiment to measure the timing of x-ray converter target debris in the DARHT II accelerator

The DARHT II accelerator utilizes a fast closing vacuum valve to block x-ray converter target debris from entering the accelerator. An optical scattering diagnostic was developed to measure the arrival time of the debris at the fast valve to verify that the valve closure time is adequate to block the debris.