C. Carlson

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.

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.

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.

Emittance growth in linear induction accelerators

The Dual-Axis Radiographic Hydrotest (DARHT) facility uses bremsstrahlung radiation source spots produced by the focused electron beam produced by 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 multiple sub-millimeter source spots, we are exploring ways to improve the emittance. Some of the possible causes for the discrepancy have been investigated using PIC codes, although most of these are discounted based on beam measurements. An effect that has not yet been eliminated is emittance growth caused by a mismatch of the beam to the magnetic transport. In this presentation we discuss the results of theory, simulations, and our latest experiments and measurements.

Spot size measurement of the DARHT First Axis radiographic source

Summary form only given. DARHT is the dual axis radiographic hydrodynamic test facility at Los Alamos National Laboratory. The radiographic spot size is a critical parameter in the performance of the facility to produce quality radiographs. Images of the radiographic spot of the first axis of the DARHT facility have been acquired using a pinhole geometry, scintillator, and CCD framing cameras. The experimental setup, data acquisition and analysis of the time integrated radiographic spot size will be presented.

DARHT-II Long-Pulse Electron Beam

Summary form only given. The second axis of the dual-axis radiographic hydrotest (DARHT) facility will provide four radiographic images within ~1.6 microseconds. This will be accomplished by slicing four short pulses out of a ~1.6-microsecond long electron beam produced by the DARHT-II linear induction accelerator and directing them onto a bremsstrahlung converter target. Commissioning of the full 17-MeV configuration the DARHT-II accelerator will begin in spring of 2007 following tests of a new high-perveance (2 kA at 2.5 MV) diode. We will present the results of diode performance measurements as well as initial measurements of the fully accelerated electron beam parameters.