M. Schulze

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.

Experimental study of proton-beam halo induced by beam mismatch in LEDA

We report measurements of transverse beam halo in mismatched proton beams in a 52-quadrupole FODO transport channel following the 6.7-MeV LEDA RFQ. Beam profiles in both transverse planes are measured using beam-profile diagnostic devices that consist of a movable carbon filament for measurement of the dense beam core, and scraper plates for the halo measurement. The gradients of the first four quadrupoles can be independently adjusted to mismatch the RFQ output beam into the beam-transport channel.

Measurements of halo generation for a proton beam in a FODO channel

An experimental effort has been undertaken to investigate the production of halo particles in a proton beam having significant space charge forces. The LEDA RFQ was used to inject a pulsed 6.7 MeV 15-75 mA beam into a linear FODO channel. Four matching quads at the input of this 52-quadrupole transport line were used to generate specific mismatch oscillations, believed to be a key mechanism in the generation of beam halo. A suite of diagnostics that provide beam profile measurements over a wide dynamic range enabled a detailed comparison of measurements with theoretical models.

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.

Status of the darht 2nd axis accelerator at the Los Alamos national laboratory

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.

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.

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.

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.

Evaluation of methods to increase beam pulse width on the DARHT Axis-II accelerator

The second axis (Axis II) of the Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility at Los Alamos National Laboratory (LANL) is an electron linear induction accelerator (LIA) using 74 induction cells, each driven by a separate pulse-forming-network-based Marx (PFN). The ability to perform precise multipulse radiography is heavily influenced by the temporal beam-energy spread, beam-pulse width, related beam motion, and other focusing and target factors. The nominal beam-pulse flattop width is about 1.6 μs. A wider beam pulse would allow for increased spacing between kicked pulses and hence more information in radiographic experiments. A flattop pulse-width increase of even 50 to 100 ns would be of significant value. In this paper, we present analyses, simulations and data for three low-cost, relatively easy means of lengthening the beam pulse for multipulse radiography. The first involves optimizing the PFN charging voltages such that their respective cell pulse widths are nearly the same. The second method studies the utility of timing the cell voltage pulses such that their trailing edges are aligned instead the leading edges. And the third method, presents experimental data on the effects of increasing the cell core reset currents which increases the available volt-second (V-s) products in the cores yielding increased pulse width.

Linear-induction-accelerator beam-energy-spread minimization: Cell models and timing optimization

The second axis (Axis II) of the Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility at Los Alamos National Laboratory (LANL) is a linear induction accelerator (LIA) using 74 cells, each driven by a separate pulsed-power modulator. The summation of the injector and 74 cell voltages is the beam-energy temporal profile. The ability to perform precise multi-pulse radiography is heavily influenced by the temporal beam energy spread, related beam motion, and other focusing and target factors. Beam loading affects both the shape and magnitude of each cells voltage during the pulse. Ideally, each pulsed-power modulator/cell pair is tuned such that the loaded-cell voltage is flat with minimal amplitude variation during the pulse. However, changes in operating parameters on Axis II (beam current, operating cell voltage) have altered the amount of flattop variation resulting in more energy spread than when commissioned. In this paper, we present an optimization and synthesis method which minimizes the beams temporal energy spread by adjusting the timing of cell voltages, either advancing or retarding them, such that the injector voltage plus the summed cell voltages in the LIA result in a flatter energy profile. The method accepts as inputs the beam current, injector voltage, cell-voltages, and synthesizes loaded cell voltages as needed. Simulations and experimental data for both unloaded and loaded-cell timing optimizations are presented. For the unloaded cells, the pre-optimization baseline energy spread was reduced by over 30 % as compared to baseline. For the loaded-cell case, the measured energy spread was reduced by 49% compared to baseline.