B.T. McCuistian

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.

First beam at DARHT-II

The second axis of the Dual Axis Radiographic HydroTest (DARHT) facility will provide up to four short (< 150 ns) radiation pulses for flash radiography of high-explosive driven implosion experiments. To accomplish this the DARHT-II linear induction accelerator (LIA) will produce a 2-kA electron beam with 18-MeV kinetic energy, constant to within /spl plusmn/ 0.5% for 2-/spl mu/s. A fast kicker will cleave four short pulses out of the 2-/spl mu/s flattop, with the bulk of the beam diverted into a dump. The short pulses will then be transported to the final-focus magnet, and focused onto a tantalum target for conversion to bremsstrahlung pulses for radiography. DARHT-II is a collaborative effort between the Los Alamos, Lawrence Livermore, and Lawrence Berkeley National Laboratories of the University of California.

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.

Status of the DARHT 2nd Axis at 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 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.

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.

DARHT-II commissioning status

The dual axis radiographic hydrotest facility (DARHT) at Los Alamos National Laboratory produces flash radiographs of hydrodynamic experiments using two linear induction accelerators situated on orthogonal axes. The first axis accelerator is operational and has produced radiographs of hydrodynamic experiments. The second axis accelerator, which is in the commissioning stage, will generate an 18-MeV, 2-kA, 2-/spl mu/s electron beam. These parameters are reduced during the initial phase of commissioning to /spl ges/12-MeV, /spl ges/1-kA, and /spl ges/350-ns. These reduced parameters allow lower voltages and fewer components than what will be necessary for the final machine. This paper presents experimental results of commissioning the second axis DARHT accelerator, including an overview of the pulsed power system plus electrical performance data from the 3-MV injector Marx generator and the 200-kV, 2-/spl mu/s induction accelerator cells. Beam transport data will also be presented.

Pulsed current threshold of Multilam/sup TM/

The Atlas pulsed-power facility presently in the design and construction phase at Los Alamos will be capable of delivering a damped sinusoidal current pulse with a peak current of 27-32 MA at 4 /spl mu/s. Multilam connections are used in several locations, primarily due to maintenance requirements of removable connections. Multilam is a commercially available strip of louvered material used to make electrical connections. The manufacturer provides a continuous current rating as well as a pulsed current rating for each type of Multilam available. However, the current pulse rating may not be appropriate if the application pulse width is different from the pulse width used to obtain the rating. LA0/0.25/55 AG Multilam from MultiContact(R) has been tested for damage threshold at currents in excess of the given pulse rating. The time to peak of the damped sinusoidal current pulse applied to the Multilam was 2.5 /spl mu/s, where the pulse rating is typically for pulses in the order of milliseconds. The observed damage threshold for this Multilam with a /spl sim/7 /spl mu/s applied current pulse is found to be 150% of the reported rating.

Atlas current connection tests performed on Pegasus II

Atlas is a 23 MJ pulsed-power facility presently in the design and construction phase at Los Alamos. Atlas will be capable of delivering a damped sinusoidal current pulse with a peak current of 27-32 MA at 4 /spl mu/s. In the center of the Atlas machine, the current converges radially to a load through the powerflow channel which is a flat and conical transmission line. The current connections to the power flow channel are at a radius of approximately one-meter, where the maximum linear current density will be of the order of 51 kA/cm. The Pegasus II facility at Los Alamos has been used to provide peak currents of 5 MA in 6 /spl mu/s to test various types of current connections applicable to the connections for the Atlas powerflow channel. A total of 32 current connections, scaled to produce current densities and actions similar to Atlas conditions, were tested. Since the powerflow channel will be replaced after each shot, most of the connections were tested under single shot conditions.