W. Broste

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.

Liner target interaction experiments on Pegasus II

The Los Alamos High Energy Density Physics program uses capacitively driven low voltage, inductive-storage pulse power (including the 4.3 MJ Pegasus II capacitor bank facility) to implode cylindrical targets for hydrodynamics experiments. Once a precision driver liner was characterized an experimental series characterizing the aluminum target dynamics was performed. The target was developed for shock-induced quasi-particle ejecta experiments including holography. The concept for the liner shock experiment is that the driver liner is used to impact the target liner which then accelerates toward a collimator with a slit in it. A shock wave is set up in the target liner and as the shock emerges from the back side of the target liner, ejecta are generated. By taking a laser hologram the particle distribution of the ejecta are hoped to be determined. The goal for the second experimental series was to characterize the target dynamics and not to measure and generate the ejecta. Only the results from the third shot, Pegasus II-26 fired April 26th, 1994, from the series are discussed in detail. The second experimental series successfully characterized the target dynamics necessary to move forward towards our planned quasi-ejecta experiments.

Precision current measurements on Pegasus II using Faraday rotation

The authors measure the current on the Los Alamos pulsed power machine, Pegasus II, using the Faraday rotation technique in a twisted, low-birefringence optical fiber. This technique yields results which are reproducible to within about 1%. When comparing their results with data from a Rogowski loop and from B-dot loop detectors, they find discrepancies larger than the uncertainties in the measurements. They have calibrated their system in three different ways: (1) the Pegasus II experiment was driven into a shorted load in a ring-down test to measure the load inductance. The measured Faraday data were fitted to a damped sinusoidal equation and compared with current calculated from the measured voltage and capacitance; (2) a single capacitor drove about 3 kA of current into a 403 turn solenoid coil. A Pearson transformer calibrated to about 1% measured the current supplied to the coil and the Faraday data were compared with the Pearson data; and (3) on a separate machine, a calibrated Rogowski coil provided direct comparison with fiber optic Faraday measurements. The Verdet constant has been measured for bulk silica glass at a wavelength of 633 nm by several researchers. The authors extrapolated these averaged results of 4.61 radians/MA to their wavelength of 830 nm and corrected it for the 4% germania dopant in the glass from which their optical fiber was fabricated. They obtained results from all three methods consistent with a Verdet constant 6% smaller than the extrapolated value. They are continuing to investigate this discrepancy and are working with NIST to measure the Verdet constant in their glass at 830 nm.

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.

Precision solid liner experiments on Pegasus II

Pulsed power systems have been used in the past to drive solid liner implosions for a variety of applications. In combination with a variety of target configurations, solid liner drivers can be used to compress working fluids, produce shock waves and study material properties in convergent geometry. The utility of such a driver depends in part on how well-characterized the drive conditions are. This, in part, requires a pulsed power system with a well-characterized current waveform and well-understood electrical parameters. At Los Alamos, the authors have developed a capacitively driven, inductive store pulsed power machine, Pegasus, which meets these needs. They have also developed an extensive suite of diagnostics which are capable of characterizing the performance of the system and of the imploding liners. Pegasus consists of a 4.3 MJ capacitor bank, with a capacitance of 850 /spl mu/f fired with a typical initial bank voltage of 90 kV or less. The bank resistance is about 0.5 m/spl Omega/, and bank plus power flow channel has a total inductance of about 24 nH. In this paper, the authors consider the theory and modeling of the first precision solid liner driver fielded on the Pegasus pulsed power facility.

Megabar liner experiments on Pegasus II

Using pulsed power to implode a liner onto a target can produce high shock pressures for many interesting application experiments. With the Pegasus II facility in Los Alamos, a detailed theoretical analysis has indicated that the highest attainable pressure is around 2 Mbar for a best designed aluminum liner. Recently, an interesting composite liner design has been proposed which can boost the shock pressure performance by a factor of 4 over the aluminum liner. This liner design was adopted in the first megabar (Megabar-I) liner experiment carried out on Pegasus last year to verify the design concept and to compare the effect of Rayleigh-Taylor instabilities on liner integrity with the code simulations. The authors present briefly the physical explanation why the composite liner provides the best shock pressure performance. The theoretical modeling and performance of Megabar-I liner are discussed. Also presented are the first experimental results and the liner design modification for their next experiment.

DARHT AXIS II Beam Position Monitors

One of Los Alamos National Laboratory’s (LANL’s) primary responsibilities for national security is to certify the readiness of our nation’s nuclear stockpile. Since the end of underground testing in 1994, LANL has used non‐nuclear experiments and computational models to certify our stockpile. The Dual Axis Radiographic Hydrodynamic Test (DARHT) Facility is the next tool scientists will utilize for stockpile certification. DARHT will soon be capable of producing a three dimensional, time resolved radiographic image of a nuclear weapon pit during implosion. Data from these radiographic images will be used to validate the computational models used to study nuclear weapons. The first axis of DARHT with its single‐pulse capability has been in use for about 2 years. Data returned from DARHT’s First axis has been exceptional, producing the highest resolution radiographic image ever for a pit test.

Origin of the B-dot jump observed in precision liner experiments

In the liner-ejecta experiments carried out at the Los Alamos pulsed power facility Pegasus II, a solid liner was magnetically imploded to impact on a target cylinder to produce the shock-induced ejecta. As a result of improved time resolution for the B-dot (dB/dt) probes fielded last fall, the authors began to notice a sharp jump in the B-dot curve occurring at a time very close to the expected liner-target collision time. This jump was also found in the time derivative of the calculated current (dI/dt) obtained from code simulation. They have shown that the jump is indeed caused by the collision as a sudden change of the liner velocity would induce a sudden jump in the time derivative of the inductance. They have derived a general formula for calculating the jump in dI/dt and verified that the result computed from it is in good agreement with the code simulation. Useful diagnostic applications of the B-dot jump are discussed.