D. Platts

Compact flash X-ray units

Flash X-ray units are used to diagnose pulsed power driven experiments on the Pegasus machine at Los Alamos. Several unique designs of Marx powered flash X-ray units have been developed to meet the requirements of the Pegasus experiments. All of these units are compact, battery powered, fiber optically controlled, and EMP shielded. Some of these units are operated with a windowless X-ray tube in the Pegasus machine vacuum tank thereby making the full bremsstrahlung spectrum available for both hard and soft X-ray images. Other units obtain multiple X-ray flashes that are almost collinear by employing an X-ray tube configuration which allows closely spaced X-ray emitting anodes. These units all emit a 10 ns FWHM X-ray pulse. Their Marx banks store from 12 to 100 Joules of electrical energy. The X-ray output ranges from 20 to 100 mR at 3 m with endpoint energies from 100 to 500 keV.

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.

Performance of the multi-pulse X-ray imaging system for the pulsed power hydrodynamic experiments at LANL

Pulsed power driven cylindrical shock physics experiments are being performed at the PEGASUS facility at Los Alamos National Laboratory. A time dependent, axial X-ray imaging capability forms a subset of the measurements to quantify material behavior during shock propagation. 20 ns pulsed W target X-ray sources with about 10 mR at 1 m form the backlighter. Inorganic scintillators generate time dependent, visible images, which are relayed to a shuttered, microchannel plate intensifier imaging system coupled to electronic video readouts. The dynamic range, sensitivity, scene contrast, and system spatial resolution are optimized to specific experiments via optimization of fluor response to both X-ray energy and light output time response. In addition, a series of calibration data are taken to permit characterizing density information through postevent image processing. This dataset includes flat field and step wedge images. The flat field is particularly important due to the relatively large spatial variation in the X-ray dose resulting from the close proximity (60 cm) of the source to the imager.

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.

Pegasus II experiments and plans for the Atlas pulsed power facility

Atlas will be a high-energy (36 MJ stored), high-power (/spl sim/10 TW) pulsed power driver for high energy-density experiments, with an emphasis on hydrodynamics. Scheduled for completion in late 1999, Atlas is designed to produce currents in the 40-50 MA range with a quarter-cycle time of 4-5 /spl mu/s. It will drive implosions of heavy liners (typically 50 g) with implosion velocities exceeding 20 mm//spl mu/s. Under these conditions, very high pressures and magnetic fields are produced. Shock pressures in the 50 Mbar range and pressures exceeding 10 Mbar in an adiabatic compression will be possible. By performing flux compression of a seed field, axial magnetic fields in the 2000 T range may be achieved. A variety of concepts have been identified for the first experimental campaigns on Atlas. Experimental configurations, associated physics issues, and diagnostic strategies are all under investigation as the design of the Atlas facility proceeds. Near-term proof-of-principle experiments employing the smaller Pegasus II capacitor bank have been identified, and several of these experiments have now been performed. This paper discusses a number of recent Pegasus II experiments and identifies several areas of research presently planned on Atlas.

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.

Analysis of radial radiography for the liner stability series at Pegasus: PGII-59, PGII-62, and PGII-63

Three liner stability experiments were performed at the Pegasus II pulsed power facility to determine the asymmetric variations in the material density of a cylindrical liner during an electro-magnetically driven implosion. The initial campaign consisted of three experiments, designated LS-1, LS-2, and LS-3. LS-1 and LS-2 were driven with a peak current of approximately 4.2 MA, whereas the peak current for LS-3 was approximately 6.4 MA. All three liners initially were 0.4 mm wall aluminum cylinders with a mean radius of 2.38 cm and a height of 2.0 cm. The inner surface of each liner was coated with a thin (18-23 /spl mu/m) layer of gold to aid in the determination of the position of the inner surface of the liner. Radial radiography was used to characterize the z-dependent and /spl theta/-dependent instabilities that were observed as the liner contracted.

An overview of the Atlas pulsed-power systems

Atlas is a facility being designed at Los Alamos National Laboratory (LANL) to perform high energy-density experiments in support of weapon-physics and basic-research programs. It is designed to be an international user facility, providing experimental opportunities to researchers from national laboratories and academic institutions. For hydrodynamic experiments, it will be capable of achieving pressures exceeding 20-Mbar in a several cm/sup 3/ volume. With the development of a suitable opening switch, it will also be capable of producing soft X-rays. The 36 MJ capacitor bank will consist of 240 kV Marx modules arranged around a central target chamber. The Marx modules will be discharged through vertical triplate transmission lines to a parallel plate collector inside the target chamber. The capacitor bank is designed to deliver a peak current of 45 to 50 MA with a 4- to 5-/spl mu/s risetime. The Marx modules are designed to be reconfigured to a 480 kV configuration for opening switch development. Predicted performance with a typical load is presented. Descriptions of the major subsystems are also presented.

Density profiles of liner-driven implosions at Pegasus II

Efforts have been undertaken to determine the one-dimensional density profile of liner-driven cylindrical targets from analysis of the transverse radiographs of liner-driven implosion experiments at Pegasus II. The technique reported here uses forward modeling to determine the density of the different materials in a cylindrically symmetric target after liner impact. The material density was determined by performing a least-squares fit of a model function in the cylindrical geometry to match the horizontal line-out of the radiographic intensity data. Estimates of the effective X-ray spectrum are required for this analysis because of the strong energy dependence of the X-ray interaction cross sections with the different materials in the experiment. The implosions described in this report maintain good cylindrical symmetry. The analysis is performed with inexpensive, commercially available software. In this work, the density profiles are determined for two separate experiments, designated DH-5 and LLNL-3.