H. Oona

RANCHERO explosive pulsed power experiments

The authors are developing the RANCHERO high explosive pulsed power (HEPP) system to power cylindrically imploding solid-density liners for hydrodynamics experiments. Their near-term goal is to conduct experiments in the regime pertinent to the Atlas capacitor bank. That is, they will attempt to implode liners of /spl sim/50 g mass at velocities approaching 15 km/sec. The basic building block of the HEPP system is a coaxial generator with a 304.8 mm diameter stator, and an initial armature diameter of 152 mm. The armature is expanded by a high explosive (HE) charge detonated simultaneously along its axis. The authors have reported a variety of experiments conducted with generator modules 43 cm long and have presented an initial design for hydrodynamic liner experiments. In this paper, they give a synopsis of their first system test, and a status report on the development of a generator module that is 1.4 m long.

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.

PLASMA FLOW SWITCH AND FOIL IMPLOSION EXPERIMENTS ON PEGASUS II

Pegasus II is the upgraded version of Pegasus, a pulsed power machine used in the Los Alamos AGEX (Above Ground EXperiments) program. A goal of the program is to produce an intense (> 100 TW) source of soft x-rays from the thermalization of the kinetic energy of a 1 to 10 MJ plasma implosion. The radiation pulse should have a maximum duration of several 10`s of nanoseconds and will be used in the study of fusion conditions and material properties. The radiating plasma source will be generated by the thermalization of the kinetic energy of an imploding cylindrical, thin, metallic foil. This paper addresses experiments done on a capacitor bank to develop a switch (plasma flow switch) to switch the bank current into the load at peak current. This allows efficient coupling of bank energy into foil kinetic energy.

PROCYON: 18-MJ, 2-/spl mu/s pulsed power system

The Procyon high explosive pulsed power (HEPP) system was designed to drive plasma Z-pinch experiments that produce Megajoule soft X-ray pulses when the plasma stagnates on axis. In the proceedings of the Ninth IEEE Pulsed Power Conference, the authors published results from system development tests. At this time, they have fielded seven tests in which the focus was on either vacuum switching or load physics. Four of the tests concentrated on the performance of a plasma flow switch (PFS) which employed a l/r mass distribution in the PFS barrel. Of the four tests, two had dummy loads and one had an implosion load. In addition, one of the tests broke down near the vacuum dielectric interface, and the result demonstrated what Procyon could deliver to an 18 nH load. The authors summarize PFS results and the 18 nH test which is pertinent to upcoming solid/liquid liner experiments. On their other three tests, they eliminated the PFS switching and powered the Z-pinch directly with the HEPP system. From the best of these direct drive tests, they obtained 1.5 MJ of radiation in a 250 ns pulse, their best radiation pulse to date. They also summarize direct drive test results. More details are given in other papers in this conference for both the PFS and direct drive experiments, and an updated analysis of their opening switch performance is also included. The remainder of this paper describes the parameters and capabilities of their system, and they use the data from several experiments to provide more precise information than previously available.

The Ranchero explosive pulsed power system

We are developing a high explosive pulsed power system concept that we call Ranchero. Ranchero systems consist of series-parallel combinations of simultaneously initiated coaxial magnetic flux compression generators, and are intended to operate in the range from 50 MA to a few hundred MA currents. One example of a Ranchero system is shown. The coaxial modules lend themselves to extracting the current output either from one end or along the generator midplane. In this paper we concentrate on the system that we will use for our first imploding liner tests, a single module with end output. The module is 1.4 m long and expands the armature by a factor of two to reach the 30 cm OD stator. Our first heavy liner implosion experiments will be conducted in the range of 40-50 MA currents. Electrical tests, to date, have employed high explosive (HE) charges 43 cm long. We have performed tests and related 1D MHD calculations at the 45-MA current level with small loads. From these results, we determine that we can deliver currents of approximately 50 MA to loads of 8 nH.

High Current, Low Jitter, Explosive Closing Switches

Isentropic compression experiments (ICE) using a high explosive pulsed power (HEPP) system have been developed to obtain isentropic equation of state data for metals at megabar pressures [1][2]. The HEPP system comprises a magnetic flux compressor, an explosively-driven opening switch and a series of closing switches; fast rising current pulses are produced, with rise times of ~500 ns at current densities exceeding many MA/cm. These currents create continuous magnetic loading of the metals under study. The success of these experiments depends on the precise control of the current profile, and that in turn depends on the precise timing of the closing switches to within 50 ns at currents of the order 10 MA and voltages of ~150 kV. We first used Procyon closing switches [3] but found their timing to be unacceptably imprecise for ICE with a jitter of typically 600 ns. We suspected that the switch timing was sensitive to applied voltage; this was subsequently confirmed by experiment, as we will show. A simple shock model was developed to explain the voltage sensitivity of closure time, dt/dV, and from the model we designed a low jitter switch that uses the shock-induced electrical conduction of polyimide. The predicted dt/dV was exactly equal to the measured value, thus confirming the model. This new switch design proved successful and met the 50 ns criterion; it is now used routinely in HEPP-ICE experiments.

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.

Computational Modeling Of The Trailmaster Procyon System

The goal of the Los Alamos Foil Implosion project (Trail-master) is the development of an intense source of soft x-rays for materials and fusion studies. The x-ray source in the Trailmaster project is a foil initiated z-pinch. The next system in the Trailmaster project is designed to deliver 15 MA of current to the imploding linear creating approximately 1 MJ of soft x-ray radiation in a submicrosecond pulse. This system, designated Procyon, will consist of a Mark 9 helical explosive generator, an explosively formed fuse (EFF) opening switch, detonator closing switches, a vacuum powerflow channel, a plasma flow switch (PFS), and the imploding foil load. In the present paper we will focus on the computational modeling of the overall Procyon system. This effort includes circuit and zero-dimensional point mass (slug) modeling, 1-D and 2-D radiation MHD calculations and 3-D radiation transport and view factor modeling of the vacuum powerflow channel. 7 refs., 10 figs.

Advances in isentropic compression experiments (ICE) using high explosive pulsed power

We are developing a prototype high explosive pulsed power (HEPP) system to obtain isentropic Equation of State (EOS) data with the Asay technique. Asay, JR (1999). Our prototype system comprises a flat-plate explosive driven magnetic flux compression generator (FCG), an explosively formed fuse (EFF) opening switch, and a series of explosively-actuated closing switches. The FCG is capable of producing /spl sim/10 MA into suitable loads, and, at a length of 216 mm, the EFF will sustain voltages in excess of 200 kV. The load has an inductance of /spl sim/3 to 10 nH, allowing up to /spl sim/7 MA to be delivered in times of /spl sim/0.5 /spl mu/s. This prototype will produce isentropic compression profiles in excess of 2 Mbar in a material such as tungsten. Our immediate plan is to obtain isentropic EOS data for copper at pressures up to /spl sim/1.5 Mbar with the prototype system; eventually we hope to reach several tens of Mbar with more advanced systems.

Analysis of explosively formed fuse experiments

Explosively formed fuse (EFF) opening switches have been used in a variety of applications to divert current in high explosive pulsed power (HEPP) experiments. Typically, EFFs operate at 0.1-0.2 MA/(cm switch width), and have an /spl sim/2 /spl mu/s risetime to a resistance of 10s-100s m/spl Omega/. We have demonstrated voltage standoff of /spl sim/7 KV/(die pattern) in some configurations, and typical switches have up to 100 die patterns. In these operating regimes, we can divert large currents (10-20 MA) to low impedance loads, and produce voltage waveforms with risetime and shape determined by the shape of the resistance curve and amount of magnetic flux in the circuit. Progress in quantitatively modeling EFF performance with magnetohydrodynamic (MHD) codes has been slow, and much of our understanding regarding the operating principles of EFF switches still comes from small-scale experiments coupled with hydrodynamic (hydro) calculations. These experiments are typically conducted at currents of /spl sim/0.5 MA in a conductor 6.4 cm wide. A plane-wave detonation system is used to drive the EFF conductor into the forming die, and current and voltage are recorded. The resulting resistance profiles are compared to the hydro calculations to get insight into the operating mechanisms. Our original goals for EFF development were limited in scope, and in pursuing specific large systems, we have left behind a valuable body of small-scale test data that has been largely unused. We now have a charter to achieve a complete understanding of EFF devices, and our first step has been to review existing data. In this paper, we present some of the results of these investigations.