E.A. Lopez

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.

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.

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.

Rail-gap switch modifications and test data for the Atlas capacitor bank

Atlas is a facility being designed at the Los Alamos National Laboratory (LANL) to perform high energy-density experiments in support of weapons-physics and basic-research programs. The capacitor bank design consists of a 36 MJ array of 240 kV Marx modules. The system is designed to deliver a peak current of 40-50 MA with a 4-5 /spl mu/s risetime. Evaluation, testing and qualification of key components of the Marx module are being conducted. One key element of the Marx module is the low inductance, high-voltage, high-current, high-coulomb transfer spark-gap switch needed for this application, 304 of which will be used in the Atlas capacitor bank. Because of the Marx module configuration, overall system inductance requirements and the need for a triggered switch, the design team initially selected the Maxwell Technologies rail-gap switch. The switch has been used in other high-voltage, high-current, high-coulomb transfer applications and would meet the Atlas facility requirements with some modifications. Testing of the Maxwell rail-gap switch under expected Atlas conditions is in progress. For the Atlas application, the rail-gap switch required some mechanical design modifications, which are discussed. Maxwell provided two modified switches for testing and evaluation. Results of this testing, before and after modifications, and inherent maintenance improvements to meet overall system reliability are discussed.

Design of the Atlas 240 kV Marx modules

A prototype 240 kV, oil-insulated Marx module has been designed and constructed at the Los Alamos National Laboratory (LANL). The prototype will be used for testing and certifying the design of the Marx module and certain components, including the closing switches, series resistor, and the capacitors themselves. The prototype will also be used to evaluate proposed mechanical systems designs. Information gained from the construction and testing of the 4-capacitor prototype will be folded into the design of the 16-capacitor maintenance unit. The prototype module consists of four 60 kV capacitors, two closing switches, one shunt resistor, and one series resistor. Cables are used to deliver the current to a dummy load scaled to match Atlas system parameters. The Marx unit is contained in a structure made from G-1O, suspended from a steel frame that also serves to support components of the trigger, charging, and control system. Appropriate safety and charging systems are an integral part of the prototype design.

Instabilities in foil implosions and the effect on radiation output

One of the aims of the Athena program at the Los Alamos National Laboratory is the generation of a high fluence of soft X-rays from the thermalization of a radially imploding foil. In experiments in the Athena program, a large axial current is passed through a cylindrical aluminum foil. Under the action of the Lorentz force, the resulting plasma accelerates toward the axis, thermalizes, and produces a fast soft X-ray pulse with a blackbody temperature up to several hundred electron volts. We present visible light images and X-ray data designed to study the effects of foil mass, current, and initial perturbations on the instability growth during foil implosion. Representative data is presented from several experiments using the Pegasus capacitor bank system and the explosively driven Procyon system. These experiments are labeled Peg 25 and Peg 33 for the Pegasus experiments and PDD1, PDD2 and PRFO for the Procyon experiments. In these experiments, all foils had radii of 5 cm but varied in mass and initial conditions. Experimental data from several shots were compared with each other and to a radiation magnetohydrodynamic (RMHD) computation. The data obtained from these experiments and the analysis has given us understanding of the physical mechanisms involved and insight for future experiments and has lead us to propose methods for minimizing the instability growth and maximizing the radiation output. In particular, we observed that wrinkles and other physical anomalies in the initial shape of foil do not appear to contribute to the growth of the instabilities.

Caballero: a high current flux compressor system for 100 MJ solid liner experiments

Pulse power systems delivering in excess of 100 MJ represent one of the next major challenges to the pulse power community. Explosive pulse power systems using magnetic flux compression provide a direct path to such demonstrations. Furthermore, as energy requirements grow, single use explosive systems may represent the only affordable source of ultra-high energy environments. Currently two flux compressor configurations are under consideration for powering solid liner implosions at currents above 100 MA and at energies above 100 MJ. A simultaneously initiated coaxial flux compressor (Ranchero) is described in a companion paper. A modular, center initiated disk configuration, generally patterned after the DEMG is the other candidate. Either can drive loads directly or can conceptually be connected in parallel with flat plate transmission lines to increase current delivery. Phenomenological models and conceptual designs for DEMG systems have been previously reported. In this paper we report the results of the experimental test of a first generation disk generator system. Individual disk segments have been tested with framing camera diagnostics to evaluate overall performance, dynamics and fabrication failure points. In general no bulk failures were observed in several shots and the critical weld joints maintained their integrity for at least 4 /spl mu/s after arrival of the detonation front. Single module pulse power experiments have been conducted at reduced initial current (1.5-2.0 MA) with a fixed inductance load of 0.22 nH.

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.

Explosively formed fuse opening switches for multi-megajoule applications

High explosive pulsed power (HEPP) systems are capable of generating very high energies in magnetic fields. Such stored energy is usually developed on time scales of a few tens or hundreds of microseconds. Many applications require shorter pulses and opening switches provide one way to use the large energy available for faster applications. With current flowing in an inductive circuit, introducing resistance produces voltage that can be used to drive current into a load. For an opening switch with a fast rising resistance, the load current rise time is determined by the R/L time constant of the circuit. A significant fraction of the circuit energy must be dissipated in the process, and in applications where very large energies must be dealt with only a few types of switches can be used. Experiments with high explosive driven opening switches have produced a few switches that can carry tens of MA current, and open on the time scale of one or a few /spl mu/s. We have specialized in a type of switch that we call an explosively formed fuse (EFF), and the use of this switch in the is MJ Procyon system is the subject of this paper. Operation of the EFF switch at levels of /spl sim/3 TW for 2 /spl mu/s has become routine, and we describe its characteristics and give data from a number of tests.