D.T. Torres

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.

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.

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.

A new 40 MA Ranchero explosive pulsed power system

We are developing a new high explosive pulsed power (HEPP) system based on the 1.4 m long Ranchero generator which was developed in 1999 for driving solid density z-pinch loads. The new application requires approximately 40 MA to implode similar liners, but the liners cannot tolerate the 65µs, 3 MA current pulse associated with delivering the initial magnetic flux to the 200 nH generator. To circumvent this problem, we have designed a system with an internal start switch and four explosively formed fuse (EFF) opening switches. The integral start switch is installed between the output glide plane and the armature. It functions in the same manner as a standard input crowbar switch when armature motion begins, but initially isolates the load. The circuit is completed during the flux loading phase using post hole convolutes. Each convolute attaches the inner (coaxial output transmission line to the outside of the outer coax through a penetration of the outer coaxial line. The attachment is made with the conductor of an EFF at each location. The EFFs conduct 0.75 MA each, and are actuated just after the internal start switch connects to the load. EFFs operating at these parameters have been tested in the past. The post hole convolutes must withstand as much as 80 kV at peak dI/dt during the Ranchero load current pulse. We describe the design of this new HEPP system in detail, and give the experimental results available at conference time. In addition, we discuss the work we are doing to test the upper current limits of a single standard size Ranchero module. Calculations have suggested that the generator could function at up to ∼120 MA, the rule of thumb we follow (1 MA/cm) suggests 90 MA, and simple flux compression calculations, along with the ∼4 MA seed current available from our capacitor bank, suggests 118 MA is the currently available upper limit.

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.

Performance of the MK-IX Explosive-Driven Flux-Compression Generator

In the seventh IEEE Pulsed Power Conference in Monterey, CA (1989), Fowler and others published the performance characteristics for the recently developed MK-IX explosive-driven magnetic flux-compression generator (FCG). Since that time we have used this generator in a variety of applications, and have published the results as one of the features of each individual experiment. In this paper, we collect the results from these applications to review the performance of the FCG. The application requiring the largest routine current delivery was the Los Alamos Procyon system. In that system, the MK-IX was the prime power supply and we worked at current levels of -21.5 MA in 73 nH. We also used a MK-IX as a booster to supply initial flux for the CN-III generator, which generated a final current of -160 MA. In Procyon applications, the generator operated with a combined system jitter of 6% over a seven shot series where the MK-IX load was nominally identical. We discuss these and other results, and note how the jitter would be reduced in future applications.

High current performance of the 43 cm long Ranchero generator

A Ranchero flux compression generator (FCG) was recently tested at the 76 MA level. Ranchero generators were designed to be cost effective high current devices, and a variety of configurations have been tested. The Ranchero armature is a 152 mm diameter aluminum cylinder with a 6 mm thick wall. The high explosive (HE) is detonated simultaneously on axis, and the armature is expanded by a factor of two. At the final 300 mm diameter, the circumference is over 950 mm, which has been anticipated to accommodate currents approaching 100 MA. The test was performed with a 43 cm long module having an initial inductance of 56 nH, and a load inductance of 0.55 nH, consisting of a short load feed slot and probe grooves. A 12 mF capacitor bank at ~16.5 kV provided the initial 3.75 MA seed current. The performance of the generator with this load was calculated using a 2D MHD ALE code, and agreement was excellent. Generator design, test data, and details of the MHD calculations are given. The most important aspect of the results is verification of codes at the 76 MA level. The same codes do not show excessive losses up to the 100 MA level, and it is a much smaller extrapolation to that level at this time.

Ranchero overview and expectations for performance at currents over 50 MA (LA-UR 11-04345)

Ranchero is a coaxial magnetic flux compression generator (FCG) initiated simultaneously on-axis, which is intended to produce currents approaching 100 MA. We continue with both applications and development, striving for both cost effectiveness and high performance. In this paper we discuss on-going work, provide details of the sophisticated detonation system, and show high current predictions from our active modeling effort. We intend further tests that will push the limits of Ranchero performance.