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Explosive pulsed power generators are devices that either convert the chemical energy stored in explosives into electrical energy or use the shock waves generated by explosives to release energy stored in ferroelectric and ferromagnetic materials. The objective of this book is to acquaint the reader with the principles of operation of explosive generators and to provide details on how to design, build, and test three types of generators: flux compression, ferroelectric, and ferromagnetic generators, which are the most developed and the most near term for practical applications. Containing a considerable amount of new experimental data that has been collected by the authors, this is the first book that treats all three types of explosive pulsed power generators. In addition, there is a brief introduction to a fourth type ix explosive generator called a moving magnet generator. As practical applications for these generators evolve, students, scientists, and engineers will have access to the results of a considerable body of experience gained by almost 10 years of intense research and development by the authors.
The purpose of this design study is to adapt the High Explosive Pulsed Power Isentropic Compression Experiment (HEPP-ICE) to milligram quantities of materials at stresses of ≈100 GPa. For this miniature application we assume that a parallel plate stripline of ≈2.5 mm width is needed to compress the samples. In any parallel plate load, the rising currents flow preferentially along the outside edges of the load where the specific impedance is a minimum [1]. Therefore, the peak current must be between 1 and 2 MA to reach a stress of 100 GPa in the center of a 2.5 mm wide parallel plate load; these are small relative to typical HEPP-ICE currents. We show that a capacitor bank alone exceeds the requirements of this miniature ICE experiment and a flux compression generator (FCG) is not necessary. The proposed circuit will comprise one half of the 2.4-MJ bank, i.e., the 6-mF, 20-kV, 1.2 MJ capacitor bank used in the original HEPP-ICE circuit. Explosive opening and closing switches will still be required because the rise time of the capacitor circuit would be of the order of 30 [mu]s without them. For isentropic loading in these small samples, stress rise times of ≈200 ns are required.
Using the Los Alamos high explosive pulsed power (HEPP) system, isentropic equation of state (EOS) data may be obtained for a wide range of materials. Current pulses with risetimes of (almost equal to)500 ns and current densities exceeding 400 MA/m, create continuous magnetic loading of samples at megabar pressures. We will summarize the technique and the problems that had to be overcome to perform the HEPP-ICE experiments at these pressures. We will then present our EOS results obtained with the conventional Lagrangian analysis and the Hayes 'Backward' integration method, and compare the data with the published principal isentrope of OFHC copper.
Accurate, ultra-high pressure isentropic equation of state (EOS) data, are required for a variety of applications and materials. Asay reported a new method to obtain these data using pulsed magnetic loading on the Sandia Z-machine. Fast rising current pulses (risetimes from 100 to 30011s) at current densities exceeding many MNcm, create continuous magnetic loading up to a few Mbar. As part of a collaborative effort between the Los Alamos and Lawrence Livermore National Laboratories we are adapting our high explosive pulsed power (HEPP) methods to obtain isentropic EOS data with the Asay technique. This year we plan to obtain isentropic EOS data for copper and tantalum at pressures up to -2 Mbar; eventually we hope to reach several tens of Mbar. We will describe the design of the HEPP systems and show out attempts to obtain EOS data to date.
With the cost of high performance, capacitor-discharge, pulse power systems continuing around $1--2 per joule and with energy requirements for experiments such as fast compression of magnetized plasmas ranging to 100 MJ and beyond, the need for economical, super-energy pulse power systems is being recognized. Explosively powered flux compressors, capable of delivering 100 MJ to a plasma physics experiment, can be designed, fabricated, and fielded at costs of less than $0.01J per shot. While less economical than laboratory pulsed power systems, if system life exceeds a few hundred full-energy shots, explosive pulse power techniques allow initial experiments to be performed quickly and economically at energies that are prohibitively costly, and hence unavailable, using conventional techniques. A variety of configurations for flux compressors suitable for 100-MJ operation can be considered. Among these, the disk configuration, pioneered by researchers at the All Russian Institute of Experimental Physics has demonstrated both high current and high energy capabilities.
High explosive pulsed power (HEPP) techniques can address a wide range of pulsed power needs. The basis for HEPP techniques is the use of high explosives to reduce the inductance of a current-carrying circuit, thus multiplying the current due to magnetic flux conservation. For the past twenty years at Los Alamos, our high energy density physics (HEDP) program has followed a path leading to more sophisticated and higher current (and often power) systems. Twenty years ago, we had the capability of conducting tests at 10, or even 30 MA, with no power conditioning and low inductance loads. The time scale of the experiment was the time it took to compress the flux explosively, and our fastest generator with high current capability was a plate generator. The operating time of the generator is less than 15 [mu]s, and flux loading requires either an additional ≈60 [mu]s or a reduced-efficiency inductive coupling scheme. We could also deliver shortened pulses to select loads by completing our generator circuit, initially, with a relatively high inductance circuit element, then switching in a lower inductance with 2-3 [mu]s left of the generator pulse. Figure 1 shows the results of such a test. The test was conducted in 1974 to investigate our capability to drive plasma z-pinch experiments for the production of soft x-rays, and was a pulsed power success. However, our understanding of vacuum power flow issues was not mature enough at that time to design a functioning plasma z-pinch load. There was a renewed need for such a system in 1980, and at that time we began assembling a complete set of techniques required for success. We first fielded a baseline test using a simplified version of the HEPP system that generated the Figure 1 data. Subsequent tests followed a 'bite size' philosophy. That is, we first designed a complete system for a level of complexity at which we believed success could be achieved. We conducted tests of that system, and once it was working in all respects, we designed the next generation system. The ultimate goal of this process was to develop a source of ≈1 MJ of soft x-rays. The process culminated, after the development of two intermediate level systems, with the development of the Procyon system. This system produced x-ray pulses of up to 1.7 MJ at temperatures up to 97 eV. Following those experiments, our attention turned to powering solid-density z-pinch liners, requiring even higher current systems. At Los Alamos, we developed the Ranchero system for that purpose, and we have collaborated with HEPP experts in Russia to power similar liner loads using disk generator systems. Our Ranchero system includes a module tested at ≈50 MA, that should operate easily at 70-90 MA. We designed Ranchero to allow modules arrayed in parallel to generate currents over 200 MA, and we are confident that we can do experiments now at 50-200 MA in the same way that we could do tests at 10-30 MA with plate generators 20 years ago. We have recently stepped back from our quest for higher energy and power systems to consider what applications we can address using relatively low cost plate generators coupled with advances achieved in our HEDP system development. We will describe relevant HEPP components, and discuss two promising applications.
Veloce is a medium-voltage, high-current, compact pulsed power generator developed for isentropic and shock compression experiments. Because of its increased availability and ease of operation, Veloce is well suited for studying isentropic compression experiments (ICE) in much greater detail than previously allowed with larger pulsed power machines such as the Z accelerator. Since the compact pulsed power technology used for dynamic material experiments has not been previously used, it is necessary to examine several key issues to ensure that accurate results are obtained. In the present experiments, issues such as panel and sample preparation, uniformity of loading, and edge effects were extensively examined. In addition, magnetohydrodynamic (MHD) simulations using the ALEGRA code were performed to interpret the experimental results and to design improved sample/panel configurations. Examples of recent ICE studies on aluminum are presented.