Metastable Intermolecular Composite (MIC) materials are comprised of a mixture of oxidizer and fuel with particle sizes in the nanometer range. To better understand the reaction mechanisms of burning MIC materials, dynamic electrical conductivity measurements have been performed on a MIC material for the first time. Simultaneous optical measurements of the wave front position have shown that the reaction and conduction fronts are coincident within 160 μm. Unlike detonating high explosives (HE) where the conductivity profile is represented by an initial peak followed by an exponential decay of conductivity, the MIC conductivity profile is a gradual, irregular ramp which increases from zero over many microseconds. This suggests that the reaction zone thickness is different in MICs compared to detonating HE. Static measurements of conductivity of pressed MIC pellets suggest that the conduction is associated with chemical reaction in the MIC.

Metastable intermolecular composite (MIC) materials are comprised of a mixture of oxidizer and fuel with particle sizes in the nanometer range. Dynamic electrical conductivity measurements have been performed on a reacting MIC material. Simultaneous optical measurements of the wavefront position have shown that the reaction and conduction fronts are coincident within 160μm. It has been observed that MICs, like high explosives, are insulators before reaction is initiated. Once reaction is induced, there is a conduction zone that corresponds with the reaction zone behind the reaction front. Unlike detonating high explosives (HEs) where the conductivity profile is represented by an initial peak followed by an exponential decay of conductivity, the MIC conductivity profile is a gradual, irregular ramp which increases from zero over many microseconds. This supports other studies that show the MIC reaction process to be significantly different from detonating HEs. Static measurements of conductivity of pressed MIC pellets suggest that the electrical conduction is associated with chemical reaction in the MIC and not compaction effects alone.

period, wedeveloped andfielded theMK-VIII, MK-IX,a sweeping wavecoaxial (coax) generator4, andtheCN-III IntheSeventh IEEEPulsed PowerConference in generator5, amongothers. We also developed three high Monterey, CA (1989), Fowler andothers published the energy systems forz-pinch implosion research, Laguna6, performance characteristics fortherecently developed Procyon7, andRanchero8. FortheCN-III, theMK-IX MK-IX Explosive-Driven Magnetic Flux-Compression served asabooster tosupply initial fluxtothehigh Generator (FCG).Since thattimewe haveusedthiscurrent generator. Ranchero experiments never required a generator inavariety ofapplications, andhavepublished booster, buttheMK-IXcanbeusedwhenhigher current theresults asoneofthefeatures ofeachindividual Ranchero experiments arecalled for9. ForLagunaand experiment. Inthis paper, we collect theresults from Procyon, theMK-IXwastheprimepowersupply ina these applications toreview theperformance oftheFCG. system withanexplosively formed fuse(EFF)opening Theapplication requiring thelargest routine currentswitch asapowerconditioner'0. We willdiscuss the delivery wastheLosAlamosProcyon system. Inthatperformance oftheMK-IXinbothapplications, andfocus system, theMK-IXwastheprime powersupply andwe ontheProcyon system. Procyon consisted ofa12shot worked atcurrent levels of-21.5 MA in73nH.We alsoseries, ofwhicheight hadnominally identical feedand usedaMK-IXasabooster tosupply initial fluxforthe loadconfigurations fortheMK-IX. CN-III generator, whichgenerated afinal current of-160 There weretwosurprises inreviewing files that dated MA. InProcyon applications, thegenerator operated with backtothemideighties. Itwasdisappointing todiscover acombined system jitter of6% overasevenshotseriesthat wehadgiven little attention toMK-IXperformance wheretheMK-IXloadwasnominally identical. We inourdataanalysis. Fromthedata readily available, we discuss these andother results, andnotehowthejitter hadfewhighquality initial current traces toconsider for wouldbereduced infuture applications. this paper.Thisprecluded ournormalizing generator performance toequivalent initial currents onProcyon

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.

Data on the high-pressure melting temperatures of metals is of great interest in several fields of physics including geophysics. Measuring melt curves is difficult but can be performed in static experiments (with laser-heated diamond-anvil cells, for instance) or dynamically (i.e., using shock experiments). However, at the present time, both experimental and theoretical results for the melt curve of lead are at too much variance to be considered definitive. As a result, we decided to perform a series of shock experiments designed to provide a measurement of the melt curve of lead up to about 50GPa in pressure. At the same time, we developed and fielded a reflectivity diagnostic, and conducted measurements on tin as well. The results show that the melt curve of lead is somewhat higher than the one previously obtained with static compression and heating techniques.

High explosive pulsed power (HEPP) systems are capable of accessing very high energy densities and can reach conditions that are not possible with capacitor bank systems. The Procyon system was developed and used for experiments over a period of six years, and is exemplary of the capabilities of HEPP systems for state-of-the-art research. In this paper we will summarize some of the more interesting aspects of the work done in the past but will suggest ideas toward applications for future research. One of the main, unique features of HEPP systems is that they integrate easily to a particular physics experiment and the power flow can be optimized for a specific test. Magnetic flux compression generators have been an ideal power source for both high current plasma physics and hydrodynamic experimental loads. These experiments have contributed greatly to the understanding of high temperature and density plasmas and more recently to the understanding of instability growth in thick ({approx}1 mm) imploding metal cylinders. Common to all these experiments is the application of a large current pulse to a cylindrically symmetric load. The resulting Lorenz force compresses the load to produce hydrodynamic motion and/or high temperature, high density plasma. In the plasma physicsmore » experiments, plasma thermalizes on axis and a black body distribution of x-rays is produced. To get better access to the radiation pulse, the load electrode geometry was modified. For example, by shaping the plasma implosion glide planes, a mass depletion region was formed along one electrode at pinch time which generated a very large voltage drop across a 1-2 mm segment of the pinch, and also produced a high energy ion beam on axis. These results were predicted by magneto-hydro-dynamic (MHD) codes and verified with framing camera and x-ray, pinhole, camera pictures. We have not previously published these features but will take another look and propose possible scenarios for studying and generating high intensity ion beams. The conditions generated in the implosion load region may be ideal for generating K and L-shell radiation via ion-atom collisions. In recent years, and in a previous conference, the simulation community has shown interest for Ar K-shell radiation and other soft x-ray sources. We will speculate on ways to use this system to generate a high fluence pulse of Ar K-shell radiation, and also to use the high intensity ion beam to study the mechanisms involved in the ion-atom collisions process. These processes can be used to enhance x-ray radiation from a variety of elements.« less

The authors have developed a system for driving hydrodynamic liners at currents approaching 30 MA. Their 43 cm module will deliver currents of interest, and when fully developed, the 1.4 m module will allow similar currents with more total system inductance. With these systems they can perform interesting physics experiments and support the Atlas development effort.

The 1997 international Dirac II Series held at Los Alamos National Laboratory involved low temperature electrical transport and optical experiments in magnetic fields exceeding 800%, produced by explosive flux compression using Russian MC-1 generators. An overview of the scientific and technical advances achieved in this Series is given, together with a strategy for future work in this challenging experimental environment. A significant outcome was achieved in transport studies of microfabricated thin-film YBCO structures with the magnetic field in the CuO plane. Using a GHz transmission line technique at an ambient temperature of 1.6 K, an onset of dissipation was observed at 150 T (a new upper bound for superconductivity in any material), with a saturation of resistivity at 240 T. Comparison with the Pauli limit expected at B=155 T in this material suggests that the critical field in this geometry is limited by spin paramagnetism. In preparation for a Diract III series, a systematic temperature-dependent transport study of YBCO using in-plane magnetic fields of 150 T generated by single-turn coils, at temperatures over the range 10-100 K, has been undertaken in collaboration with the Japanese Megagauss Laboratory. The objective is to map out the phase diagram for this geometry, which is expected to be significantly different than the Werthamer-Helfand-Hohenberg model, due to the presence of paramagnetic limiting. Nanofabricated magnetometers have also been developed in a UNSW-LANL collaboration for use in Dirac III for Fermi surface measurements of YBCO in megagauss fields, which are described.

At Los Alamos, the authors have primarily applied Explosively Formed Fuse (EFF) techniques to high current systems. In these systems, the EFF has interrupted currents from 19 to 25 MA, thus diverting the current to low inductance loads. The magnitude of transferred current is determined by the ratio of storage inductance to load inductance, and with dynamic loads, the current has ranged from 12 to 20 MA. In a system with 18 MJ stored energy, the switch operates at a power up to 6 TW. The authors are now investigating the use of the EFF technique to apply high voltages to high impedance loads in systems that are more compact. In these systems, they are exploring circuits with EFF lengths from 43 to 100 cm, which have storage inductances large enough to apply 300 to 500 kV across high impedance loads. Experimental results and design considerations are presented. Using cylindrical EFF switches of 10 cm diameter and 43 cm length, currents of approximately 3 MA were interrupted producing {approximately}200 kV. This indicate s the switch had an effective resistance of {approximately}100 m{Omega} where 150--200 m{Omega} was expected. To understand the lower performance, several parameters were studied, including: electrical conduction through the explosive products; current density; explosive initiation; insulator type; conductor thickness; and so on. The results show a number of interesting features, most notably that the primary mechanism of switch operation is mechanical and not electrical fusing of the conductor. Switches opening on a 10 to 10 {micro}s time scale with resistances starting at 50 {micro}{Omega} and increasing to perhaps 1 {Omega} now seem possible to construct, using explosive charges as small as a few pounds.

Ranchero is an explosively driven magnetic flux-compression generator that has been developed, over the last four years, as a versatile power source for high energy density physics experiments. It is coaxial, and comprises a 15 cm-diameter armature and a 30-cm stator, each aluminum. The length may be varied to suit the demands of each experiment; thus far, lengths of 0.43 m and 1.4 m have been used. The stator is filled and driven by a high performance cast explosive, and the ultimate performance of the device is limited by the smoothness of the armature expansion. The armature explosive is initiated on axis by PETN hemispheres, spaced at intervals of about 18 mm and 24.5 mm; each is simultaneously detonated by a slapper detonator system. Calculations of armature expansion predicted ripples less than 0.2 mm, and this was confirmed in early experiments. Yet, ripples approaching tens of millimeters were observed in some more recent experiments. The authors discuss the possible origins of the se large ripples, and the methods the authors have used to correct them.

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.

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, EFF's operate at 0.1-0.2 MA/(cm switch width), and have an /spl sim/2 /spl mu/s risetime to a resistance of 10's-100's 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.

Accurate, ultra-high pressure isentropic equation of state (EOS) data, are required for a variety of applications and materials. Asay (1999) 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 300 ns) at current densities exceeding many MA/cm, create continuous magnetic loading up to a few Mbar. As part of a collaborative effort between the Los Alamos and Lawrence Livermore National Laboratories, the authors are adapting their high explosive pulsed power (HEPP) methods to obtain isentropic EOS data with the Asay technique. This year, they plan to obtain isentropic EOS data for copper and tantalum at pressures up to /spl sim/2 Mbar; eventually we hope to reach several tens of Mbar. They describe the design of the HEPP systems and show their attempts to obtain EOS data to date.

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, the authors' high energy density physics (HEDP) program has followed a path leading to more sophisticated and higher current (and often power) systems. Twenty years ago, they 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 their fastest generator with high current capability was a plate generator. The operating time of the generator is less than 15 /spl mu/s, and flux loading requires either an additional /spl sim/60 /spl mu/s or a reduced-efficiency inductive coupling scheme. They could also deliver shortened pulses to select loads by completing their generator circuit, initially, with a relatively high inductance circuit element, then switching in a lower inductance with 2-3 /spl mu/s left of the generator pulse.

As experiments done with explosively driven switches and magnetic flux compression generators become complex, the details become increasingly important. In most experiments the detonation of explosives is done through layers of material that include metal and plastic, which may retard the detonation, and at the same time the insulating materials must maintain their integrity at high voltages. We have initiated some small-scale experiments that use a few hundred grains of explosives to study effects on shocked materials. These studies look at effects on detonation through various materials as a function of their thickness, and will be compared with hydrodynamic computer modeling done with the MESA2D code. Another related series of experiments observed the voltage breakdown of insulators under shock conditions. In this set of experiments insulators made of polyethylene, Teflon and Mylar were placed between two electrodes and exposed to 120 kV during a shock. The timing of the shock was determined from light produced at a flash gap. Photo-diodes coupled to optical fibers were used to transmit the signals to the diagnostic bunker. A Pearson probe was used to monitor the current at the insulator during the breakdown. The timing of the breakdown relative to the shock arrival time was recorded. The breakdown data as a function of materials and geometry are provided in this report. Also, these data are compared with computer simulations that may suggest material conditions at the time of insulator failure.

Recent innovations in sample lithography and measurement techniques have made possible electrical transport measurements on both high-Tc superconductor and semiconductor samples in the harsh environment of implosively generated magnetic fields B up to 800T, with reliable data obtained up to 300T. Thin films of the cuprate superconductor YBa2Cu3O7−δ have been studied with B directed parallel to the CuO planes (J‖B⊥c-axis). At an ambient temperature of 1.6K and a probe frequency of 0.9GHz we find an onset of dissipation at Bon=(150±20)T, the highest field yet reported. In addition, the measured upper critical field Bc2=(240±30)T is significantly above the paramagnetic limit for this material. Measurements on AlxGa1−xAs/GaAs parabolic quantum well structures, designed to probe the existence of a magnetically induced triplet superconducting phase, and on HgSe:Fe samples are also discussed.

The experimental results of Faraday rotation (FR) measurements in dilute magnetic semiconductors in high magnetic field (`Dirac Series' – Los Alamos) are presented. The magnetic field is produced by an explosive-driven flux-compression generator (150T). Measurements have been carried with samples of Cd1−xMnxTe with x=0.43 using 633nm light at liquid helium temperature. The FR increases in such samples when the magnetic field exceeds 60T. Interband exchange interaction and the direct influence of the external magnetic field on the exchange interaction must be considered to interpret the experimental results.

Thin-film YBa2Cu3O7−δ has been studied with in-plane magnetic fields B to 850 T, with good signal-to-noise to 300 T, using a GHz-transmission-line technique for electrical transport measurements in explosive-driven microsecond pulsed magnetic fields. At an ambient temperature of 1.6 K with B⊥c axis we find an onset of dissipation at 150 T, the highest field reported, with a saturation of resistivity at 240 T. Comparison with the Pauli limit Bp=155 T expected for this material suggests that the critical field is limited by spin paramagnetism.Received 20 February 1998DOI:https://doi.org/10.1103/PhysRevB.57.R14084©1998 American Physical Society