G.F. Kiuttu

Design, fabrication, and operation of a high-energy liner implosion experiment at 16 megamperes

We discuss the design, fabrication, and operation of a liner implosion system at peak currents of 16 MA. Liners of 1100 aluminum, with initial length, radius, and thickness of 4 cm, 5 cm, and 1 mm, respectively, implode under the action of an axial current, rising in 8 /spl mu/s. Fields on conductor surfaces exceed 0.6 MG. Design and fabrication issues that were successfully addressed include: Pulsed Power-especially current joints at high magnetic fields and the possibility of electrical breakdown at connection of liner cassette insulator to bank insulation; Liner Physics-including the angle needed to maintain current contact between liner and glide-plane/electrode without jetting or buckling; Diagnostics-X-radiography through cassette insulator and outer conductor without shrapnel damage to film.

An Armature-Stator Contact Resistance Model for Explosively Driven Helical Magnetic Flux Compression Generators

Although helical magnetic flux compression generators (HFCGs) have been in use for more than four decades, no one has been able to satisfactorily model their behavior. To bring computed currents into agreement with experimental values, tuning factors or anomalous flux loss factors are used. Such factors are not universal, and they must be adjusted for each generator design, or for different operational parameters (e.g., seed current or load inductance) for a given design. Many HFCG modeling codes have been reported over the years with various types of these empirical factors. One of the recognized issues for HFCGs is magnetic flux loss near the moving contact point between expanding armature and helical stator coil winding. In our new model, we have analytically estimated the rate of magnetic field diffusion in the vicinity of the contact point. When converted to a flux loss rate, we find that it usually scales nonlinearly with the instantaneous current, and that the resulting effective resistance is proportional to the square root of the current. This result applies even at relatively small operating currents. Whereas the usual HFCG resistances drop as the generator length decreases, the contact resistance generally increases throughout operation. While small initially, we find that it usually dominates late in time and ultimately limits the gain of most generators. In this paper, we present the derivation of the contact resistance model and show its effectiveness in estimating current gain for simple HFCG designs using a simple spreadsheet program. The model has also been implemented in the 11/2-D FCG-model code, CAGEN, and an accompanying paper presents CAGEN results for a wide range of HFCGs, benchmarking the new model. The formulation for our model is universal; i.e., there are no adjustable factors, and it has generally enabled calculation of HFCG currents to within 20% of experimentally reported values.

Helical explosive flux compression generator research at the Air Force research laboratory

The inherent high energy density of explosives make them an obvious choice for pulsed power systems requiring high peak power and energy in compact packages. Ongoing research at the Air Force Research Laboratorys Directed Energy Directorate into helical explosive flux compression generators is discussed. These generators provide the initial pulsed power drive for a high voltage, long pulse system, which is the subject of a companion paper. The helical generator research described here centers on experiments utilizing two distinct generator designs, based on 7.6 cm. and 15.2 cm diameter aluminum armatures, respectively. Experiments using several different stator coil winding schemes with these armatures are described.

Recent Advances in Modeling Helical FCGS

Helical explosively driven magnetic flux compression generators (FCGs) have been intensively investigated for more than four decades, because of their ability to amplify electrical current and magnetic energy with high gain and relatively small size. Whereas coaxial-geometry FCGs have lent themselves to reasonably accurate modeling, helical FCGs have always been considered anomalously lossy, with calculated performance invariably exceeding observed performance - often by factors of two or more in peak output current. With the advent of the analytically derived Kiuttu contact resistance model (KCRM), it has become possible to approximately account for the losses in the vicinity of the contact point between armature and stator without resorting to any empirical tuning factors. Such factors have generally been required by other modeling and simulation codes to achieve agreement with experimental data. Since its introduction, the KCRM has been extended to include the region immediately in front of the contact point as well, thus improving its accuracy. Another key element in modeling the performance of helical FCGs is proper accounting of the proximity effect between adjacent turns of the solenoidal stator winding. This effect alters the magnetic field and current density distributions from their isolated, approximately locally uniform distributions, leading to an effective increase in flux diffusion rates. In order to quantitatively assess this effect, we have run a number of two- dimensional quasi-magnetostatic simulations for varying stator geometries and extracted simplified approximations that can be used in one-dimensional diffusion calculations. We have also examined the details of the circuit model definition (i.e., flux-based from Faradays Law, or the diffusion equation, and energy-based from Poyntings Theorem). The generator equation, derived from the circuit model, involves lumped-element approximations for resistance and inductance, and we have shown that the combination of inductance and resistance, which yields experimental current and time derivative of current, is not unique, and that each lumped element must be consistently defined. We have incorporated these various models and effects into the CAGEN (1&1/2-D) modeling code. As a result, we have been able to accurately calculate the performance of a wide variety of FCGs without using any additional adjustment factors. Representative results, as well as descriptions of the models, will be presented.

Development and Testing of a High-Gain Magnetic Flux Compression Generator

The performance of a high-gain FCG is often limited by internal electrical breakdown caused by the high voltage generated during operation. Modern diagnostic techniques provide the opportunity to diagnose internal breakdowns so that generator designs can be improved. This paper describes the internal breakdowns observed in the JAKE FCG developed at the AFRL during the late 1990s. A revision to the stator winding pattern of the JAKE generator has led to improved control of the internal voltage. Designated JILL, the revised generator has substantially better flux transport efficiency, particularly at higher seed current. The techniques employed to design the new stator winding and the results of development testing are presented.

Benchmarking the Care'n Co. Flux Compression Generator Code, CAGEN, Implementing the Kiuttu Contact Resistance Model

The PC-based program CAGEN has been described before and has since continued development. The most recent innovation implemented in CAGEN is the Kiuttu Contact Resistance Model (KCRM). This model is described elsewhere in these proceedings and represents the most important step forward for FCG modeling observed in many years. In this paper, the performance of a very large range of helical flux compression generators is computed using CAGEN, with no adjustable tuning factors. These generators span from the small Lawrence Livermore National Laboratory Minigen and the Lobo, each less than 100 cubic centimeters volume, to the quite large Los Alamos National Laboratory Mark IX, which is more than 220 liters in size. The examples range over a factor of 1000 in output current and over a factor of 100,000 in output energy, and represent different construction techniques. The results of eight such benchmark calculations using CAGEN, with the KCRM, are never in error more than 18% with respect to reported experimental current values.

Flux Compression Generator Development at the Air Force Research Laboratory

The Air Force Research Laboratory (AFRL) maintains an extensive capability for the design, analysis, construction and testing of explosive pulsed power (EPP) components. Three flux compression generators (FCGs) were designed as part of an EPP technology development effort sponsored by AFRL and the Defense Advanced Research Projects Agency (DARPA). A secondary-stage, high-current FCG was designed to deliver 10 MA into a nominal load inductance of 80 nH from an initial generator inductance of 1.6 muH that is seeded with 1 MA. We have also developed a coaxial FCG to deliver more than 20 MA into a 2 nH load. The initial flux in the coaxial chamber (60 nH at 1.5 MA) is compressed uniformly using a copper armature, which is simultaneously initiated using a slapper detonator. Either of these two FCGs can be seeded with a third generator design: a high-gain, helical FCG. This model serves as our workhorse generator capable of delivering 2 MA into a 0.5 muH inductive load. It has also been operated into load inductances ranging from 0.1 to 2.0 muH with comparable flux delivery. All experiments are conducted on an explosive test range located on Kirtland Air Force Base [1]. The design effort is supported by powerful computer modeling using CAGEN [2], CALE and MACH2. Design features for all three FCGs are presented in this paper with results from recent explosive tests.

Full Axial Coverage Radiography of Deformable Contact Liner Implosion Performed with 8 cm Diameter Electrode Apertures

diameter ratio, radial convergence, uniformity, and implosion velocity suitable forcompressing anFRC[3]. We obtained full axial coverage radiography ofa Ourrecent progress hasbeentoreplace themorestandard deformable contact imploding liner. Thisradiographic data sliding liner-electrode contacts withdeformable linerindicates thefeasibility ofusing avarying thickness inalong electrode contacts, whichenables theuseoflarge cylindrical solid liner, driven asa 12megampZ-pinch, to electrode apertures, suitable forFRCinjection. SeeFig. 1 achieve factor - 16cylindrical convergence, while using 8cm foraillustration ofthis concept. diameter aperture electrodes. TheAlliner was30cmlong, with9.78cminner diameter forits full length, 10.0cmouter Research ontheuseofimploding liners to diameter forthecentral 18cm ofitslength, andouter compress plasmas hasbeenreported byanumberof diameter increased linearly to10.2cmat1cmfromeitherresearchers. Thisincludes suggesting thegeneral concept electrode, andto11cmatelectrode contacts. Theelectrode ofusing liners tocompress plasma, andresearch on apertures allow injection ofField Reversed Configurations in shorter orlowervelocity liner implosions [4-17], and proposed future experiments onmagnetized target fusion. implosion ofaCu-Wliner withexplosives tocompress Indexterms: capacitor bank, Field Reversed Configuration, flux to200T[18]. FRC,Magnetized Target Fusion, MTF,imploding liner, radiography, megamp Uniform-thickness liner Variable-thickness

Predicting the threshold for onset of internal electric breakdown in helical flux compression generators using CAGEN and CALE

The objective of this work is to provide predictability for the threshold of the onset of internal electrical breakdown for helical magnetic flux compression generators (HFCG) to enable higher performance and higher voltage designs and reduce the cost of relying on empirical design methods. The Caren LLC has used the code CAGEN [1] in conjunction with CALE [2] to investigate the phenomenon of internal electrical breakdown in these devices. The CAGEN was modified to compute the peak electric field strength between the armature and the stator as well as between adjacent wires of the stator. Shock compressed gas within the HFCG plays an essential role so the 2-D hydro-code CALE was necessary in order to get a full prediction of the thresholds. An overview of the modeling and the comparison of experiment are presented. An issue is the role bifurcations play in the electric field breakdown. A new code BSBIF [3] models some of the effects. In order to confirm the modeling, a set of very special HFCGs were fabricated and then fired by Hyperspectral Sciences, Inc. The fabrication details [4], the experimental set-up, and the measurements [5] were entirely the work of HSI.

Dynamic deformation of a solenoid wire due to internal magnetic pressure, revised

Deformation of the wire used in the windings of an inertially confined (single use) solenoid used to produce a pulsed high magnetic field is potentially the limiting factor for the magnitude and duration of the magnetic field produced. The rising magnetic pressure at the wire surface becomes large enough to cause the cross section of the wire to plastically deform on a time scale shorter than the overall solenoid disassembly time. This may result in short circuiting due to insulator breakage and/or physical contact of adjacent windings. An analytic approximation modeling the deformation dynamics is presented which takes into account both inertial and material yield strength effects. The model is validated by comparison to two dimensional magnetohydrodynamic simulations of the process by Numerexs MS Windows version of AFRLs MACH2. Cases ranging from those where yield strength has a negligible effect on the deformation to where yield strength is significant are considered. This paper expands on work presented at the previous IEEE IPPC [E.L. Ruden et al., 2001].