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The HERMES III accelerator is an 18–20-MeV linear induction accelerator constructed at Sandia National Laboratories in the late 1980s and which continues operation to this day. As part of recent modernization efforts, the laser triggering system on the accelerator has been replaced with a newly designed solid-state system. This system consists of ten Nd:YAG lasers, each having a nominal output energy of 40–45 mJ at a wavelength of 266 nm. The beam from each laser is split such that it triggers two of the Rimfire gas switches on the accelerator. Compared to the previous laser triggering system, this arrangement makes it possible to more readily tailor the final output pulse shape, and the overall reliability of this key accelerator system is now improved. The design of the new laser triggering system is presented here, along with details pertaining to the energy budgeting, optical beam paths, and electrical triggering of the lasers. The initial operational data from the HERMES III accelerator using this new triggering system are also presented.

We describe a direct electro-optical approach to measuring a strong 118 MV/m narrow pulse width (~33 ns) electric field in the magnetically insulated transmission line (MITL) of a pulsed power accelerator. To date, this is the highest direct external electric field measured electro-optically in a pulsed power accelerator, and it is between two to three orders of magnitude higher than values reported in comparable high energy scientific experiments. The MITL electric field is one of the most important operating parameters in an accelerator and is critical to understanding the properties of the radiation output. However, accurately measuring these high fields using conventional pulsed power diagnostics is difficult due to the strength of interfering particles and fields. Our approach uses a free-space laser beam with a dielectric crystal sensor that is highly immune to electromagnetic interference and does not require an external calibration. Here we focus on device theory, operating parameters, laboratory and pulsed power accelerator experiments as well as challenges that were overcome in the measurement environment.

The field-reversed configuration heating experiment (FRCHX) was designed to form closed-field-line magnetized target plasmas for magnetoinertial fusion and other high energy density plasma research. These plasmas are in a field-reversed configuration and are formed via a reversed-field theta pinch on an already magnetized background plasma. To extend the duration and temporal uniformity of the pinch, the capacitor bank driving the reversed-field discharge is crowbarred near the current peak. Four parallel rail-gap switches are used on the FRCHX for this application to ensure a low-inductance crowbar discharge path and to accommodate the large magnitude of the discharge current (often greater than 1 MA). Historically, parallel operation of spark gap switches in a crowbarring arrangement has often proved to be difficult due to the very low voltage present on the bank and across the switches at the time of peak current. In a low-inductance design, triggering can be further complicated by the rapid collapse of what little voltage there is across the switches as soon as the first spark gap begins conduction. This paper reports on the efforts that were made to develop a low-inductance crowbar switch for the FRCHX and to ultimately enable successful triggering and operation of the four parallel rail-gap switches used in the crowbar. The design of the low-inductance parallel switch assembly is presented first, followed by a description of the triggering scheme employed to ensure conduction of all four switches.

The objective of the field-reversed configuration heating experiment (FRCHX) is to obtain a better understanding of the fundamental scientific issues associated with high-energy density laboratory plasmas (HEDLPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion, laboratory astrophysical research, and intense radiation sources, among others. To create HEDLP conditions, a field-reversed configuration (FRC) plasma of moderate density is first formed via reversed-field theta pinch. It is then translated into a cylindrical aluminum flux conserver (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically driven implosion of the solid liner. A requirement is that, once the FRC is stopped within the solid liner, the trapped flux inside the FRC must persist while the compression process is completed. With the present liner dimensions and implosion drive bank parameters, the total time required for implosion is ${\sim}{\rm 25}~\mu{\rm s}$ . Lifetime measurements of recent FRCHX FRCs indicate that trapped lifetimes following capture are now approaching ${\sim}{\rm 14}~\mu{\rm s}$ (and therefore, total lifetimes after formation are now approaching ${\sim}{\rm 19}~\mu{\rm s}$ ). By separating the mirror and translation coil banks into two so that the mirror fields can be set lower initially, the liner compression can now be initiated 7–9 $\mu{\rm s}$ before the FRC is formed. A discussion of FRC lifetime-limiting mechanisms and various experimental approaches to extending the FRC lifetime will be presented.

Magnetized target fusion (MTF) is a means to compress plasmas to fusion conditions that uses magnetic fields to greatly reduce electron thermal conduction, thereby greatly reducing compression power density requirements. The compression is achieved by imploding the boundary, a metal shell. This effort pursues formation of the field-reversed configuration (FRC) type of magnetized plasma, and implosion of the metal shell by means of magnetic pressure from a high current flowing through the shell. We reported previously on experiments demonstrating that we can use magnetic pressure from high current capacitor discharges to implode long cylindrical metal shells (liners) with size, symmetry, implosion velocity, and overall performance suitable for compression of FRCs. We also presented considerations of using deformable liner-electrode contacts of Z-pinch geometry liners or theta pinch-driven liners, in order to have axial access to inject FRCs and to have axial diagnostic access. Since then, we have experimentally implemented the Z-pinch discharge driven deformable liner-electrode contact, obtained full axial coverage radiography of such a liner implosion, and obtained 2frac12 dimensional MHD simulations for a variety of profiled thickness long cylindrical liners. The radiographic results indicate that at least 16 times radial compression of the inner surface of a 0.11-cm-thick Al liner was achieved, with a symmetric implosion, free of instability growth in the plane of the symmetry axis. We have also made progress in combining 2frac12-D MHD simulations of FRC formation with imploding liner compression of FRCs. These indicate that capture of the injected FRC by the imploding liner can be achieved with suitable relative timing of the FRC formation and liner implosion discharges.

Two experiments showing continuous, real-time measurements of the radial convergence of a high-aspect-ratio aluminum flux conserver are presented. These results were obtained by measuring the compression of both axial and radial components of an internal low-intensity magnetic field. Repeatable flux conserver compressions of this type, uniform to 10:1 compression ratio, form a step toward achieving magnetized target fusion, where a plasma of appropriate temperature and density would be introduced into the flux conserver for compression to fusion conditions. While X radiographs show this compression ratio was achieved, the magnetic field probe signals were cut off earlier. Axial component measurements resulted in compression ratios of 7:1 and 6.3:1, for the first and second compressions, before the magnetic probe signals were lost. Radial component measurements disagree with the axial probe results. Although the discrepancy between axial and radial probe measurements is not completely understood, possible explanations are presented.

We are planning experiments using a field reversed configuration plasma injected into a metal cylinder, which is subsequently electrically imploded to achieve a fusing plasma. Diagnosing this plasma is quite challenging due to the short timescales, high energy densities, high magnetic fields, and difficult access. We outline our diagnostic sets in both a phase I study (where the plasma will be formed and translated), and phase II study (where the plasma will be imploded). The precompression plasma (diameter of only 8–10 cm, length of 30–40 cm) is expected to have n∼1017 cm−3, T∼100–300 eV, B∼5 T, and a lifetime of 10–20 μs. We will use visible laser interferometry across the plasma, along with a series of fiber-optically coupled visible light monitors to determine the plasma density and position. Excluded flux loops will be placed outside the quartz tube of the formation region, but inside of the diameter of the θ-pinch formation coils. Impurity emission in the visible and extreme ultraviolet range will b...

Summary form only given. The objective of the Field-Reversed Configuration Heating Experiment (FRCHX) is to obtain a better understanding of the fundamental scientific issues associated with high energy density plasmas (HEDPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion (MIF), laboratory astrophysical research, and intense radiation sources, among others. To create the HEDP, a field-reversed configuration (FRC) plasma of moderate density is first formed via reversed-field theta pinch. It is then translated into a cylindrical aluminum shell (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically-driven implosion of the shell. A requirement is that once the FRC is stopped within the shell, the trapped flux inside the FRC must persist while the compression process is completed. With the present shell dimensions and drive bank parameters, the total time required for implosion is ~25 microseconds. Lifetime measurements of recent FRCHX FRCs indicate trapped lifetimes now approaching ~14 microseconds, and with recent experimental modifications the liner compression can be initiated considerably earlier before formation is completed in order to close that gap further. A discussion of FRC lifetime-limiting mechanisms will be presented along with a description of FRCHX and recent changes that have been made to it. Results from recent experiments aimed at lengthening FRC lifetime will also be presented.

We described efforts to advance the state of the art for Magnetized Target Fusion (MTF) using Field Reversed Configuration (FRC) plasmas. The work is primarily experimental. We document production, translation, and trapping of FRCs in a coaxial system, and initial efforts to drive cylindrical aluminum liners with the Shiva Star fast capacitor bank to compress magnetic flux and magnetized plasma. We describe the pulsed power systems used to create and manipulate the plasma and the diagnostics employed to determine magnetic field and plasma characteristics. We also describe modifications to the plasma system for puffed gas injection, radio frequency (RF) systems for pre-preionization of the gas, and use of biased rings to attempt to control the rotation and subsequent instability of the plasma.

Experiments to form, compress, and accelerate compact toroids are described. A 1-m-diam, two-stage, puffed gas, magnetic field embedded coaxial plasma gun is used. Emphasis is on conical compression. Discharges were in the several mega-ampere, few microsecond rise time range. Magnetic probe data suggest that l/(r{center_dot}{delta}r) compression of the toroid field is achieved, consistent with theoretical prediction. The magnetic field pulse and electron density pulse due to the compact toroid correlate in space and time. The compact toroid species is the injected gas species and precedes electrode plasma by several microseconds. The poloidal magnetic field precedes the azimuthal magnetic field. The time of arrival of the axial magnetic field compared with the axial position is consistent with the mean current axial position trajectory obtained from inductance growth. 8 refs., 6 figs.

We have magnetically driven a tapered-thickness spherical aluminum shell implosion with a 12.5 MA axial discharge. The initially 4 cm radius, 0.1 to 0.2 cm thick, [plus minus]45[sup [degree]] latitude shell was imploded along conical electrodes. The implosion time was approximately 15 [mu]sec. Radiography indicated substantial agreement with 2D-MHD calculations. Such [ital calculations] for this experiment [ital predict] final inner-surface implosion velocity of 2.5 to 3 cm/[mu]sec, peak pressure of 56 Mbar, and peak density of 16.8 g/cm[sup 3] ([gt]6 times solid density). The principal experimental result is a demonstration of the feasibility of electromagnetic-driven spherical liner implosions in the cm/[mu]sec regime.

For high energy density magnetized target fusion experiments at the Air Force Research Laboratory FRCHX machine, obtaining multi-frame soft x-ray images of the field reversed configuration (FRC) plasma as it is being compressed will provide useful dynamics and symmetry information. However, vacuum hardware will be destroyed during the implosion. We have designed a simple in-vacuum pinhole nosecone attachment, fitting onto a Conflat window, coated with 3.2 mg∕cm(2) of P-47 phosphor, and covered with a thin 50-nm aluminum reflective overcoat, lens-coupled to a multi-frame Hadland Ultra intensified digital camera. We compare visible and soft x-ray axial images of translating (~200 eV) plasmas in the FRX-L and FRCHX machines in Los Alamos and Albuquerque.

The AFRL Shiva Star capacitor bank (1300 microfarads, up to 120 kilovolts) operated typically with 4 to 5 megajoules of electrically stored energy, with axial discharge currents of 10 to 15 megamps, and current rise times of approximately 10 microseconds, has been used to drive metal shell (solid liner) implosions in several geometries, including long cylindrical designs, which are suitable for compression of axial magnetic fields to multi-megagauss levels. Such imploding liners are also suitable for compressing magnetized plasmas to magneto-inertial fusion conditions. Magneto-Inertial Fusion (MIF) approaches take advantage of embedded magnetic field to improve plasma energy confinement by reducing thermal conduction relative to conventional inertial confinement fusion (ICF). MIF reduces required implosion speed and convergence ratio relative to ICF. AFRL, its contractors and collaborating institutions LANL, UNM, and UNR have developed one version of magnetized plasmas at pre-compression densities, temperatures, and magnetic fields that may be suitable for such compression. These are Field Reversed Configurations (FRCs). This effort reliably formed, translated, and captured FRCs in magnetic mirrors inside10 cm diameter, 30 cm long, mm thick metal shells or liners in preparation for subsequent compression by liner implosion; imploded a liner with an interior magnetic mirror field, obtaining evidence for compression of 1.36 T field to approximately 500 T; performed a full system experiment of FRC formation, translation, capture, and imploding liner compression operation; identified by comparison of 2D-MHD simulation and FRC capture experiments factors limiting the closed- field lifetime of FRCs to about half that required for good liner compression of FRCs to multi-keV, 1019 ion/cm3, high energy density plasma (HEDP) conditions; and designed and prepared hardware to increase that closed field FRC lifetime to the required amount. Those lifetime extension experiments have obtained imaging evidence of FRC rotation (which is a phenomenon that limits such closed field lifetimes), and of initial rotation control measures slowing and stopping such rotation. These and the results of subsequent closed field plasma lifetime and compression experiments and related simulations will be discussed.

The goal of the Field-Reversed Configuration Heating Experiment (FRCHX) is to demonstrate magnetized plasma compression and thereby provide a low cost approach to high energy density laboratory plasma (HEDLP) studies, which include such topics as magneto-inertial fusion (MIF). A requirement for the field-reversed configuration (FRC) plasma is that the trapped flux in the FRC must maintain confinement of the plasma within the capture region long enough for the compression process to be completed, which is approximately 20 microseconds for FRCHX. Current lifetime measurements of the FRCs formed with FRCHX show lifetimes of only 7 ∼ 9 microseconds once the FRC has entered the capture region. A description of the pulsed power systems that comprise FRCHX will be presented along with an overview of the magnetic and plasma diagnostics fielded on the experiment. Results from recent FRCHX experiments will then be presented, and possible reasons for the lifetime limitations will be discussed along with several approaches for overcoming these limitations.

Research on forming, compressing, and accelerating milligram‐range compact toroids using a meter diameter, two‐stage, puffed gas, magnetic field embedded coaxial plasma gun is described. The compact toroids that are studied are similar to spheromaks, but they are threaded by an inner conductor. This research effort, named marauder (Magnetically Accelerated Ring to Achieve Ultra‐high Directed Energy and Radiation), is not a magnetic confinement fusion program like most spheromak efforts. Rather, the ultimate goal of the present program is to compress toroids to high mass density and magnetic field intensity, and to accelerate the toroids to high speed. There are a variety of applications for compressed, accelerated toroids including fast opening switches, x‐radiation production, radio frequency (rf) compression, as well as charge‐neutral ion beam and inertial confinement fusion studies. Experiments performed to date to form and accelerate toroids have been diagnosed with magnetic probe arrays, laser interf...

Experiments on FRC formation and translation into the interior of a metal shell or liner have been conducted at AFRL. Flux exclusion, collimated light, and interferometer data on magnetized plasma injection will be presented. These are a pre-requisite for FRC compression by liner implosion, experiment progress on which will also be presented. FRC translation, capture, and compression experiments all use primarily axial ∼ 2 Tesla guide and mirror fields established inside the liner, using ∼ 5 millisecond rise time discharges into an array of pulsed magnet coils surrounding the liner implosion portion of the device. A 12 MA, 4.5 MJ axial discharge drives the liner implosion for compression experiments. The FRC capture experiments use 3 capacitor discharges into a segmented theta coil surrounding the FRC formation region to establish a bias field, accomplish pre-ionization of deuterium gas, and provide the reverse field main theta discharge (∼ 1 Megamp) which forms the FRC. This is aided by two cusp field discharges. The guide and mirror fields enable translation of the FRC and its capture in the liner interior region. Diagnostics include pulsed power (current and voltage), magnetic field, field exclusion, He Ne laser interferometry, imaging and spectroscopy, radiography, and both activation and time-of-flight neutron detection. Design features and operating parameters are guided by 2D-MHD simulations.

Arrays of Si PIN diodes are fielded on radiation producing experiments on the Shiva Star pulsed power generator at AFRL's Directed Energy Directorate. This work is a continuation of earlier work done on Shiva Star1. Each diode is filtered to respond to a part of the band of interest and the signals are then deconvolved to generate the radiation spectrum as a function of time. The plasma temperature may be inferred from this method, assuming Bremsstrahlung radiative processes, and impurities may be detected by a departure from the mean temperature.

Summary form only given. The Los Alamos National Laboratory (LANL) collaboration with Air Force Research Laboratory (AFRL) collaboration is close to a physics demonstration of compressional heating in a Magneto Inertial Fusion (MIF) plasma target. These first Magnetized Target Fusion (MTF) experiments will use solid aluminum flux compressor shells. The experimental high density Field Reversed Configuration (FRC) can be made to translate fast enough so that FRC lifetime is not an issue. We show some initial translation data from the Los Alamos FRC experiment FRXL that characterize the translated target plasma. We have taken advantage of the LANL experience so that a near duplicate of FRXL has come up in several months. The solid liner MTF is only one of several magnetized, pulsed MIF fusion schemes that are being pursued. We outline the present status of MTF including target formation, translation to a trapping region, and compression results.

Interferometric measurements of the line‐averaged free electron density of plasma compact toroids as part of preliminary toroid formation experiments are presented. These experiments are in preparation for compact toroid acceleration experiments to be performed at Phillips Laboratory’s High Energy Plasma Division. The primary instrument used was a Michelson (double pass) interferometer operating with a HeNe laser at a wavelength of 633 nm. Density measurements of hydrogen and argon plasmas are correlated with magnetic field and spectroscopic measurements to characterize the toroid mass, velocity, and impurity distribution.

Experiments with coaxial plasma guns at currents in excess of ten megamperes have resulted in the production of high‐voltage pulses (0.5 MV) and hard x radiation (10–200 keV). The x‐radiation pulse occurs substantially after the high‐voltage pulse suggesting that high‐energy electrons are generated by dynamic processes in a very high speed (≳106 m/s), magnetized plasma flow. Such flows, which result from acceleration of relatively low‐density plasma (10−4 vs 1.0 kg/m3) by magnetic fields of 20–30 T, support high voltages by the back electromotive force‐u×B during the opening switch phase of the plasma flow switch. A simple model of classical ion slowing down and subsequent heating of background electrons can explain spectral evidence of 30‐keV electron temperatures in fully stripped aluminum plasma formed from plasma flows of 1–2 × 106 m/s. Similar modeling and spectral evidence indicates tungsten ion kinetic energies of 4.5 MeV and 46 keV electron temperatures of a highly stripped tungsten plasma.

Summary form only given. We present experimental data indicating the feasibility of using a varying thickness in a long cylindrical solid liner, driven as a Z-pinch, to achieve factor /spl sim/17 cylindrical convergence, while using large aperture electrodes. The Al liner was 30 cm long, with 9.78 cm inner diameter for its full length, 10.0 cm outer diameter for the central 18 cm of its length, and outer diameter increased linearly to 10.2 cm at 1 cm from either electrode, and to 11 cm at electrode contacts. The two electrodes had 8 cm diameter holes or apertures, to allow injection of field reversed configurations (FRCs) in proposed future experiments on magnetized target fusion (MTF). 2D-MHD simulations as well as flash radiography and axial view fast optical photography indicate that this varying thickness results in a deforming, nearly non-sliding, liner-electrode contact. The Z-pinch geometry discharge was driven by the AFRL Shiva Star 1300 microfarad capacitor bank, charged to 84 kilovolts, with /spl sim/44 nanoHenry initial inductance, sub-milliohm external resistance, plus a safety fuse as described. The current history was similar to that for similar, uniform thickness long liner implosions with the more traditional sliding liner-electrode contact, reported. The current exceeded 11 megamps with /spl sim/10 microsecond risetime. The implosion time was 22.5 microseconds. The inner surface implosion velocity exceeded 0.5 cm/microsecond, and kinetic energy was /spl sim/1 megajoule.

Summary form only given. We describe a novel dense plasma focus experiment at the Shiva Star facility (operated at 1 MJ-2 MJ capacitor bank energy), which uses a compact toroid (CT) magnetized plasma flow switch to initiate the focus implosion downstream from a shielded vacuum insulator. The CT armature stably and reproducibly translates up to 3 MA from the vacuum feed region through coaxial electrodes to a puffed-gas central load. The inertia of the 1 mg CT and the work that must be done in compressing the internal magnetic fields during the translation provide a delay in current delivery to the pinch of 5-/spl mu/s ps, which matches the bank quarter cycle time relatively well. Effectiveness of the current delivery was monitored by primarily by inductive probes in the PFS region, fast photography of the focus, and X-ray and neutron measurements of the pinch. No evidence of current loss was observed.

Summary form only given, as follows. As the result of international cooperation, we previously developed and tested a 30 MJ generator which can be used to drive high-power stationary plasma radiation sources. For further improvement of the generator design and increase in output energy it is necessary to study in more detail the physical processes and to minimize the main loss mechanisms. In this paper we present experimental results and compare them with numerical simulations. It is shown that the main losses of magnetic flux are associated with nonlinear diffusion of the field in conductors and flux disruption at section joints. Expected performance parameters for an updated design with decreased flux losses are given.

A series of four liner implosion experiments, denoted the Near Term Liner Experiments (NTLX) was recently conducted on the Shiva Star capacitor bank at the Air Force Research Laboratory (AFRL). Measurement of the driving currents in these experiments is required for post-shot analysis of the liner implosion and experiments conducted in the target cylinder. A Faraday rotation measurement was fielded on Shiva Star to measure the current and compare with the current measured by a Rogowski coil technique. The Faraday rotation technique measured the 16 MA currents in these experiments with better than 1 % precision. In addition, six B-dot probes were fielded at equal angles around a circle in the powerflow channel outside the liner to measure the symmetry of the liner impact on the target cylinder. The B-dot probes measure the local I-dot, which has a jump when the liner impacts the target cylinder. A high-pass filter allows one to measure this jump more accurately. From the relative timing of the jump signals, the offset of the liner axis and the circularity of liner are inferred.

Summary form only given, as follows. A series of Near Term Liner Experiments (NTLX) was recently, conducted on the Shiva Star capacitor bank at the AFRL. These experiments consisted of an aluminum liner that is magnetically imploded onto a central target by self-induced radial Lorentz forces. The central target consists of an inner Lucite cylinder surrounded by an outer Sn layer. Shock propagation within the Lucite is measured to provide information for hydrodynamic code verification and validation. Target design utilized the adaptive mesh refinement (AMR) Eulerian hydrodynamics code RAGE in 2- and 3D. 1D simulations of the liner driver utilizing the RAVEN MHD code set initial liner/target interaction parameters, which are then used as initial conditions for the RAGE calculations. Both codes utilized standard SESAME equation-of-state data to close the system of equations. At liner/target impact, a convergent shock is generated that drives subsequent hydrodynamics experiments. In concentric targets, cylindrically symmetric shocks will converge on axis, characterizing the symmetry of the liner driver. By shifting the target center away from the liner symmetry axis, variations in shock propagation velocity generate off-center shock convergence. Comparison of experimentally measured and simulated shock trajectories are discussed as are convergence effects associated with cylindrical geometry. Efforts are currently underway to compare equation-of-state effects by utilizing a Gruneisen EOS instead of the original SESAME tables. Further comparisons are made to quasi-analytical solutions in cylindrical geometry [Harlow, F.H. and Amsden, A.A., Fluid Dynamics, Los Alamos National Laboratory Report LA-4700, (1971)] to examine shock acceleration due to radial convergence.

Summary form only given. The design, calculations, and experimental progress on forming Field Reversed Configuration's (FRCs) suitable for subsequent compression by an imploding metal liner to Magnetized Target Fusion (MTF) conditions are presented and discussed. The desired initial FRC parameters are density /spl sim/1017 cm/sup -3/, temperature /spl sim/100 to 300 eV, length /spl sim/30 cm, separatrix radius /spl sim/2.5 cm, magnetic field between separatrix and liner /spl sim/5 Tesla. This effort is being pursued using the 36 mf, 250 kilojoule Colt capacitor bank at LANL, and using the 110 mf, 750 kilojoule formation capacitor bank (FB), which is auxiliary to the 1300 mf, 9 megajoule Shiva Star capacitor bank at AFRL, as well as smaller auxiliary banks to drive smaller, related solenoids. Crowbarring of both the Colt and FB facilities will be done, using triggered pressurized railgap switches. Initial FRC diagnostics include laser interferometry and magnetic field probe arrays (to observe field exclusion, that is, diamagnetic effect). Later experiments will include spectroscopy for impurity detection and temperature measurement, and Thompson scattering. The 30 cm long, 5 cm initial radius, 0.11 cm thick Al liner is imploded by magnetic pressure from an 11 megamp axial discharge driven by the 1300 mf Shiva star capacitor bank. Initial successful implosions are reported Transactions on Plasma Science (Degnan, 2001). Other aspects of FRC formation and compression were discussed by K. Schoenberg, R. Siemon et al. (1998). 2D-MHD simulations of planned FRC formation are also presented. These predict that with 100 milliTorr deuterium gas prefill, which corresponds to /spl sim/6/spl times/10/sup 15/ cm/sup -3/ initial ion/atomic density, /spl sim/0.5 Tesla initial bias axial magnetic field, and /spl sim/3 ms risetime, 5 Tesla reverse bias field, that the desired density and temperature FRC is formed.

Summary form only given. The design, calculations, and experimental progress for theta pinches to produce field reversed configuration's (FRC's) suitable for subsequent compression by an imploding metal liner to magnetized target fusion (MTF) conditions are presented and discussed. The desired initial FRC parameters are density /spl sim/10/sup 17/ cm/sup -3/, temperature /spl sim/100 to 300 eV, length /spl sim/30 cm, separatrix radius -2.5 cm, magnetic field between separatrix and liner /spl sim/5 Tesla.

As part of ongoing hydrodynamic code validation efforts, a series of Near Term Liner Experiments (NTLX) was designed for the Shiva Star capacitor bank at the Air Force Research Laboratory (AFRL). A cylindrical aluminum liner that is magnetically imploded onto a central target by self-induced radial Lorentz forces drove the experiments. The behavior of the target was simulated using the adaptive mesh refinement (AMR) Eulerian hydrodynamics code RAGE in 2- and 3-D. One-dimensional simulations of the liner driver utilizing the RAVEN MHD code were used to predict the liner density and temperature profiles as well as the velocity at impact time. At liner/target impact, a convergent shock is generated that drives subsequent hydrodynamics experiments. In concentric targets, cylindrically symmetric shocks will converge on axis. This characterizes the symmetry of the liner driver. By shifting the target center away from the liner symmetry axis, material dependencies in the shock propagation velocity generate off-center shock convergence. Both codes show excellent agreement over the majority of the shock and interface trajectories. However, a small but significant discrepancy between codes does occur during the last few millimeters of run in when convergence effects are greatest. Comparisons with experimental data show similar shock velocities being measured. However, the simulated shock arrives at a given trajectory location nearly 100 ns earlier than experimentally measured.

Compression and acceleration of magnetized plasmas is relevant to fusion for two reasons. Certain types of magnetized plasmas can be compressed and accelerated without fluid instability growth. These are magnetized plasmas rings or compact toroids1,2. Because of their stability, they can be compressed and accelerated over meters of distance and several microseconds of time, enabling economic scaling to much higher energy operation. Other types of implosions and compressions, e.g., Z-pinches, are limited by instability growth3 to much shorter acceleration distances (a few cm) and times (less than 100 nanoseconds), making it very expensive to scale their operating energies to the fusion regime. An important second advantage of magnetized plasmas is that discussed by Lindemuth and Kirkpatrick in their magnetized target fusion (MTF) concept4. Reduced electron thermal conduction losses and increased alpha energy deposition result in reduced requirements of fuel density-radius product for achieving fusion ignition. In this paper, we discuss two experimental efforts at the Phillips Laboratory relevant to this topic. These are our Compact Toroid2,5,6,7 and Solid Liner/Working Fluid8,910,11 efforts. Though these efforts have potential fusion application, their present support is for the applications of intense X-ray generation and achieving high density and pressure in the laboratory, respectively.

Detailed calculations of the formation, guide, and mirror applied magnetic fields in the FRC compression-heating experiment (FRCHX) were conducted using a commercially available generalized finite element solver, COMSOL Multiphysics(®). In FRCHX, an applied magnetic field forms, translates, and finally captures the FRC in the liner region sufficiently long to enable compression. Large single turn coils generate the fast magnetic fields necessary for FRC formation. Solenoidal coils produce the magnetic field for translation and capture of the FRC prior to liner implosion. Due to the limited FRC lifetime, liner implosion is initiated before the FRC is injected, and the magnetic flux that diffuses into the liner is compressed. Two-dimensional axisymmetric magnetohydrodynamic simulations using MACH2 were used to specify optimal magnetic field characteristics, and this paper describes the simulations conducted to design magnetic field coils and compression hardware for FRCHX. This paper presents the vacuum solution for the magnetic field.

The Phillips Laboratory's MARAUDER Compact Toroid acceleration experiments are described. The acceleration discharge is the second stage of a two-stage co-axial plasma gun discharge. The first stage discharge forms a compact torus. First stage discharge parameters are 110 microfarad, 60 to 80 kv initial voltage, 40 nanohenry initial inductance, 6 milliohm series resistance, 2 to 2.6 Megamps peak current, 1 to 2 milligrams initial gas mass (puff injected), 0.1 Tesla initial radial field (double solenoid injected). Second stage discharge parameters are 108 microfarad, 40 to 80 kv initial voltage, 25 nanohenry initial inductance. The two-stage gun geometry parameters are 44.8 cm inner conductor radius, 52.4 cm first stage outer conductor radius, 62.8 cm second stage outer conductor radius, transition from first stage to second stage at 20 cm axial distance downstream from gas injection plane, and variable (up to 80 cm) axial length of second stage. Data from fast photography, arrays of radial, azimuthal, and axial magnetic probes, He-Ne and microwave interferometry, time resolved optical spectroscopy, time gated and spectral line filtered photography, and X-ray detectors will be presented and discussed. The acceleration discharge appears to sharpen and strengthen radial magnetic field reversals, suggesting driven reconnection. The effects of varying the second stage geometry on acceleration discharge current delivery will be discussed.

Summary form only given. Magnetic loop probes with areas ranging from 0.2 to 0.9 cm2 were used to observe azimuthal, radial, and axial magnetic fields versus time at 28 locations in compact toroid formation discharges. The probes were enclosed in 6-mm-outer-diameter, 4-mm-inner-diameter glass tubes which were inserted through the 90-cm-diameter inner (electrically hot) electrode. Probe signals were sent to an array of Tektronix 7903 oscilloscopes via fiber optic links. Typical field values ranged from 0.1 to 2 T. Typical discharge currents had a 3-ms risetime. Comparison with 2D-MHD predictions was made, and good agreement was shown

The plasma flow switch utilizes the nonlinear and nonuniform dynamics of a plasma discharge in vacuum to accumulate magnetic energy in times of several microseconds and then release this energy to a load in times of a few hundred nanoseconds. Experiments have been performed with capacitor banks up to 6 MJ, providing currents in excess of 107 A and peak voltages over 0.5 MV. Theoretical models include simple slug dynamics coupled to lumped-circuit analyses, magnetoacoustic considerations of one- and two-dimensional aspects of the plasma flow, and two-dimensional magnetohydrodynamic code calculations. The present article reviews both experimental and theoretical efforts, discusses the use of the plasma flow switch to drive plasma liner implosions and high-energy ion flows, and indicates directions for plasma flow switch applications to very high current, high-energy inductive pulsed power systems.

Using a 1313-?F, 3-nH, 120-kV, 9.4-MJ SHIVA Star capacitor bank, we have performed vacuum inductive store/plasma flow switch (PFS) driven implosions of low mass (200-400 ?g/cm2) cylindrical foil liners of 2-cm height and 5-cm radius. This technique employs a coaxial discharge through a plasma armature, which stores magnetic energy over 3-4 ?s and rapidly switches it to an imploding load as the plasma armature exits the coaxial gun muzzle. The current transferred to the load by the PFS has a rise time of less than 0.2 ?s. With 5-MJ stored energy, we have driven fast liner implosions with a current of over 9 MA, obtaining an isotropic equivalent 2.7-TW 0.5-MJ X-ray yield.