Magneto-inertial fusion (MIF) approaches take advantage of an embedded magnetic field to improve plasma energy confinement by reducing thermal conduction relative to conventional inertial confinement fusion (ICF). MIF reduces required precision in the implosion and the convergence ratio. Since 2008 (Wurden et al 2008 IAEA 2008 Fusion Energy Conf. (Geneva, Switzerland, 13-18 October) IC/P4-13 LA-UR-08-0796) and since our prior refereed publication on this topic (Degnan et al 2008 IEEE Trans. Plasma Sci. 36 80), AFRL and LANL have developed further one version of MIF. We have (1) reliably formed, translated, and captured field reversed configurations (FRCs) in magnetic mirrors inside metal shells or liners in preparation for subsequent compression by liner implosion; (2) imploded a liner with interior magnetic mirror field, obtaining evidence for compression of a 1.36 T field to 540 T; (3) performed a full system experiment of FRC formation, translation, capture, and imploding liner compression operation; (4) identified by comparison of 2D-MHD simulation and experiments factors limiting the closed-field lifetime of FRCs to about half that required for good liner compression of FRCs to multi-keV, 10(19) ion cm(-3), high energy density plasma (HEDP) conditions; and (5) designed and prepared hardware to increase that closed-field FRC lifetime to the required amount. Those lifetime experiments are now underway, with the goal of at least doubling closed-field FRC lifetimes and performing FRC implosions to HEDP conditions this year. These experiments have obtained imaging evidence of FRC rotation, and of initial rotation control measures slowing and stopping such rotation. Important improvements in fidelity of simulation to experiment have been achieved, enabling improved guidance and understanding of experiment design and performance.

In 1991, Turchi et al. [1] reported evidence for a 2,000 km/s aluminum plasma that originated from the upstream boundary of a wire array armature in a plasma flow switch (PFS) [2]. The 2008 article by Turchi et al. [3] posits that if such high Z plasma could instead be composed of deuterium or a deuterium-tritium mixture then the resultant multi-keV plasma would make an effective target for magnetized plasma compression to fusion conditions. This report documents several exploratory tests executed in an effort to achieve significant energy transfer from a plasma flow switch to a deuterium plasma. The first phase of this research concentrated on extension of the earlier work [1, 2] to a lower current system that would emulate the PFS used in series with an imploding liner load. The apparatus was also modified to permit pulsed injection of deuterium gas along the insulated coaxial electrodes between the PFS armature and the vacuum power feed. In analyzing the armature behavior, the initial conditions used in 2-D axisymmetric MHD simulations to approximate the wire-array/polymer film composite armature resulted in significant uncertainty in the validity of the calculations. This uncertainty confounded efforts to improve the opening switch behavior of the armature. Low density foams, commonly used in other high energy density plasma experiments, were seen as a candidate material for the armature that would facilitate greater fidelity between simulations and the experiment. Two subsequent tests were conducted using foam armatures. In both cases, current prematurely shunted upstream in the vacuum feed. Several possible causes were explored for the shunting of the current. Among the modifications implemented, the gas injection system was altered to increase both the quantity of gas adjacent to the armature while facilitating an increased pressure gradient between the armature and the current feed. A series of low energy shots were conducted to examine the impact of several proposed design modifications on current delivery to the armature. One conclusion of these experiments was that it has been very difficult to forestall breakdown in the injected gas as required by Turchi et al. [3]. Nevertheless, two experiments were conducted to evaluate performance with foam armatures. Both experiments exhibited good current delivery to the armature, behaving initially like the low energy experiments. The magnetic flux convected downstream was greater than in any of the prior experiments, though significant work remains to demonstrate the ultra-high-speed plasma flow concept.

A system of two FCG's coupled via "flux-trapping" is described. The driver FCG, designated SAM, was custom-designed for this application. The output of SAM is a single-turn loop that is tightly coupled to the first winding section of a larger FCG, designated JILL. The single-turn driver loop, coupled to 35 turns of the input winding of JILL, provides a calculated flux gain of 28.For the first experimental test of the SAM/JILL system, the SAM generator was seeded with 1.0 kA (flux 0.29 Wb) and produced a current of 472 kA (flux 0.10 Wb) in the coupling loop at crowbar time of the JILL generator.Based on the calculated mutual inductance of 5.95 mu H, the JILL generator began operation with a seed flux of 2.81 Wb. With this seed flux, the expected output current for JILL driving a 0.8 mu H load is 1.8 MA. The measured output current was 884 kA, roughly one half of the expected current. Analysis of the I-dot data from the test shows that this low performance was due to multiple electrical breakdowns in the JILL generator during the interval when the armature-stator contact point was underneath the coupling loop.Subsequent analysis suggests that the electrical breakdowns were the result of flux compression in the coupling loop. Details of the experiment and analysis will be presented. A modification to the SAM/JILL apparatus is proposed to eliminate electrical breakdown.

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 1990's. 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.

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 (1,2). 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 at Megagauss 9 that we had shown experimentally (3) 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 that is suitable for compression of Field Reversed Configurations (FRC's). 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 FRC's 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 2D-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. We have also made progress in combining 2D-MHD simulations of FRC formation with imploding liner compression of FRC's.

Data are presented that are part of a first step in establishing the scientific basis of magnetized target fusion (MTF) as a cost effective approach to fusion energy. A radially converging flux compressor shell with characteristics suitable for MTF is demonstrated to be feasible. The key scientific and engineering question for this experiment is whether the large radial force density required to uniformly pinch this cylindrical shell would do so without buckling or kinking its shape. The time evolution of the shell has been measured with several independent diagnostic methods. The uniformity, height to diameter ratio and radial convergence are all better than required to compress a high density field reversed configuration to fusion relevant temperature and density.

Summary form only given, as follows. The initial design and performance of a magnetic pressure driven imploding solid liner with dimensions suitable for compressing a field reversed configuration (FRC) is presented and discussed. The nominal liner parameters are 30 cm length, 5 cm outer radius, /spl sim/0.1 cm thickness, Al material. The liner is imploded by magnetic pressure from an axial discharge driven by a 1300 microfarad capacitor bank. Other nominal discharge parameters are /spl sim/80 kV initial bank voltage, /spl sim/44 nanohenry initial total inductance, and /spl sim/milliohm series resistance. The discharge current exceeds 10 megamps in /spl sim/9 /spl mu/sec. Several types of calculations indicate that such a liner will implode in /spl sim/22 to 25//spl mu/sec, and will achieve a >0.3 cm//spl mu/sec implosion velocity by the time the liner has imploded to /spl sim/2.5 cm radius. This performance and these dimensions are suitable for FRC formation and compression. The diagnostics for the initial experiments include current (via Rogowski coils and inductive magnetic probes), voltage (via capacitive divider probes), flash radiography, and diagnostic magnetic field compression. Several types of simulations, including two dimensional magnetohydrodynamic simulations, are also discussed.

Axial current discharges were used to implode spherical aluminum shells with thickness tapered to match the external magnetic pressure. These implosions were used to compress hydrogen plasmas with an initial pressure >100 bar and an initial temperature >1 eV. The implosion and compressed plasma effect on a central target were observed radiographically. The target's radius vs time indicates that the plasma pressure reaches similar to 1 Mbar prior to liner-target contact. [S0031-9007(99)08774-8].

Abstract : The field of High Power Microwaves (HPM) has evolved as a result of advances in the field of pulsed power, which has made pulses of electrical energy available that can drive HPM sources to gigawatt levels. One of the most compact forms of pulsed power involves the storage of chemical energy in the form of explosive charges. Explosive magnetohydrodynamic (MHD) generators are electrical power sources, which convert the kinetic energy of moving plasma into useful electrical energy through the magnetic portion of the Lorentz force. This report describes research conducted by the Air Force Research Laboratory to test specific designs of explosively driven magnetohydrodynamic generators. The goal of the research was to investigate the use of gigawatt generators in driving reactive loads appropriate to diode, and ultimately HPM applications. Two test series were performed, the first of which consisted of experiments on a low voltage generator and the second of which had the goal of scaling the existing design to higher voltage while retaining the reactive-type load. The complex problem of diagnostics of the plasma in this explosive test was addressed using fast, temporally resolved, plasma measurements, as well as spectroscopic plasma constituent measurements.

One scheme is considered, acceleration of a ring between coaxial electrodes by a B/sub theta/ field as in a coaxial rail-gun. If the electrodes are conical, a ring accelerated towards the apex of the cone undergoes self-similar compression (focussing) during acceleration. Because the allowable acceleration force F/sub a/ = kappa U/sub m//R (kappa < 1) increases as R/sup -2/, the accelerating distance for conical electrodes is considerably shortened over that required for coaxial electrodes. In either case however, since the accelerating flux can expand as the ring moves, most of the accelerating field energy can be converted into kinetic energy of the ring leading to high efficiency.

High power microwave sources driven by intense relativistic electron beams have been a subject of much research over the last decade. In the course of research upon the issues of RF breakdown, we have developed a diagnostic capable of measuring high RF power levels in high electric field regions. This diagnostic takes advantage of the RF Stark effect. To investigate the effect, a hydrogen-filled glass tube is placed in a region of high RF power. When an RF pulse impinges on the tube, the gas in the tube is excited. By spectral observations of the decay of the gas back to the ground state, the Stark splitting of the atomic states can be measured. This splitting is proportional to both the RF electric field amplitude and frequency. In this paper we present results from a series of experiments performed at the Phillips Laboratory on the relativistic klystron. Spectral measurements were performed in the near field of the radiating antenna using an optical multi-channel analyzer with a half-meter spectrograph. These experiments demonstrate that Stark broadening can be a useful diagnostic in high field environments.

Plasma jet generators have been designed and tested which used an explosive driver and shocktube with a rectangular cross section that optimize the flow velocity and electrical conductivity. The latest in a series of designs has been tested using a reactive load to diagnose the electrical properties of the MHD generator/electromagnet combination. The results of these tests indicate that the plasma jet/MHD generator design does generate a flow velocity greater than 25 km/s and produces several gigawatts of pulsed power in a very small package size. A larger, new generator design is also presented.

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, \ifmmode\pm\else\textpm\fi{}45\ifmmode^\circ\else\textdegree\fi{} latitude shell was imploded along conical electrodes. The implosion time was approximately 15 \ensuremath{\mu}sec. Radiography indicated substantial agreement with 2D-MHD calculations. Such calculations for this experiment predict final inner-surface implosion velocity of 2.5 to 3 cm/\ensuremath{\mu}sec, peak pressure of 56 Mbar, and peak density of 16.8 g/${\mathrm{cm}}^{3}$ (>6 times solid density). The principal experimental result is a demonstration of the feasibility of electromagnetic-driven spherical liner implosions in the cm/\ensuremath{\mu}sec regime.

Research on the formation of a hot hydrogen working fluid, which may be used in multiple concentric solid-density liner implosions, is reported. In such implosions, an axisymmetric outer liner is driven by a multi-megamp axial discharge, and a coaxial inner liner is driven by a working fluid contained between the liners. The fluid is shocklessly compressed to high pressure as the outer liner implodes around it. In the work reported here a 10 to 100 Torr pressure, hydrogen filled coaxial gun discharge was used to inject plasma into a diagnostic chamber simulating an interliner volume. Spectroscopically determined electron densities of between 1017 and 1018 cm−3 and electron temperatures in the 0.5–2.0 eV range were obtained with a fair degree of reproducibility and symmetry. Two-dimensional, time-dependent magnetohydrodyna- mic computer simulations of the working fluid formation experiment have been performed, and the computations suggest that the present experiment achieves electron number densities and temperatures at the lower extreme of these limits, and neutral densities ∼ 0.3–1.0 ×1019 cm−3. The simulations further suggest that the upper range, and beyond, can be achieved in a more energetic version of the present experiment.