S. D. Frese’s research while affiliated with United States Air Force Research Laboratory and other places

The notion of employing very high magnetic fields for fusion has been extended to so-called magnetized-target fusion (MTF), which may comprise both magnetic and inertial-confinement fusion schemes, and magneto-inertial fusion (MIF) in which the inertia of the liner is explicitly recognized for compressing and holding fusion plasma at relatively high density. Recently, the U.S. Department of Energy through ARPA-E has initiated the ALPHA program for technologies that will enable the development of low-cost controlled fusion by MIF. While it is certainly possible to continue the past history of single-shot implosions of liners onto plasma targets, it has become clear that some means for performing frequent laboratory experiments at multimegajoule levels are needed for reasonable progress. To develop the necessary plasma targets for liner compression requires hundreds of shots, so technology for low cost, repetitive experiments must be created and demonstrated. Furthermore, to satisfy overall program goals, these techniques must extend to break-even experiments and economical fusion power reactors. The stabilized liner compressor (SLC) seeks to accomplish these goals by means of pneumatically driven, annular free-pistons imploding rotationally stabilized liquid metal liners. We review the basic concept, including the reactor embodiment, and discuss the liner and plasma issues for SLC.

There appears to be an optimum operating regime1, known variously as Magnetized Target Fusion (MTF) or Magneto-Inertial Fusion (MIF), between the mainline programs of magnetic-confinement and inertial-confinement fusion that offers reduced size and cost for controlled fusion reactors. It depends, however, on magnetic fields at megagauss levels. These field levels require dynamic conductors, e.g., imploding shells, aka, liners. Two broad approaches follow from the communities attracted to MIF, respectively: an ICF-related side at higher energy-density interested in ignition, enabled in part by high magnetic fields, and an MCF-side, typically interested in arrangements that represent extensions of MCF to much higher fields than conventional programs. The latter harkens to back to US and Soviet programs of the 1970's1,2 and looks for efficiency, rather than ignition.

NumerEx's Stabilized Liner Compressor (SLC) is a mechanical driver that rotates and implodes a liquid metal vortex — our liner — to compress magnetic fields and plasma to fusion energy densities. The rotation conquers drive asymmetries, and makes the liner rebound for immediate reuse after the implosion. The liquid liner will serve as a first wall in a reactor, absorbing neutrons, protecting other components, and breeding tritium.

The AFRL Shiva Star capacitor bank (1300 μΡ, up to 120 kV) used typically at 4 to 5 MJ stored energy, 10 to 15 MA current, 10 μs current rise time, has been used to drive metal shell (solid liner) implosions for compression of axial magnetic fields to multi-megagauss levels, suitable for compressing magnetized plasmas to Magneto-Inertial Fusion (MIF) conditions. MIF approaches use embedded magnetic field to reduce thermal conduction relative to inertial confinement fusion (ICF). MIF substantially reduces required implosion speed and convergence. Using a profiled thickness liner enables large electrode apertures and the injection of a field-reversed configuration (FRC) version of a magnetized plasma ring. Using a longer capture region than originally used, the FRC trapped flux lifetime was made comparable to implosion time and an integrated compression test was conducted. The FRC was compressed cylindrically by more than a factor of ten, with density up more than 100x, to >1018 cm−3 (a world FRC record), but temperatures were only in the range of 300–400 eV, compared to the intended several keV. Although compression to megabar pressures was inferred by the observed time and rate of liner rebound, we learned that heating rate during the first half of the compression was not high enough compared to the normal FRC decay rate. Principal diagnostics for this experiment were soft x-ray imaging, soft x-ray diodes, and neutron measurements. Measures that could double the trapped flux lifetime and pre-compression temperature of the FRC will be discussed.

In 1991, Turchi et al. reported evidence for a 2000 km/s aluminum plasma that originated from the upstream boundary of a wire array armature in a plasma flow switch (PFS). The 2008 article by Turchi et al. 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 experiments executed in an effort to achieve an ultrahigh-speed flow in a deuterium plasma. The first phase of this research concentrated on extension of the earlier work 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. The experiments met with limited success, exhibiting evidence of a 550 km/s plasma flow which convected a small fraction of the total magnetic field. 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. These experiments demonstrated that the hardware assembled for this investigation was unlikely to forestall breakdown in the injected gas as required by Turchi et al. 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. - he magnetic flux convected downstream was greater than in any of the prior experiments, though significant work remains to demonstrate the ultrahigh-speed plasma flow concept.

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, 1019 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.