The Stabilized Liner Compressor (SLC), described at the EAPPC-BEAMS-Megagauss conference [1], grew out of the original Linus program at the Naval Research Laboratory in the 1970s [2]. The idea then, as now, was to create a stable liner to compress magnetic fields or magnetized plasmas to high energy densities up to thermonuclear fusion conditions. Other liner compression schemes have been unsuccessful because of instabilities - in particular the Rayleigh-Taylor instability. In the case of Linus and SLC, stability is provided by rotation of the liner, which is formed from liquid metal. The current project has been supported by the Advanced Research Projects Agency - Energy (ARPA-E) under the ALPHA program, whose goal is to further technologies enabling lower-cost fusion approaches [3]. A critical tool for understanding and designing compression systems is MACH2, a 2 1/2-D ALE magnetohydrodynamics simulation code [4]. In this paper, we present the results of simulations leading to a point design for SLC demonstration, as well as an engineering description of the point design. We also describe technical issues encountered.

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 the Rayleigh- Taylor instability during final compression, and makes the liner rebound for immediate reuse after the implosion. A liquid lead and lithium liner will serve as a first wall in a reactor, absorbing neutrons, protecting other components, and breeding tritium. The time is right for the SLC as a magnetized fusion target compressor because magnetized plasma lifetimes are increasing dramatically.

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.

The Stabilized Liner Compressor (SLC) concept uses annular free-pistons, driven by high-pressure helium, to implode a rotating liquid metal liner that compresses a plasma/field target to fusion conditions [1], [2]. The free-pistons accelerate and re-capture the liquid metal, avoiding Rayleigh-Taylor instability by eliminating the free outer surface of the liquid. Sufficient rotation at the inner surface of the liquid prevents Rayleigh-Taylor instability there during the compression and re-expansion of the liner at peak energy-densities of a 50 – 100 T field. Such field levels correspond to operation near the cost-minimum for controlled fusion systems [1], [2]. The combination of free-piston drive and rotation permits stable exchange of energy between pneumatic energy storage and the plasma target. In the fusion reactor concept, modest losses in the liner implosion system are replaced by work done by magnetically-confined alpha-particles from D-T fusion reactions, thereby reducing the required nuclear gain for an economical reactor. Our project for ARPA-E is focused on extending the earlier success with the stabilized liner techniques, demonstrated at the Naval Research Laboratory c. 1979, to the higher drive-pressures (25 kpsi) and implosion speeds (>1 km/s) required to compress plasma targets anticipated in the near future. This technique would enable the frequent repetition of plasma compression experiments needed to develop the liner and plasma system for breakeven tests and fusion reactor design. We use the MACH2 code [3] for calculation of the liner dynamics and also for obtaining estimates of mechanical stresses in the very high pressure SLC prototype. Development of the SLC represents a departure from conventional pulsed electrical power techniques, substituting compact energy storage as highpressure gas (at >100 MJ/m3) for capacitive energy storage, fast gas-valves instead of electrical switches, and pulsed hydrodynamic convergence instead of multiple electrical stages for power density multiplication.

Summary form only given. 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 time is right for SLC because magnetized plasma target lifetimes are increasing dramatically. However SLC's design poses significant challenges. We must make the implosion fast enough, the structure strong enough, and the flow repeatable enough to recapture the liquid liner. We will surmount these challenges first in a fully functional laboratory-scale model designed to implode a 10 cm diameter cylindrical volume to 1 cm diameter in 25 μs. We are designing this device using multi-physics computer simulation. We are using 2½-d MACH2, with its 2-d domain and ∂/∂θ assumed zero but all three components of each vector field included, as is essential for problems with fluid rotation. The driving energy in SLC comes from a plenum of high pressure gas released by fast valves to a chamber behind a free annular piston. The piston pushes on low melting-point liquid metal that converts the piston's swept volume to the implosion of an inward-facing rotating free surface. Our simulations follow the compressible motion of the high pressure gas into the chamber and the elastic behavior of the piston as it transmits the pressure to the liquid metal, producing an initially nearly incompressible fluid flow. As the free surface of the liquid liner is driven to smaller radius and higher radial and azimuthal velocity, our simulations capture the increasingly compressible fluid behavior. We will show simulations of low pressure experiments done with water and with eutectic NaK at NRL in the late 1970's. The relatively complete data from the water experiment are well matched by our simulations in radius and symmetry. The NaK data are less complete and less well matched. We will show very high pressure simulations of Na and NaK in geometries similar to those used at NRL, with similarly symmetric results. We will also show simulations of axially longer geometries that fail to achieve the necessary symmetry to show that this is clearly a work in progress!

Summary form only given. The AFRL Shiva Star capacitor bank (1300 μF, 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 >10 18 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.

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.

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.

The purpose of this research has been to study the physics of macroscopic magnetized high-energy-density laboratory plasmas (HEDLPs) created through the compression of a high-beta compact toroid (CT) plasma having closed magnetic field lines. The high-beta CT chosen for this work is a field-reversed configuration (FRC). The basic approach is to investigate CT plasmas as they are compressed to a HED state by the electromagnetic implosion of a surrounding metallic shell or solid liner (Figure 1). The shell provides an axisymmetric, electrically-conducting boundary around the plasma and its supporting magnetic field and is imploded by means of the magnetic pressure force arising from axial current flow in the liner interacting with its associated azimuthal magnetic field. Compression of the CT will bring the plasma to fusion temperatures at higher densities and magnetic fields (multi-MegaGauss [MG]) than have previously been present in conventional magnetic fusion approaches. The resulting energy densities will be ~1 Mbar or greater and thus will place the plasma in a parameter space intermediate to MFE and IFE. This work has been a collaboration between the Air Force Research Laboratory, Los Alamos National Laboratory, and NumerEx, LLC.

Summary form only given. The U.S. Air Force Research Laboratory Directed Energy Directorate and Los Alamos National Laboratory are forming, translating, capturing, and compressing magnetic field-reversed configuration plasmas (FRCs) in stationary and imploding aluminum cylinders, with the goal of creating a high-energy-density magnetized plasma configuration. We have previously described 1 2-d axisymmetric magnetohydrodynamic (MHD) simulations of these experiments with the geometric configuration of the actual device driven by the time-dependent currents of its pulsed power systems. Our simulations begin with the theta-pinch preionization phase based on the experimentally determined breakdown/ionization time to produce the correct flux trapped in the FRC and thus improve agreement with the experimental magnetic probe data. They have become the theoretical workhorse for delivering understanding of those experiments and assessing the potential impact of design variations upon them. FRC experiments have shown evidence of plasma rotation, a process which can lead to amplification of asymmetry and subsequent collision of plasma with the wall. In cylindrical symmetry, the magnetic force from either poloidal field or toroidal field alone has no toroidal component, but when all three components of the field are present, toroidal force and hence rotational acceleration are possible. Furthermore, in the presence of toroidal current, the Hall Effect can shear radial field into the toroidal direction, converting some poloidal field into toroidal field. Hence, extended MHD (XMHD) is necessary in order to model the generation of rotation in axisymmetric simulations of plasma motions induced by toroidal coils only.

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.

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. MagnetoInertial 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, 10<^>19 ion/cm<^>3, 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.

Summary form only given. Field Reversed Configurations (FRCs) are formed by first trapping an initial bias field in an ionized plasma and then applying a large field opposite to the bias field. This reversal wraps the field lines around the ionized plasma forming a toroidal configuration that is then compressed by further increase of the reversed field.