Peter J. Turchi

Experimental and Computational Progress on Liner Implosions for Compression of FRCs

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.

Liner Stability Problems for Megagauss Fusion

Megagauss fields obtained by liner implosion may offer controlled fusion at much lower cost, size, and entry power levels than conventional fusion schemes. Such implosions are subject to elastic-plastic instability (for solid-density liners) and Rayleigh-Taylor instability in fluid liners (liquid or plasma). This paper provides budgets on allowable perturbations for the inner surface of the liner to offer a simple guide for researchers.

A Compact Torus Plasma Flow Switch

Abstract : Experiments to form and accelerate compact toroid ( CT) plasmas have been performed on the 9.4 MJ Shiva Star fast capacitor bank at Phillips Laboratory (Kirtland AFB, New Mexico) since late 1990. In this paper, we investigate the possibility of employing a CT as a very fast opening switch to drive fast Z-pinches by performing a series of 2 1/2-dimensional magnetohydrodynamic computer simulations. Comparisons are made between computer simulations of conventional and compact torus plasma flow switches. We find that a CT switch leaves less switch plasma on the electrode walls, and can deliver current to an implosion load faster and more uniformly than a conventional plasma flow switch.

Compact toroid formation experiments

Summary form only given, as follows. A compact toroid (CT) formation experiment is discussed. The device has coaxial electrode diameters of 0.9 m (inner) and 1.25 m (outer) and an electrode length of ~1.2 m, including an expansion drift section. The CT is formed by a 0.1-0.2-T initial radial magnetic field embedded coaxial puff gas discharge. The gas puff is injected with an array of 60 pulsed solenoid driven fast valves. The formation discharge is driven by a 108-μF, 40-100-kV, 86-540-kJ, 2-5-MA capacitor discharge with ~20-nH initial total discharge inductance. The hardware includes transmission line connections for a Shiva Star (1300-μF, up to 120-kV, 0.4-MJ) capacitor bank driven acceleration discharge. Experimental measurements include current and voltage; azimuthal, radial, and axial magnetic field at numerous locations; fast photography and optical spectroscopy; and microwave, CO2 laser, and He-Ne laser interferometry. Auxiliary experiments include Penning ionization gauge, pressure probe, and breakdown gas trigger diagnostics of gas injection, and Hall probe measurements of magnetic field injection

Stabilized Liner Compressor for Low-Cost Controlled Fusion at Megagauss Field Levels

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.

Controlled fusion reactor based on stabilized liner compression of magnetized plasma

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 1970s1,2 and looks for efficiency, rather than ignition.

Stabilized Liner Compressor: 21/2-D multiphysics simulations

Summary form only given. NumerExs 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 SLCs 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 pistons 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 1970s. 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!

Compact Transformer Drive for High-Current Applications

The approach called precision high energy-density liner implosion experiment (PHELIX) provides a technique to allow research on high energy-density phenomena associated with liner implosions in a scaled-down system suitable for use with proton radiography. It has been noted that the ratio of load current to bank energy is almost an order of magnitude higher using the PHELIX transformer technique than achievable with direct drive from high-energy capacitor banks. This increase in current per stored-joule offers the opportunity for using similar transformer arrangements for other applications apart from imploding liners. These potential applications include rail-guns and the dense plasma focus. Results from the dimensionless analyses previously used successfully to design PHELIX will be described for these new applications and design limitations will be discussed.

Comparison of Z-pinch and theta-pinch drive for implosion of solid liners suitable for compression of Field Reversed Configurations

Summary form only given, as follows. A comparison of Z-pinch and theta-pinch driven implosions of metal shells (solid liners) is presented. Both schemes are feasible, and they have different advantages for compression of magnetized plasmas to magnetized target fusion conditions. The Z-pinch approach has already demonstrated at least 35% conversion efficiency from stored electrical energy to implosion kinetic energy with good implosion behavior and symmetry, and at least 13 times radial convergence of the liner inner surface. The theta-pinch approach has potential advantages for purer and easier injection of Field Reversed Configurations, easier diagnostic access, and may be more easily operated repetitively.