E.L. Ruden

A high density field reversed configuration (FRC) target for magnetized target fusion: First internal profile measurements of a high density FRC

Magnetized target fusion (MTF) is a potentially low cost path to fusion, intermediate in plasma regime between magnetic and inertial fusion energy. It requires compression of a magnetized target plasma and consequent heating to fusion relevant conditions inside a converging flux conserver. To demonstrate the physics basis for MTF, a field reversed configuration (FRC) target plasma has been chosen that will ultimately be compressed within an imploding metal liner. The required FRC will need large density, and this regime is being explored by the FRX–L (FRC-Liner) experiment. All theta pinch formed FRCs have some shock heating during formation, but FRX–L depends further on large ohmic heating from magnetic flux annihilation to heat the high density (2–5×1022 m−3), plasma to a temperature of Te+Ti≈500 eV. At the field null, anomalous resistivity is typically invoked to characterize the resistive like flux dissipation process. The first resistivity estimate for a high density collisional FRC is shown here. Th...

FRX-L: A field-reversed configuration plasma injector for magnetized target fusion

We describe the experiment and technology leading to a target plasma for the magnetized target fusion research effort, an approach to fusion wherein a plasma with embedded magnetic fields is formed and subsequently adiabatically compressed to fusion conditions. The target plasmas under consideration, field-reversed configurations (FRCs), have the required closed-field-line topology and are translatable and compressible. Our goal is to form high-density (1017 cm−3) FRCs on the field-reversed experiment-liner (FRX-L) device, inside a 36 cm long, 6.2 cm radius theta coil, with 5 T peak magnetic field and an azimuthal electric field as high as 1 kV/cm. FRCs have been formed with an equilibrium density ne≈(1 to 2)×1016 cm−3, Te+Ti≈250 eV, and excluded flux ≈2 to 3 mWb.

Implosion of solid liner for compression of field reversed configuration

The design and first successful demonstration of an imploding solid liner with height to diameter ratio, radial convergence, and uniformity suitable for compressing a field reversed configuration is discussed. Radiographs indicated a very symmetric implosion with no instability growth, with /spl sim/13x radial compression of the inner liner surface prior to impacting a central measurement unit. The implosion kinetic energy was 1.5 megajoules, 34% of the capacitor stored energy of 4.4 megajoules.

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.

Experimental measurements of a converging flux conserver suitable for compressing a field reversed configuration for magnetized target fusion

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.

Confinement analyses of the high-density field-reversed configuration plasma in the field-reversed configuration experiment with a liner

The focus of the field-reversed configuration (FRC) experiment with a liner (FRX-L) is the formation of a target FRC plasma for magnetized target fusion experiments. An FRC plasma with density of 1023m−3, total temperature in the range of 150–300 eV, and a lifetime of ≈20μs is desired. Field-reversed θ-pinch technology is used with programed cusp fields at θ-coil ends to achieve non-tearing field line reconnections during FRC formation. Well-formed FRCs with density between (2–4)×1022m−3, lifetime in the range of 15–20μs, and total temperature between 300–500 eV are reproducibly created. Key FRC parameters have standard deviation in the mean of 10% during consecutive shots. The FRCs are formed at 50 mTorr deuterium static fill using 2 kG net reversed bias field inside the θ-coil confinement region, with external main field unexpectedly ranging between 15–30 kG. The high-density FRCs confinement properties are approximately in agreement with empirical scaling laws obtained from previous experiments with fi...

Rayleigh-Taylor instability with a sheared flow boundary layer

S. Chandrasekhar, in his book, Hydrodynamic and Hydromagnetic Stability (New York: Dover, 1961), derives the stability criteria for a semi-infinite uniform density incompressible inviscid fluid with uniform horizontal velocity supported in a gravitational field by one of higher density and opposite velocity. A transitional layer of inviscid fluid with a density equal to the average of the upper and lower fluids, and a horizontal velocity that varies linearly with depth from that of the upper fluid at the top to that of the lower fluid at the bottom is assumed. This analysis of the Kevin-Helmholtz (K-H) instability may be transformed into a model of the effect of such a velocity sheared boundary layer on the Rayleigh-Taylor (R-T) instability of modes with wave numbers in the direction of the sheared velocity by reversing the sign of the top-bottom density differential. Orthogonal modes are unaffected by the shear in the linear limit and are, therefore, R-T unstable unless an independent mechanism for their stabilization is present, such as a magnetic field orthogonal to the sheared velocity. The combined R-T/K-H stability analysis is, therefore, expected to be most applicable for magnetically accelerated media such as a Z pinch with an axial velocity sheared outer layer orthogonal to the outer azimuthal magnetic field which drives the implosion.

FRC lifetime studies for the Field Reversed Configuration Heating Experiment (FRCHX)

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.

Diagnostics for a magnetized target fusion experiment

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...

Addressing Short Trapped-Flux Lifetime in High-Density Field-Reversed Configuration Plasmas in FRCHX

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 ~25 μs. Lifetime measurements of recent FRCHX FRCs indicate that trapped lifetimes following capture are now approaching ~14 μs (and therefore, total lifetimes after formation are now approaching ~19 μs). 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 μs before the FRC is formed. A discussion of FRC lifetime-limiting mechanisms and various experimental approaches to extending the FRC lifetime will be presented.