A challenge for TW-class accelerators, such as Sandia's Z machine [1], is current diversion via electrode plasma formation, expansion, and gap closure in the Magnetically Insulated Transmission Lines (MITLs) [2]. To measure this electrode plasma density, a Dispersion Interferometer (DI) diagnostic has been developed with a sensitivity of $\sim 1x1014 {\mathrm{e}-1}/{\text{cm}^{\wedge} 2}$ [3]. Discussed are the required optical and electrical system upgrades for this diagnostic to make density measurements of the millimeter scale sheaths generated during the $\sim 140$ ns rise times of the electrode plasma formation experiments on Sandia's 650 kA Mykonos driver [4].

We have developed a conceptual design of a next-generation pulsed-power accelerator that is optimized for advanced high-energy-density-physics experiments. The prime power source of the machine consists of 210 impedance-matched Marx generators (IMGs). Each IMG drives a 150-ns-long coaxial-transmission-line impedance transformer. The coaxial lines provide a minimum of 300 ns of transit-time isolation between each pair of IMGs. The lines in turn drive six radial-transmission-line impedance transformers, which transport the power generated by the IMGs to a six-level vacuum-insulator stack. The stack is connected to six conical outer magnetically insulated vacuum transmission lines (MITLs); these are joined in parallel at a 12-cm radius by a triple-post-hole vacuum convolute. The convolute sums the electrical currents at the outputs of the six outer MITLs, and delivers the combined current to a single short inner MITL. The inner MITL transmits the combined current to the accelerator's physics load. Since the accelerator would be the largest and most-powerful pulsed-power machine developed to date, we refer to it as Jupiter. The conceptual design of Jupiter is 72 m in diameter, stores 140 MJ of electrical energy, and generates 960 TW of peak electrical power at the output of the IMG system. The design delivers 2700 TW, 67 MA, and 9.2 MJ in a 110-ns pulse to a 0D magnetized-liner inertial-fusion (MagLIF) target. The principal goal of the design is to achieve high-yield thermonuclear fusion; i.e., a fusion yield that exceeds the energy initially stored by the accelerator's capacitors.

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 be monitored spectroscopically, and fast bolometers will measure the total radiated power. A 20 J Thomson scattering laser beam will be introduced in the axial direction, and scattered light (from multiple spatial points) will be collected from the sides. Neutron diagnostics (activation and time-resolved scintillation detectors) will be fielded during both phases of the DD experiments.

Integrated magnetic modeling and design are important to meet the requirements for (1) formation, (2) translation, and (3) compression of a field reversed configuration (FRC) for magnetized target fusion. Off-the-shelf solutions do not exist for many generic design issues. A predictive capability for time-dependent magnetic diffusion in realistically complicated geometry is essential in designing the experiment. An eddy-current code was developed and used to compute the mutual inductances between driven magnetic coils and passive magnetic shields (flux excluder plates) to calculate the self-consistent axisymmetric magnetic fields during the first two stages. The plasma in the formation stage was modeled as an immobile solid cylinder with selectable constant resistivity and magnetic flux that was free to readjust itself. It was concluded that (1) use of experimentally obtained anomalously large plasma resistivity in magnetic diffusion simulations is sufficient to predict magnetic reconnection and FRC formation, (2) comparison of predicted and experimentally observed timescales for FRC Ohmic decay shows good agreement, and (3) for the typical range of resistivities, the magnetic null radius decay rate scales linearly with resistivity. The last result can be used to predict the rate of change in magnetic flux outside of the separatrix (equal to the back-emf loop voltage), and thus estimate a minimum θ-coil loop voltage required to form an FRC.

After considerable design and construction, we describe the status of a physics exploration of magnetized target fusion (MTF) that will be carried out with the first flux conserving compression of a high pressure field-reversed configuration (FRC). The upgraded Los Alamos (LANL) high density FRC experiment FRXL has demonstrated that an appropriate FRC plasma target can be created and translated on a time scale fast enough to be useful for MTF. Compression to kilovolt temperature is expected to form a Mbar pressure, high energy density laboratory plasma (HEDLP). Integrated hardware on the new Field Reversed Compression and Heating Experiment (FRCHX) at the Air Force Research Laboratory Shiva Star facility, has formed initial FRC's and will radially compress them within a cylindrically symmetric aluminum "liner". FRXL has shown that time scales for FRC translation to the target region are significantly shorter than the typical FRC lifetime. The hardware, diagnostics, and design rationales are presented. Pre-compression plasma formation and trapping experimental data from FRXL and FRCHX are shown.

We describe a physics scaling model used to design the high density field reversed configuration (FRC) at LANL that will translate into a mirror bounded compression region, and undergo Magnetized Target Fusion compression to a high energy density plasma. At Kirtland AFRL the FRC will be compressed inside a flux conserving cylindrical shell. The theta pinch formed FRC will be expelled from inside a conical theta coil. Even though the ideal FRC has zero helicity and toroidal magnetic field, significant non-ideal properties follow from formation within a conical (not cylindrical) theta coil. The FRC stability and lifetime properties may improve. Several experimental features will also allow unique scientific investigations of this high Lundquist number but collisional plasma.

We present an analytic treatment of the transport of magnetic field into a metallic material, when the surface field is changing in time. This has many applications in the area of high-current pulsed power. We focus on one of these in this paper, magnetized target fusion (MTF), a simple, potentially inexpensive method of creating burning fusion conditions through fast compression of dense, warm magnetized plasma. Magnetization of the plasma electrons, needed to inhibit thermal transport losses, means that compression, on the order of 10 microseconds (10-5 seconds), results in large magnetic field compression. Current density, J, proportional to the field gradient in the walls, is also found analytically. Heating in the wall is also a function of etaJ2, and so can also be evaluated with these solutions. MTF studies proposed to be conducted at the ATLAS pulsed-power facility (23 MJ, 30 MA, 240 kV), must explicitly determine energy dissipation in the wall. Vaporization, or possibly even melting, of metallic wall material could lead to mixing of such high-Z material with the hot hydrogen plasma. The ensuing radiation losses and plasma cooling would be catastrophic to any MTF scheme.

Summary form only given. A flux compression experiment is being designed for the Atlas pulsed power facility. The purpose is to investigate generation of megagauss fields with liner technology in the geometry needed for compression of a stabilized diffuse z pinch.1 To survey possible parameters quickly and conveniently, a semi-analytic model has been developed that computes liner motion under the assumption that the liner remains cylindrically symmetric during the implosion and the metal of the liner is incompressible.2 Thus the liner thickness increases during implosion in a predictable way to conserve liner mass. Equations are derived for the time variation of liner position and circuit current including the effect of back pressure from the compressed flux. The model allows using realistic Atlas circuit parameters. The equations are integrated using the Matlab program and a standard Runge Kutta method. Recently the model has been extended to account for a shunt resistor and the resulting time-dependent current that would be generated inside the liner.3 The important advantage of a shunt resistor is that an auxiliary power supply is not needed to generate the seed flux which liner motion will compress. By tapping the power of Atlas to generate the seed flux, the incremental cost of a flux compression experiment is minimized. The selection of shunt material and dimensions must consider both the heating of the shunt and the amount of trapped flux, which along with the liner kinetic energy determines the final level of compressed magnetic field. Initial results suggest that readily available materials (a steel shunt and an aluminum liner) and properly chosen dimensions give a workable combination that generates magnetic field of several megagauss.

In the 'metal liner' approach to magnetized target fusion (MTF), a preheated magnetized plasma target is compressed to thermonuclear temperature and high density by externally driving the implosion of a flux conserving metal enclosure, or liner, which contains the plasma target. As in inertial confinement fusion, the principal fusion fuel heating mechanism is pdV work by the imploding enclosure, called a pusher in ICF. One possible MTF target, the hard-core diffuse z pinch, has been studied in MAGO experiments at VNIIEF and is one possible target being considered for experiments on the Atlas pulsed power facility. Numerical MHD simulations show two intriguing and helpful features of the diffuse z pinch with respect to compressional heating. First, in two-dimensional simulations the m = 0 interchange modes, arising from an unstable pressure profile, result in turbulent motions and self-organization into a stable pressure profile. The turbulence also gives rise to convective thermal transport, but the level of turbulence saturates at a finite level, and simulations show substantial heating during liner compression despite the turbulence. The second helpful feature is that pressure profile evolution during compression tends towards improved stability rather than instability when analysed according to the Kadomtsev criteria. A liner experiment is planned for Atlas to study compression of magnetic flux without plasma, as a first step. The Atlas geometry is compatible with a diffuse z pinch, and simulations of possible future experiments show that kiloelectronvolt temperatures and useful neutron production for diagnostic purposes should be possible if a suitable plasma injector is added to the Atlas facility.

Summary form only given. The near-term goal of magnetized target fusion research is to compress plasma inside a liner to thermonuclear temperatures. Two candidate plasma targets are the field-reversed configuration (FRC), and the stabilized hard-core Z pinch or MAGO configuration. Advantages of the FRC are high beta intrinsic to the configuration, a separatrix in the magnetic field that can isolate the fusion fuel from the walls, and experimentally demonstrated translation of the FRC from a formation coil into an imploding liner geometry. Advantages of the MAGO are experimentally demonstrated operation at high-density, self-organized stability for a wide range of beta values, wall confinement of fuel as needed for compression to beta greater than unity, and a coaxial geometry well suited for in-situ formation in an Atlas chamber. The major issues for the FRC are its poorly understood magnetohydrodynamic stability and the relatively complex hardware required for formation. For MAGO the major issues are mitigation of wall impurities generated by wall-plasma interactions intrinsic to the configuration, and generally less extensive diagnostics of experimental parameters connected with high-energy-density experimental conditions. This paper will present results of modeling aimed at comparison of these targets in a liner-compression context. An FRC compression experiment is being planned for the Air Force Research Laboratory Shiva Star facility. Higher energy compression experiments of either target are possible on Atlas. To make a sound judgment about the potential of magnetized target fusion, both plasma targets should be tested experimentally as soon as funding can be obtained

initial-1 28 6.5 0.18 9.0-04 0.20 3.5E+22 22 2.6 0.83 final-1 2.8 26 0.072 3.6E-06 4.8 5.3E+24 8.10E+04 160 0.83 3.4E+12 initial-2 28 6.5 0.18 8.7E-04 0.26 1.3E+23 83 5 0.84 final-2 1.9 33 0.062 1.3E-06 9.1 5.2E+25 1.50E+06 680 0.84 9.2E+14 Table 1. Two design cases are shown above, 40% losses assumed, 0.3 usec dwell time, elongation E=6.5, 2.4-D compression. Case 1: low field, 10x convergence. Case 2: higher field, with 15x convergence.