Thomas Weber

The evolution of plasma parameters over confinement timescales is a key consideration for any confinement concept, especially the relatively short-lived Field Reversed Configuration (FRC). While it is challenging to model dynamic systems far from equilibrium, it is often possible to model the evolution and decay of FRCs through a progression of quasi-equilibrium states using simple analytic and phenomenological models. This approach can elucidate historically observed trends in decaying FRCs and be used to predict plasma parameters throughout slowly-varying dynamic processes, such as adiabatic expansion/compression. Limitations of this method are also discussed and several example cases are presented.

For the first time, a diagnostic for extreme ultraviolet (EUV) spectroscopy was fielded on a sheared-flow-stabilized (SFS) fusion Z-pinch experiment (FuZE-Q). The spectrometer collected time-gated plasma emission spectra in the 5-40 nm wavelength (30-250 eV) range for impurity identification, radiative power studies, and for plasma temperature and density measurements. The implementation of the diagnostic included fast (−10-ns risetime) pulsed high voltage electronics and a multi-stage differential pumping system that allowed the vacuum-coupled spectrometer to collect 3-independently-timed spectra per FuZE-Q shot while also protecting sensitive internal components. Analysis of line emission identified oxygen (0-III, O-IV, O-V, and O-VI), peaking in intensity shortly after maximum current (>500 kA). This work provides a foundation for future high energy spectroscopy experiments on SFS Z-pinch devices.

A colloidal quantum dot loaded polymer coated onto the surface of a sample pre-coated with epoxy was found to linearly change photoluminescence intensity around a 611.5 nm peak while under tensile strain. This peak was the epoxy's photoluminescence emission wavelength while the wavelengths around it were attributed to the colloidal quantum dot loaded polymer. From the spectra emitted from both the epoxy and the colloidal quantum dot loaded polymer, an empirical relation was made to calculate the changes in photoluminescence intensity between them. A calibration was then devised to create an optical stress-strain curve. The relationship found between both the optical and mechanical stress-strain curves indicated that this measurement technique followed the sample towards failure in the plastic region better than when only measuring from a colloidal quantum dot loaded polymer peak. For the first time, the results demonstrated here show that an epoxy's photoluminescence emission peak utilized in tandem with colloidal quantum dot loaded polymer can be used for strain sensing. Potential applications that could benefit from this finding would be: quality control, strain gauge for systems, and materials science.

National Security Technologies (NSTec) is developing dense plasma focus (DPF) systems for applications requiring intense pulsed neutron sources. Sandia National Laboratories participated in a limited number of experiments with one of those systems. In collaboration with NSTec, Los Alamos National Laboratory, and Lawrence Livermore National Laboratory, we installed additional electrical and X-ray image measurements in parallel with normal operation of the system. Dense plasma focus machines have been studied for decades, but much of the experimental interest has been on neutron and X-ray yield. The primary goal for the present work was to develop and field high-fidelity and traceably-calibrated current and voltage measurements for comparison to digital simulations. The secondary goals were to utilize the current and voltage measurements to add general understanding of vacuum insulator behavior and current sheath dynamics. We also conducted initial scoping studies of soft X-ray diagnostics. We will show the electrical diagnostics and the techniques used to acquire high-fidelity signals in the difficult environment of the 2 MA, 6 μ plasma focus drive pulse. We will show how we measure accreted plasma mass non-invasively, and the sensitivity to background fill density. We will present initial qualitative results from filtered X-ray pinhole images and spectroscopic data from the pinch region.

Summary form only given. A collaborative research effort was launched in 2000 between the Air Force Research Laboratory and Los Alamos National Laboratory to investigate the formation of high density field-reversed configuration (FRC) plasmas for the purpose of then adiabatically compressing them to a high energy density (HED) state. The goal of the experimental system developed through this collaboration was to enable a low-cost approach to achieving thermonuclear fusion of the target plasma, which would then facilitate magneto-inertial fusion studies, laboratory astrophysical studies, and numerous other basic research studies connected with HED plasmas. This system was assembled at the Shiva Star facility at the Air Force Research Laboratory and referred to as the Field-Reversed Configuration Heating Experiment (FRCHX). The target FRC plasma is formed in the experiment through a reversed-field theta pinch that uses four to five capacitor banks. Once formed, the FRC is translated a short distance and then captured inside a magnetic well co-located and coaxial with an aluminum solid liner. Two additional capacitor banks establish guide fields and the end mirror fields for the magnetic well a few milliseconds before the FRC is formed. The Shiva Star High Energy Capacitor bank is then used to compress the FRC via the electromagnetic implosion of the surrounding solid liner, which is a 30-cm long, 10-cm diameter aluminum cylinder that is tapered at the top and bottom to reduce motion these locations and maintain electrical contact. Due to the overall circuit inductance the implosion requires -25 us for stagnation to occur, thus FRC formation process is started several microseconds after the liner implosion begins. This presentation will provide an overview of the latest FRCHX field coil and vacuum stand design, the pulsed power systems used, and the diagnostics employed on the experiment. The integration of the FRCHX systems into Shiva Star will be described, as well, and possible next steps will be presented.

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.

The Field-Reversed Configuration Heating Experiment (FRCHX) was designed to form closed-field-line magnetized target plasmas for magneto-inertial fusion and other high energy density plasma research. These plasmas are in a field-reversed configuration (FRC) and are formed via a reversed-field theta pinch on an already-magnetized background plasma. To extend the duration and uniformity of the pinch, the capacitor bank driving the reversed-field discharge is crowbarred near the current peak. Four parallel rail-gap switches are used on FRCHX for this application to ensure a low-inductance crowbar discharge path and to accommodate the large magnitude of the discharge current (often greater than 1 MA). Parallel operation of spark gap switches in a crowbarring arrangement, however, has often proved to be difficult due to the very low voltage present on the bank and across the switches at the time of peak current. This paper reports on the successful efforts made to develop a low-inductance crowbar switch for FRCHX and to ultimately enable successful triggering and operation of the four parallel rail-gap switches used in the crowbar. The design of the parallel switch assembly is presented first, followed by a description of the triggering scheme employed to ensure conduction of all four switches.

Summary form only given. The generation and destruction of magnetic field is an important feature of solar, magnetosphere and cosmic plasmas, for example during reconnection, dynamo, and turbulent processes. We have experimentally measured spatially resolved profiles of magnetic flux ropes. These data include ion flow, magnetic field, current density, and plasma pressure, which allow us to verify a screw pinch equilibrium and also infer the electron fluid flow in three dimensions. Parallel currents along each flux rope result in a mutual attraction, which compresses and distorts the cylindrically symmetric equilibrium profiles. The electron and ion fluid flows turn out to be different, and we show that sheared axial electron fluid flow v_e generates magnetic field B(t) via the induction term curl X̅v_e X B = -curl X E = dB/dt. Data show a quadrupole out of plane magnetic field signature with four fold symmetry that is driven by flux rope flows with two fold symmetry. This mechanism provides a natural and general mechanism for large scale sheared flows to acquire smaller scale magnetic features, disordered structure, and possibly turbulence.

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.

Summary form only given. The Field-Reversed Configuration Heating Experiment (FRCHX) is a collaborative experiment between the Air Force Research Laboratory (AFRL) and Los Alamos National Laboratory (LANL) to explore the physics of magneto-inertial fusion (MIF) and other high energy density laboratory plasma (HEDLP) phenomena. In the experiment, a plasma in a field-reversed configuration (FRC), with density 5 × 10 16 ions/cm 3 , total temperature ~200 eV, poloidal magnetic field ~1 T, length 15 ~ 20 cm, and field exclusion radius ~2 cm is formed via a reversed-field theta discharge and then translated a short distance (~1 m) into a magnetic mirror that has been established within a 30 cm long, 10 cm diameter, 0.11 cm thick aluminum solid liner. The high energy density state (10 19 ions/cm 3 , multi-keV, MegaGauss fields) will be achieved when a 12 MA axial current, provided by the AFRL Shiva Star capacitor bank, implodes the liner and compresses the FRC within. Conventional FRC formation techniques trap only a small fraction of the initial axial bias field. Guided by extended 2D-MHD simulations, several factors limiting the closed field lifetime of the FRCs to about half that required for good liner compression have been identified, and new experimental hardware has been designed and prepared to increase that lifetime. Results from recent setup experiments will be presented, including a full-scale engineering test shot, along with a description of FRCHX's pulsed power systems and plasma diagnostics.