T.E. Weber

Flux rope dynamics in three dimensions

A large 3D data set has been assembled using the relaxation scaling experiment (RSX) device to study the dynamics of flux ropes. In a series of single flux rope experiments, we have measured induced eddy currents inside the plasma that complicate the evolution of a nominally simple current system. It is also likely that the nominal MHD force balance is violated on ion inertial length scales. These phenomena appear irreducibly three dimensional.

Laboratory observation of magnetic field growth driven by shear flow

Two magnetic flux ropes that collide and bounce have been characterized in the laboratory. We find screw pinch profiles that include ion flow vi, magnetic field B, current density J, and plasma pressure. The electron flow ve can be inferred, allowing the evaluation of the Hall J×B term in a two fluid magnetohydrodynamic Ohms Law. Flux ropes that are initially cylindrical are mutually attracted and compress each other, which distorts the cylindrical symmetry. Magnetic field is created via the ∇×ve×B induction term in Ohms Law where in-plane (perpendicular) shear of parallel flow (along the flux rope) is the dominant feature, along with some dissipation and magnetic reconnection. We predict and measure the growth of a quadrupole out-of-plane magnetic field δBz. This is a simple and coherent example of a shear flow driven dynamo. There is some similarity with two dimensional reconnection scenarios, which induce a current sheet and thus out-of-plane flow in the third dimension, despite the customary picture...

A phenomenological model on the kink mode threshold varying with the inclination of sheath boundary

In nature and many laboratory plasmas, a magnetic flux tube threaded by current or a flux rope has a footpoint at a boundary. The current driven kink mode is one of the fundamental ideal magnetohydrodynamic instabilities in plasmas. It has an instability threshold that has been found to strongly depend on boundary conditions (BCs). We provide a theoretical model to explain the transition of this threshold dependence between nonline tied and line tied boundary conditions. We evaluate model parameters using experimentally measured plasma data, explicitly verify several kink eigenfunctions, and validate the model predictions for boundary conditions BCs that span the range between NLT and LT BCs. Based on this model, one could estimate the kink threshold given knowledge of the displacement of a flux rope end, or conversely estimate flux rope end motion based on knowledge of it kink stability threshold.

Operation of parallel rail-gap switches in a high-current, low-inductance crowbar switch

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.

Results from compression of field reversed configuration using imploding solid liner

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 >1018 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 on Shiva Star

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.

Parallel Triggering and Conduction of Rail-Gap Switches in a High-Current Low-Inductance Crowbar Switch

The field-reversed configuration heating experiment (FRCHX) was designed to form closed-field-line magnetized target plasmas for magnetoinertial fusion and other high energy density plasma research. These plasmas are in a field-reversed configuration and are formed via a reversed-field theta pinch on an already magnetized background plasma. To extend the duration and temporal 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 the 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). Historically, parallel operation of spark gap switches in a crowbarring arrangement 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. In a low-inductance design, triggering can be further complicated by the rapid collapse of what little voltage there is across the switches as soon as the first spark gap begins conduction. This paper reports on the efforts that were made to develop a low-inductance crowbar switch for the FRCHX and to ultimately enable successful triggering and operation of the four parallel rail-gap switches used in the crowbar. The design of the low-inductance parallel switch assembly is presented first, followed by a description of the triggering scheme employed to ensure conduction of all four switches.

Plasma-gun-assisted field-reversed configuration formation in a conical θ-pinch

Injection of plasma via an annular array of coaxial plasma guns during the pre-ionization phase of field-reversed configuration (FRC) formation is shown to catalyze the bulk ionization of a neutral gas prefill in the presence of a strong axial magnetic field and change the character of outward flux flow during field-reversal from a convective process to a much slower resistive diffusion process. This approach has been found to significantly improve FRC formation in a conical θ-pinch, resulting in a ∼350% increase in trapped flux at typical operating conditions, an expansion of accessible formation parameter space to lower densities and higher temperatures, and a reduction or elimination of several deleterious effects associated with the pre-ionization phase.

PPPS-2013: Addressing short trapped-flux lifetime in high-density field-reversed configuration plasmas in FRCHX

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.

Experimental measurements of magnetic field generation from sheared flows

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.