Thomas P Intrator

Conversions between magnetic and kinetic energy occur over a range of plasma scales in astrophysical and solar dynamos and reconnection in the solar corona and the laboratory. Canonical flux tubes reconcile all plasma regimes with concepts of twists, writhes, and linkages. We present measurements of canonical flux tubes, their helicity, and their helicity transport in a gyrating plasma kink. The helicity gauge is removed with general techniques valid even if only a limited section of the plasma is measured. Temporal asymmetries in the helicities confirm irreducible 3D fields in the kink.

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.

The dynamics of magnetic reconnection is investigated in a laboratory experiment consisting of two magnetic flux ropes, with currents slightly above the threshold for the kink instability. The evolution features periodic bursts of magnetic reconnection. To diagnose this complex evolution, volumetric three-dimensional data were acquired for both the magnetic and electric fields, allowing key field-line mapping quantities to be directly evaluated for the first time with experimental data. The ropes interact by rotating about each other and periodically bouncing at the kink frequency. During each reconnection event, the formation of a quasiseparatrix layer (QSL) is observed in the magnetic field between the flux ropes. Furthermore, a clear correlation is demonstrated between the quasiseparatrix layer and enhanced values of the quasipotential computed by integrating the parallel electric field along magnetic field lines. These results provide clear evidence that field lines passing through the quasiseparatrix layer are undergoing reconnection and give a direct measure of the nonlinear reconnection rate. The measurements suggest that the parallel electric field within the QSL is supported predominantly by electron pressure; however, resistivity may play a role.

The momentum balance of a plasma pinch in the Reconnection Scaling Experiment (RSX) is examined in three dimensions using several repositionable, insertable probes. A new camera-based system described here triangulates the locations of the probe tips so that their measurements are spatially registered. The optical system locates probes to within +/- 1.5 mm of their absolute 3D position in the vessel and to within +/- 0.7 mm relative to other probes, on the order of the electron inertial length (1-2 mm). (C) 2014 AIP Publishing LLC.

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.

Stochastic transport of a two-dimensional (2D) dusty plasma liquid with a perpendicular magnetic field is studied. Superdiffusion is found to occur especially at higher magnetic fields with β of order unity. Here, β = ω(c)/ω(pd) is the ratio of the cyclotron and plasma frequencies for dust particles. The mean-square displacement MSD = 4D(α)t(α) is found to have an exponent α > 1, indicating superdiffusion, with α increasing monotonically to 1.1 as β increases to unity. The 2D Langevin molecular dynamics simulation used here also reveals that another indicator of random particle motion, the velocity autocorrelation function, has a dominant peak frequency ω(peak) that empirically obeys ω(peak)(2) = ω(c)(2) + ω(pd)(2)/4.

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 x B term in a two fluid magnetohydrodynamic Ohm's 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 del x v(e) x B induction term in Ohm's 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 delta B-z. 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 that considers flows only in the reconnection plane. These data illustrate a general and deterministic mechanism for large scale sheared flows to acquire smaller scale magnetic features, disordered structure, and possibly turbulence. (C) 2014 AIP Publishing LLC.

A compact ionization gauge has been developed to carry out in situ measurements of high density (10(20)-10(22) m(-3)) neutral gas flow dynamics with high temporal and spatial resolution. Key design aspects are discussed including gauge sensitivity and time response scaling with decreased probe dimensions, high-pressure operation, improved driver circuit bandwidth, and techniques for constructing a miniaturized probe head. Gas adsorption was discovered to significantly alter emission current and gauge sensitivity over timescales of several seconds. This effect must be taken into consideration when making time-resolved, high-density measurements. Over short timescales gauge response was predicted by scaling the sensitivity of a nominal Bayard-Alpert gauge to account for variations in probe dimensions and species-dependent ionization cross-section. Time-resolved neutral density profiles have been acquired in the Magnetized Shock Experiment at Los Alamos National Laboratory, providing data on the initial conditions of the ionization, plasmoid formation, and translation processes. It is shown that the desired density profiles can be achieved using a dynamic gas fill and that density can be scaled independently of the spatial profile. (C) 2014 AIP Publishing LLC.

The kinetic features of secondary magnetic reconnection in a single flux rope undergoing internal kink instability are studied by means of three-dimensional Particle-in-Cell simulations. Several signatures of secondary magnetic reconnection are identified in the plane perpendicular to the flux rope: a quadrupolar electron and ion density structure and a bipolar Hall magnetic field develop in proximity of the reconnection region. The most intense electric fields form perpendicularly to the local magnetic field, and a reconnection electric field is identified in the plane perpendicular to the flux rope. An electron current develops along the reconnection line in the opposite direction of the electron current supporting the flux rope magnetic field structure. Along the reconnection line, several bipolar structures of the electric field parallel to the magnetic field occur making the magnetic reconnection region turbulent. The reported signatures of secondary magnetic reconnection can help to localize magnetic reconnection events in space, astrophysical and fusion plasmas.

Flux ropes form basic building blocks for magnetic dynamics in many plasmas, are macroscopic analogues of magnetic field lines, and are irreducibly three dimensional (3D). We have used the relaxation scaling experiment (RSX) to study flux ropes, and have found many new features involving 3D dynamics, kink instability driven reconnection, nonlinearly stable but kinking flux ropes, and large flows.