C. D. Cothran

Two fluid effects on three-dimensional reconnection in the Swarthmore Spheromak Experiment with comparisons to space data

Several new experimental results are reported from spheromak merging studies at the Swarthmore Spheromak Experiment [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] with relevance to three-dimensional (3D) reconnection in laboratory and space plasmas. First, recent velocity measurements of impurity ions using ion Doppler spectroscopy are reported. Bidirectional outflow at nearly the Alfven speed is clearly observed. Second, experimental measurements of the out-of-plane magnetic field in a reconnection volume showing a quadrupolar structure at the ion inertial scale are discussed. Third, a measurement of in-plane Hall electric field and nonideal terms of the generalized Ohm’s law in a reconnection volume of a weakly collisional laboratory plasma is presented. Time resolved vector magnetic field measurements on a 3D lattice [B(r,t)] enables evaluation of the various terms. Results show that the Hall electric field dominates everywhere (J×B∕ne) and also exhibits a quadrupolar structure at the ion inertial scale; ...

Numerical Study Of The Formation, Ion Spin-Up And Nonlinear Stability Properties Of Field-Reversed Configurations

Results of three-dimensional (3D) numerical simulations of field-reversed configurations (FRCs) are presented. The emphasis of this work is on the nonlinear evolution of magnetohydrodynamic (MHD) instabilities in kinetic FRCs and the new FRC formation method by counter-helicity spheromak merging. Kinetic simulations show nonlinear saturation of the n = 1 tilt mode, where n is the toroidal mode number. The n = 2 and n = 3 rotational modes are observed to grow during the nonlinear phase of the tilt instability due to the ion spin-up in the toroidal direction. The ion toroidal spin-up is shown to be related to the resistive decay of the internal flux and the resulting loss of particle confinement. Three-dimensional MHD simulations of counter-helicity spheromak merging and FRC formation show good qualitative agreement with the results from the SSX–FRC experiment. The simulations show the formation of an FRC in about 20–30 Alfven times for typical experimental parameters. The growth rate of the n = 1 tilt mode is shown to be significantly reduced compared with the MHD growth rate due to the large plasma viscosity and field-line-tying effects.

Experimental Observation Of Energetic Ions Accelerated By Three-Dimensional Magnetic Reconnection In A Laboratory Plasma

Magnetic reconnection is widely believed responsible for heating the solar corona as well as for generating X-rays and energetic particles in solar flares. On astrophysical scales, reconnection in the intergalactic plasma is a prime candidate for a local source (!100 Mpc) of cosmic rays exceeding the Greisen-Zatsepin-Kuzmin cutoff (∼10 19 eV). In a laboratory astrophysics experiment, we have made the first observation of particles accelerated by magnetic reconnection events to energies significantly above both the thermal and the characteristic magneto- hydrodynamic energies. These particles are correlated temporally and spatially with the formation of three- dimensional magnetic structures in the reconnection region. Subject headings: acceleration of particles — cosmic rays — magnetic fields — methods: laboratory — MHD — plasmas Magnetic reconnection occurs when two bodies of highly conductive plasma bearing oppositely directed embedded mag- netic fields merge (Brown 1999; Priest & Forbes 2000). In the simplest, two-dimensional picture, the interface where the in- flowing magnetofluid stagnates contains a current sheet to sup- port the curl of the magnetic field and an electric field to support the consumption of magnetic flux (see Fig. 1). Within the bulk of each inflow, the motion of the field and fluid are coupled owing to the high conductivity. At the interface, this condition no longer holds, and field lines convected into this region break and reconnect across the layer, producing a global change in field topology. The reconnection outflow, coplanar and trans- verse to the inflow, exits the reconnection region with a speed not exceeding that of a magnetohydrodynamic (MHD) or Alf- ven wave of the coupled fluid and field. In 1/2

Three-dimensional reconnection and relaxation of merging spheromak plasmas

Plasma relaxation inside a highly conducting cylindrical boundary is studied both experimentally and computationally. Dynamics are initiated by the introduction of two equal helicity spheromaks at either end of the cylinder. In the experiment, dense, high-magnetic-flux spheromaks are injected into the flux conserving volume with magnetized plasma guns. In the simulation, identical spheromaks initially occupy both halves of the cylinder and a perturbation is introduced. Merging commences with a single three-dimensional null-point that moves radially out of the flux conserving volume at velocities up to 0.2 of the reconnection outflow velocity. Relaxation to the minimum energy state occurs in about ten Alfven times. An important conclusion is that even though the dynamical activity is limited to a few modes, this activity is sufficient to promote relaxation to the final, minimum energy state. The dynamical activity appears to conserve magnetic helicity while magnetic energy is converted to flow and heat. The final state arrived at dynamically is identical to that described by C. D. Cothran et al. [Phys. Rev. Lett. 103, 215002 (2009)] using static, eigenvalue analysis.

Fast high resolution echelle spectroscopy of a laboratory plasma

An echelle diffraction grating and a multianode photomultiplier tube are paired to construct a high resolution (R=λ∕δλ≈2.5×104) spectrograph with fast time response for use from the UV through the visible. This instrument has analyzed the line shape of CIII impurity ion emission at 229.687nm over the lifetime (≈100μs) of the hydrogen plasmas produced at SSX. The ion temperature and line of sight average velocity are inferred from the observed thermal broadening and Doppler shift of the line. The time resolution of these measurements is about 1μs, sufficient to observe the fastest magnetohydrodynamic activity.

Fluid and kinetic structure of magnetic merging in the Swarthmore Spheromak Experiment

Received 1 July 2005; revised 27 September 2005; accepted 26 October 2005; published 6 December 2005. [1] Measurement of the in-plane Lorentz force and the outof-plane magnetic field associated with the Hall electric field near the reconnection zone in the Swarthmore Spheromak Experiment (SSX) confirms expectations, based on simulation, theory and spacecraft data, that the quadrupolar out-of-plane magnetic field is a signature of collisionless effects in magnetic reconnection with a weak guide field. Citation: Matthaeus, W. H., C. D. Cothran, M. Landreman, and M. R. Brown (2005), Fluid and kinetic structure of magnetic merging in the Swarthmore Spheromak Experiment, Geophys. Res. Lett., 32, L23104, doi:10.1029/ 2005GL023973.

Rapid multiplexed data acquisition: Application to three-dimensional magnetic field measurements in a turbulent laboratory plasma

Multiplexing electronics have been constructed to reduce the cost of high-speed data acquisition at the Swarthmore Spheromak Experiment (SSX) and Redmond Plasma Physics Laboratory. An application of the system is described for a three-dimensional magnetic probe array designed to resolve magnetohydrodynamic time scale and ion inertial spatial scale structure of magnetic reconnection in a laboratory plasma at SSX. Multiplexing at 10 MHz compresses 600 pick-up coil signals in the magnetic probe array into 75 digitizer channels. An external master timing system maintains synchronization of the multiplexers and digitizers. The complete system, calibrated and tested with Helmholtz, line current, and magnetofluid fields, reads out the entire 5×5×8 probe array every 800 ns with an absolute accuracy of approximately 20 G, limited mainly by bit error.

Three‐dimensional structure of magnetic reconnection in a laboratory plasma

[1] The local three-dimensional structure of magnetic reconnection has been measured for the first time in a magnetohydrodynamic (MHD) laboratory plasma at the Swarthmore Spheromak Experiment. An array of 600 magnetic probes which resolve ion inertial length and MHD time scale dynamics on a single shot basis measured the magnetic structure of partial spheromak merging events. Counter-helicity spheromaks merge rapidly, and reconnection activity clearly self-generates a local component of B which breaks the standard 2D symmetry at the ion inertial scale. Consistent with prior results, no reconnection is observed for co-helicity merging.

Spectroscopic measurements of temperature and plasma impurity concentration during magnetic reconnection at the Swarthmore Spheromak Experiment

Electron temperature measurements during counterhelicity spheromak merging studies at the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] are presented. VUV monochromator measurements of impurity emission lines are compared with model spectra produced by the non-LTE excitation kinematics code PRISMSPECT [J. J. MacFarlane et al., in Proceedings of the Third Conference on Inertial Fusion Science and Applications (2004)] to yield the electron temperature in the plasma with 1 μs time resolution. Average Te is seen to increase from 12 to 19 eV during spheromak merging. Average C III ion temperature, measured with a new ion Doppler spectrometer (IDS) [C. D. Cothran et al., Rev. Sci. Instrum. 77, 063504 (2006)], likewise rises during spheromak merging, peaking at ∼22 eV, but a similar increase in Ti is seen during single spheromak discharges with no merging. The VUV emission line measurements are also used to constrain the concentrations of various impurities in the SSX plasma, ...

Dipole trapped spheromak in a prolate flux conserver

This paper reports the observation and characterization of a spheromak formed in the Swarthmore Spheromak Experiment (SSX, [M. R. Brown, Phys. Plasmas 6, 1717 (1999)]) and trapped in a simple dipole magnetic field. The spheromak is studied in a prolate (tilt unstable) 0.4m diameter, 0.6m length copper flux conserver in SSX. This plasma does not tilt, despite the prolate flux conserver. The spheromak is characterized by a suite of magnetic probe arrays for magnetic structure B(r,t), ion Doppler spectroscopy for Ti and flow, and interferometry for ne. Three-dimensional magnetohydrodynamics simulations of this configuration verify its gross sta-bility.