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=lambda/delta lambda approximate to 2.5x10(4)) spectrograph with fast time response for use from the UV through the visible. This instrument has analyzed the line shape of C III impurity ion emission at 229.687 nm over the lifetime (approximate to 100 mu s) 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 1 mu s, sufficient to observe the fastest magnetohydrodynamic activity

Three-Dimensional Structure Of Magnetic Reconnection In A Laboratory Plasma

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

Generalized Ohm\u27s Law In A 3-D Reconnection Experiment

We report the measurement of non-ideal terms of the generalized Ohm\u27s law at a reconnection site of a weakly collisional laboratory magnetohydrodynamic plasma. Results show that the Hall term dominates the measured terms; resistive and electron inertia terms are small. We suggest that electron pressure (not measured) supports the observed quasistatic reconnection rate, and that anomalous resistivity, while not ruled out, is not required to account for the results

Off-shell Corrections and Moments of the Deep Inelastic Nuclear Structure Functions

We present an improved method for handling off-shell effects in deep inelastic nuclear scattering. With a firm understanding of the effects of the nuclear wave function, including these off-shell corrections as well as binding and nucleon-nucleon correlations, we can begin to examine the role of QCD in nuclei through an analysis of the moments of the nuclear structure function. Our analysis is aimed at extracting the Q^2 dependence of the moments of the nucleon structure function by using the recent high x world Iron data and by properly removing nuclear effects from the perturbative contribution. In addition, we compare quantitatively the behavior of the extracted moments with a simple O(1/Q^2) phenomenological form and we determine the mass term for this parametrization.Comment: 13 pages, 3 figures, submitted to Physics Letters

Three-Dimensional Magnetohydrodynamics Simulations Of Counter-Helicity Spheromak Merging In The Swarthmore Spheromak Experiment

Recent counter-helicity spheromak merging experiments in the Swarthmore Spheromak Experiment (SSX) have produced a novel compact torus (CT) with unusual features. These include a persistent antisymmetric toroidal magnetic field profile and a slow, nonlinear emergence of the n = 1 tilt mode. Experimental measurements are inconclusive as to whether this unique CT is a fully merged field-reversed configuration (FRC) with strong toroidal field or a partially merged doublet CT configuration with both spheromak- and FRC-like characteristics. In this paper, the SSX merging process is studied in detail using three-dimensional resistive MHD simulations from the Hybrid Magnetohydrodynamics (HYM) code. These simulations show that merging plasmas in the SSX parameter regime only partially reconnect, leaving behind a doublet CT rather than an FRC. Through direct comparisons, we show that the magnetic structure in the simulations is highly consistent with the SSX experimental observations. We also find that the n = 1 tilt mode begins as a fast growing linear mode that evolves into a slower-growing nonlinear mode before being detected experimentally. A simulation parameter scan over resistivity, viscosity, and line-tying shows that these parameters can strongly affect the behavior of both the merging process and the tilt mode. In fact, merging in certain parameter regimes is found to produce a toroidal-field-free FRC rather than a doublet CT. (C) 2011 American Institute of Physics. [doi:10.1063/1.3660533

Flow Dynamics And Plasma Heating Of Spheromaks In SSX

We report several new experimental results related to flow dynamics and heating from single dipole-trapped spheromaks and spheromak merging studies at SSX. Single spheromaks (stabilized with a pair of external coils, see Brown, Phys. Plasmas 13 102503 (2006)) and merged FRC-like configurations (see Brown, Phys. Plasmas 13, 056503 (2006)) are trapped in our prolate (R = 0.2 m, L = 0.6 m) copper flux conserver. Local spheromak flow is studied with two Mach probes (r(1) = rho(i) ) calibrated by time-of-flight with a fast set of magnetic probes at the edge of the device. Both Mach probes feature six ion collectors housed in a boron nitride sheath. The larger Mach probe will ultimately be used in the MST reversed field pinch. Line averaged flow is measured by ion Doppler spectroscopy (IDS) at the midplane. The SSX IDS instrument measures with 1 mu s or better time resolution the width and Doppler shift of the C-III impurity (H plasma) 229.7 nm line to determine the temperature and line-averaged flow velocity (see Cothran, RSI 77, 063504 (2006)). We find axial flows up to 100 km/s during formation of the dipole trapped spheromak. Flow returns at the wall to form a large vortex. Recent high-resolution IDS velocity measurements during spheromak merging show bi-directional outflow jets at +/- 40 km/s (nearly the Alfven speed). We also measure T-i \u3e= 80 eV and T-e \u3e= 20 eV during spheromak merging events after all plasma facing surfaces are cleaned with helium glow discharge conditioning. Transient electron heating is inferred from bursts on a four-channel soft x-ray array. The spheromaks are also characterized by a suite of magnetic probe arrays for magnetic structure B(r,t), and interferometry for n(e) . Finally, we are designing a new oblate, trapezoidal flux conserver for FRC studies. Equilibrium and dynamical simulations suggest that a tilt-stable, oblate FRC can be formed by spheromak merging in the new flux conserver

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 (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 magnetohydrodynamic energies. These particles are correlated temporally and spatially with the formation of three-dimensional magnetic structures in the reconnection region

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 SpheromakExperiment[M. R. Brown, Phys. Plasmas6, 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 Alfvén 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; resistive and electron inertia terms are small. Each of these is related to and compared with similar measurements in a solar or space context

Fluid And Kinetic Structure Of Magnetic Merging In The Swarthmore Spheromak Experiment

Measurement of the in-plane Lorentz force and the out-of-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

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. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3492726