Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session DI2: Electron Holes and ReconnectionInvited
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Chair: William Amatucci, Naval Research Laboratory Room: 210 CDGH |
Monday, October 31, 2016 3:00PM - 3:30PM |
DI2.00001: Helicity Transformation under the Collision and Merging of Magnetic Flux Ropes Invited Speaker: Timothy DeHaas A magnetic flux rope is a tube-like, current carrying plasma embedded in an external magnetic field. The magnetic field lines resemble threads in a rope, which vary in pitch according to radius. Flux ropes are ubiquitous in astrophysical plasmas, and bundles of these structures play an important role in the dynamics of the space environment. They are observed in the solar atmosphere [1] and near-earth environment [2] where they are seen to twist, merge, tear, and writhe. In this MHD context, their global dynamics are bound by rules of magnetic helicity conservation, unless, under a non-ideal process, helicity is transformed through magnetic reconnection, turbulence, or localized instabilities. These processes are tested under experimental conditions in the Large Plasma Device (LAPD). The device is a twenty-meter long, one-meter diameter, cylindrical vacuum vessel designed to generate a highly reproducible, magnetized plasma. Reliable shot-to-shot repetition of plasma parameters and over four hundred diagnostic ports enable the collection of volumetric datasets (measurements of n$_{\mathrm{e}}$, T$_{\mathrm{e}}$, V$_{\mathrm{p}}$, \textbf{B}, \textbf{J}, \textbf{E}, \textbf{u}$_{\mathrm{\mathbf{flow}}})$ as two kink-unstable flux ropes form, move, collide, and merge. Similar experiments on the LAPD have utilized these volumetric datasets, visualizing magnetic reconnection through a topological quasi-separatrix layer, or QSL [3]. This QSL is shown to be spatially coincident with the reconnection rate [4], \begin{figure}[htbp] \centerline{\includegraphics[width=0.43in,height=0.17in]{150720161.eps}} \label{fig1} \end{figure} $\int {E\cdot dl} ,$ and oscillates (although out of phase) with global helicity. Magnetic helicity is observed to have a negative sign and its counterpart, cross helicity, a positive one. These quantities oscillate 8{\%} peak-to-peak, and the changes in helicity are visualized as 1) the transport of helicity $(\phi B+E\times A)$ and 2) the dissipation of the helicity $-2E\cdot B$. [1] J. W. Cirtain, \textit{et al}. Nature 493, 501--503 (2013). [2] P.D. Henderson, \textit{et al}. Ann. Geophys., 24, 651 (2006) [3] W. Gekelman, \textit{et al.} ApJ, 753:131, (2012) [4] W. Gekelmen, \textit{et al. }Phys. Rev. Lett. 116, 235101 (2016) [Preview Abstract] |
Monday, October 31, 2016 3:30PM - 4:00PM |
DI2.00002: Recent progress of magnetic reconnection research in the MAST spherical tokamak Invited Speaker: Hiroshi Tanabe In the last three years, magnetic reconnection research in the MAST spherical tokamak achieved major progress by use of new 32 chord ion Doppler tomography, 130 channel YAG- and 300 channel Ruby-Thomson scattering diagnostics. In addition to the significant plasma heating up to $\sim$1keV [1], detailed full temperature profile measurements including the diffusion region have been achieved for the first time. 2D imaging measurements of $T_i$ and $T_e$ profiles have revealed that magnetic reconnection mostly heats ions globally in the downstream region of outflow jet and electrons locally at the X-point [2]. The higher toroidal field in MAST ($B_t>0.3$T) strongly inhibits cross-field thermal transport scaling as $1/B_t^2$ and the characteristic peaked $T_e$ profile at the X point is sustained on a millisecond time scale. In contrast, ions are mostly heated in the downstream region of outflow acceleration inside the current sheet width ($c/\omega_{pi}\sim$ 0.1m) and around the stagnation point formed by reconnected flux mostly by viscosity dissipation and shock-like compressional damping of the outflow jet. Toroidal confinement also contributes to the characteristic $T_i$ profile, forming a ring structure aligned with the closed flux surface. There is an effective confinement of the downstream thermal energy due to a thick layer of reconnected flux. The characteristic structure is sustained for longer than an ion-electron energy relaxation time (\tau^E_{ei}\sim4-11$ms) and the energy exchange between ions and electrons contributes to the bulk electron heating in the downstream region. The toroidal guide field mostly contributes to the formation of a localized electron heating structure at the X-point but not to bulk ion heating downstream. [1] Y. Ono {\it et al}., Phys. Plasmas {\bf 22}, 055708 (2015). [2] H. Tanabe {\it et al}., Phys. Rev. Lett. {\bf 115}, 215004 (2015). [Preview Abstract] |
Monday, October 31, 2016 4:00PM - 4:30PM |
DI2.00003: Experimental Demonstration of Resistive Electron Plasmoids in a Reconnecting Current Sheet Invited Speaker: Jonathan Jara-Almonte Magnetic reconnection is an important process occurring in nearly all magnetized plasmas that involves the complex coupling of multiple physical scales. Significant progress has been made in understanding the cross-scale physics of magnetic reconnection around localized reconnection sites, but how reconnection couples to global physics is still an open question. Recently, the spontaneous formation of plasmoids has been proposed as a mechanism for bridging widely disparate scales, thereby permitting fast reconnection in large systems. Numerous works have demonstrated the existence of collisionless plasmoids in both space and laboratory plasmas, however to-date, direct evidence for collisional plasmoids has been confined to numerical simulations and analytic theory, although remote-sensing observations of solar and fusion plasmas have provided some indirect evidence. However, it is known that many naturally occurring plasmas, such as the solar chromosphere or the interstellar medium, are both large and collisional, thus requiring collisional plasmoids. In part, the current lack of experimental or in situ observational evidence for collisional plasmoids is due to the large Lundquist numbers required for plasmoid formation within the resistive MHD framework. In this work [1], experimental evidence for resistive electron plasmoid formation during magnetic reconnection in the two-fluid regime is given. Using the Magnetic Reconnection Experiment (MRX), driven reconnection is studied in collisional current sheets wherein the electric field is balanced solely by classical Spitzer resistivity. Despite low Lundquist numbers, these collisional current sheets are observed to be unstable to the spontaneous formation of plasmoids, which is explained by the importance of electron physics when in the two-fluid regime. The number of plasmoids is observed to scale with the Lundquist number. Due to the onset of plasmoids, both the local reconnection electric field and the globally normalized reconnection rate are observed to increase above an already fast rate. \begin{thebibliography}{9} \bibitem{jaraalmonte} J. Jara-Almonte \textit{et al.}, submitted to Phys. Rev. Lett. \end{thebibliography} [Preview Abstract] |
Monday, October 31, 2016 4:30PM - 5:00PM |
DI2.00004: Plasmoids formation in a laboratory and large-volume flux closure during simulations of Coaxial Helicity Injection in NSTX-U Invited Speaker: Fatima Ebrahimi In NSTX-U, transient Coaxial Helicity Injection (CHI) is the primary method for current generation without reliance on the solenoid. A CHI discharge is generated by driving current along open field lines (the injector flux) that connect the inner and outer divertor plates on NSTX/NSTX-U, and has generated over 200 kA of toroidal current on closed flux surfaces in NSTX. Extrapolation of the concept to larger devices requires an improved understanding of the physics of flux closure and the governing parameters that maximizes the fraction of injected flux that is converted to useful closed flux. Here, through comprehensive resistive MHD NIMROD simulations conducted for the NSTX and NSTX-U geometries, two new major findings will be reported. First, formation of an elongated Sweet-Parker current sheet and a transition to plasmoid instability has for the first time been demonstrated by realistic global simulations. [F. Ebrahimi and R. Raman, PRL 114, 205003 (2015)] This is the first observation of plasmoid instability in a laboratory device configuration predicted by realistic MHD simulations and then supported by experimental camera images from NSTX. Second, simulations have now, for the first time, been able to show large fraction conversion of injected open flux to closed flux in the NSTX-U geometry. [F. Ebrahimi, R. Raman Nuclear Fusion 56, 044002 (2016)] Consistent with the experiment, simulations also show that reconnection could occur at every stage of the helicity injection phase. The influence of 3D effects, and the parameter range that supports these important new findings is now being studied to understand the impact of toroidal magnetic field and the electron temperature, both of which are projected to increase in larger ST devices. [Preview Abstract] |
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