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Aug 2009

Volume 16, Issue 8, Articles (08xxxx)

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Phys. Plasmas 16, 082701 (2009); http://dx.doi.org/10.1063/1.3195065 (14 pages)

I. V. Igumenshchev, F. J. Marshall, J. A. Marozas, V. A. Smalyuk, R. Epstein, V. N. Goncharov, T. J. B. Collins, T. C. Sangster, and S. Skupsky
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Dependence of the low to high confinement mode transition power threshold and turbulence flow shear on injected torque

D. J. Schlossberg, G. R. McKee, R. J. Fonck, K. H. Burrell, P. Gohil, R. J. Groebner, M. W. Shafer, W. M. Solomon, and G. Wang

Phys. Plasmas 16, 080701 (2009); http://dx.doi.org/10.1063/1.3192766 (4 pages) | Cited 2 times

Online Publication Date: 11 August 2009

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The power required to induce a bifurcation from a low-confinement mode to a high-confinement mode in DIII-D tokamak [ J. L. Luxon, Nucl. Fusion 42, 614 (2002) ] plasmas is found to depend sensitively on the injected neutral beam torque and consequent toroidal rotation. Plasmas exhibit a factor of 2–4 reduction in this power threshold, dependent on ion ∇B drift direction. Correlated with this change, turbulence velocity measurements near 0.9<r/a<1.0 for balanced injection demonstrate significantly larger poloidal flow shear at a given injection power, relative to cocurrent injection, facilitating the confinement transition.
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52.35.Ra Plasma turbulence
52.30.-q Plasma dynamics and flow
52.55.Fa Tokamaks, spherical tokamaks
52.55.-s Magnetic confinement and equilibrium
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The magnetic field reconnection site and dissipation region

P. L. Pritchett and F. S. Mozer

Phys. Plasmas 16, 080702 (2009); http://dx.doi.org/10.1063/1.3206947 (4 pages) | Cited 4 times

Online Publication Date: 12 August 2009

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Magnetic field reconnection is said to involve an ion diffusion region surrounding an electron diffusion region. Because of uncertainties in the meanings of these terms and on the physical parameters that characterize them, this paper defines the reconnection site as a region having an electron scale size and containing magnetic fields from four topologies, and the dissipation region, as having an ion scale size and surrounding the reconnection site. Two-dimensional, asymmetric, open, particle-in-cell simulations, with and without guide fields, examine these regions. It is found that significant values of (E+UI×B) and/or (E+Ue×B) are not confined to either the reconnection site or the dissipation region. The reconnection site is uninteresting because, for asymmetric reconnection, it does not contain processes that serve to locate it, such as electron acceleration, parallel electric fields, super-Alfvenic electron flow, maximum electron beta, electron nongyrotopy, or demagnetized thermal electrons. However, the surrounding dissipation region exhibits these features.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.55.-s Magnetic confinement and equilibrium
52.65.-y Plasma simulation

Generation of non-Gaussian statistics and coherent structures in ideal magnetohydrodynamics

M. Wan, S. Oughton, S. Servidio, and W. H. Matthaeus

Phys. Plasmas 16, 080703 (2009); http://dx.doi.org/10.1063/1.3206949 (4 pages) | Cited 10 times

Online Publication Date: 13 August 2009

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Spectral method simulations of ideal magnetohydrodynamics are used to investigate production of coherent small scale structures, a feature of fluid models that is usually associated with inertial range signatures of nonuniform dissipation, and the associated emergence of non-Gaussian statistics. The near-identical growth of non-Gaussianity in ideal and nonideal cases suggests that generation of coherent structures and breaking of self-similarity are essentially ideal processes. This has important implications for understanding the origin of intermittency in turbulence.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Ra Plasma turbulence
52.65.-y Plasma simulation

Nonlinear skin effect in a collisionless plasma

A. M. Froese, A. I. Smolyakov, and D. Sydorenko

Phys. Plasmas 16, 080704 (2009); http://dx.doi.org/10.1063/1.3211196 (4 pages) | Cited 2 times

Online Publication Date: 21 August 2009

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The skin effect in a warm collisionless plasma is studied via one-dimensional advective particle-in-cell simulations. It is found that the skin depth exhibits nonlinear behavior at low frequencies. A simple model describing the nonlinear behavior of the skin depth is proposed and shown to match the results of the simulations well. The transitions between local, anomalous, and nonlinear skin effect regimes are investigated in the parameter space of wave frequency and amplitude.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.65.Rr Particle-in-cell method
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