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Varying Stochasticity in the Core of the MST RFP

Author: Joshua A. Reusch
Requested Type: Consider for Invited
Submitted: 2009-12-04 15:21:22

Co-authors: J.K. Anderson, D.J. Den Hartog, F. Ebrahimi, C.B. Forest, M.B. McGarry, D.D. Schnack, H.D. Stephens

Contact Info:
University of Wisconsin at Madison
1150 University Ave.
Madison, WI   53706
USA

Abstract Text:
The low safety factor in the RFP allows for a large number of overlapping magnetic islands and nearly global stochasticity due to resonant magnetic tearing modes. Parallel losses along stochastically wandering field lines enhance electron thermal diffusion. In the MST we have recently observed, in both experiment and in simulation, that the level of stochasticity in the core plasma varies significantly over the course of quasi-periodic global reconnection events known as sawteeth. Furthermore, the Rechester-Rosenbluth stochastic transport model overestimates the core thermal diffusion except right at the sawtooth crash, suggesting that the core is only fully stochastic during this period of the sawtooth cycle. In the experiment, recent upgrades to the Thomson scattering system have allowed us to identify three distinct phases of transport in the sawtooth cycle. In the phase before the crash, core electron transport is dominated by remnant tearing mode islands that flatten the temperature profile across the island structures. At the sawtooth crash, the temperature profile flattens globally across the minor radius. This flattening is not correlated with any particular tearing mode, but is due instead to fully developed stochasticity induced by the sudden increase in amplitude of many resonant modes. After the crash, a large helical temperature structure appears and persists for a few milliseconds, consistent with a hot island very near the magnetic axis. To further our understanding of these plasmas, simulations with the nonlinear 3D resistive MHD code DEBS were performed with the neoclassical resistivity profile and the Lundquist number observed in the experiment (S~4x10^6). These simulations show remarkable agreement with experimental data from MST. The evolution of the mean field through the sawtooth cycle is well reproduced, as are the mode spectra and the time evolution of the mode amplitudes. The same three phases of the experimental sawtooth cycle are observed in the simulations by tracing the magnetic field lines. Initially, there is a remnant island structure due to an internally resonant tearing mode that partially overlaps the next nearest mode. At the crash, the magnetic field becomes completely stochastic over the entire plasma volume. Shortly after the crash, a large externally resonant mode creates an island structure very near the magnetic axis that contains fully healed magnetic surfaces. This combination of experimental data and simulation provides strong evidence that the core of the RFP is rarely fully stochastic. This work supported by the US DOE.

Characterization: E1,E10

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