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Magnetic-Diffusion-Induced Electron Transport in the MST RFP

Author: Joshua A. Reusch
Requested Type: Consider for Invited
Submitted: 2011-06-10 17:35:02

Co-authors: J.K. Anderson, D.J. Den Hartog, C.B. Forest, C.P. Kasten, D.D. Schnack, H.D. Stephens

Contact Info:
UW - Madison
1150 University Ave.
Madison, WI   53703

Abstract Text:
In this work, the results from two extensive nonlinear resistive MHD simulations, run at parameters matching those of 400kA discharges in MST (S~4x106), are used to investigate electron thermal diffusion, χe, in the RFP. The first simulation is a zero beta simulation in which the measured χe is compared to the thermal diffusion due to parallel losses along diffusing magnetic field lines, χst=VllDmag. Agreement is only found if the reduction in χst due to trapped particles is taken into account. In the second simulation, the pressure field was evolved self consistently assuming Ohmic heating and anisotropic thermal conduction. The preliminary results from this simulation show that fluctuations in the simulated temperature are very similar in character and time evolution to temperature fluctuations measured in MST. The zero-β simulation is useful for isolating the contribution of the magnetic stochasticity to the overall thermal diffusion measured in the experiment. The evolution of the Dmag profile was determined for over 20 sawteeth so that the ensemble averaged evolution could be compared to the sawtooth ensembled data from MST. The result of this comparison shows that χst, which should represent the minimum possible thermal diffusion, is larger than χe at most times. However, if electrons are trapped in a magnetic well they cannot carry energy along the diffusing magnetic field lines and thus χst should be reduced by the circulating particle fraction in order to be compare with the experimental measurements. This reduction brings χst to within uncertainty of χe in the mid radius at most times throughout the sawtooth cycle. In the core, the measured χe greater than χst leading up to and including the sawtooth crash, suggesting other transport mechanisms such as temperature flattening due to magnetic islands, may be important or even dominant transport mechanisms at this time. In the finite beta simulation, the plasma pressure was allowed to evolve self consistently. The striking thing about this simulation is the evolution of the temperature fluctuations in time. Before the sawtooth, an m=1, n=6 fluctuation flattens the temperature profile while after the sawtooth an m=1, n=5 fluctuation appears near the core generating a hot island. At the crash, m=0, n=1 temperature fluctuations can also be seen spanning the minor radius of the plasma. All of this is consistent with temperature fluctuation measurements made in MST. This work supported by the US DOE and NSF.

Characterization: A7,D1


University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011