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Characteristics of non-collisional ion heating in the MST RFP

Author: Richard M Magee
Requested Type: Consider for Invited
Submitted: 2009-12-04 15:42:06

Co-authors: B. E. Chapman, D. Craig, D. J. Den Hartog, G. Fiksel, S. T. A. Kumar, J. S. Sarff

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

Abstract Text:
In the Madison Symmetric Torus (MST) reversed-field pinch (RFP), discrete magnetic reconnection events liberate up to ~ 200 MW of power from the equilibrium magnetic field, of which 10-25% is channeled to the ions, producing a four-fold increase in Ti to values much greater than Te. Multiple theoretical models of the heating mechanism have been proposed, however to date no single theory appears adequate. Despite this, it has already been exploited on the MST to achieve sustained high ion temperatures (> keV). By triggering current density profile control on a reconnection event, the heat can be captured while the confinement degradation of subsequent events is suppressed. A theoretical understanding of the underlying physics of anomalous ion heating could lead to additional RFP performance enhancements, and recent measurements on MST promise to further constrain theory. Preliminary charge exchange recombination spectroscopy measurements of the impurity ions reveal a long-lived temperature anisotropy above a density threshold of ne ~ 1.0e13 cm-3, where Tperp/Tpar > 1. This could be evidence that the heating is primarily acting in the perpendicular direction, although the lack of isotropization suggests additional energy transfer mechanisms may be active. Rutherford scattering measurements of the bulk ion temperature reveal a square-root mass dependence to the heating efficiency. This is close to the predictions of both stochastic and ion-cyclotron-resonance heating models. These and other ion heating observations will be discussed in the context of theoretical models.

Characterization: E3

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