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carmody_epr_2014.pdf2014-08-15 12:16:38Daniel Carmody

Microinstability-driven Transport in the Madison Symmetric Torus

Author: Daniel R. Carmody
Requested Type: Consider for Invited
Submitted: 2014-05-28 15:16:22

Co-authors: M.J. Pueschel, P.W. Terry

Contact Info:
UW - Madison
1150 University Ave
Madison, Wisconsin   53715
United States

Abstract Text:
In standard operation, the reversed field pinch (RFP) is dominated by large-scale tearing modes driven by gradients in the current profile. However, through the use of pulsed poloidal current drive (PPCD), a current profile control technique, it has been possible to reduce global tearing mode activity and to increase the energy confinement time by up to a factor of ten. With global fluctuations suppressed, small scale drift-wave instabilities may be able to make significant contributions to particle and energy transport. In this work, we explore the role of microinstabilities in driving transport in the Madison Symmetric Torus (MST) using the gyrokinetic code GENE. We use a concentric circle equilibrium model to simulate two PPCD discharges in MST, with plasma currents of 200 kA and 500 kA, and find the dominant linear instabilities to be an ion temperature gradient (ITG) and trapped electron mode (TEM), respectively, the latter driven by the density gradient. We also present results from nonlinear gyrokinetic calculations for the 500 kA case in the outer core region. We calculate ion and electron heat and particle fluxes and find a strong Dimits-like shift in the threshold gradient for transport, with a critical value roughly three times the linear one. The suppression of transport even in the presence of strong linear growth rates is associated with strong zonal flow activity, likely a result of strong magnetic shear in the RFP. Furthermore, the critical nonlinear density gradient is close to the experiment value, suggesting that microturbulence may be the process limiting confinement in these discharges.

Characterization: 2.0

Comments:
Possibly group with other MST work.

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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