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anderson_2013_epr_final.pdf2013-02-23 12:54:49Jay Anderson

Energetic Ion Confinement and Stability in the RFP

Author: Jay K Anderson
Requested Type: Consider for Invited
Submitted: 2012-12-07 17:35:31

Co-authors: V. Belykh, S. Eilerman, G. Fiksel, C.B. Forest, J.J. Koliner, L. Lin, D. Liu, V.V. Mirnov, M.D. Nornberg, S. Polosatkin, J.A. Reusch, J.S. Sarff, Y. Tsidulko, J. Waksman

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

Abstract Text:
The envisioned burning plasma experiment, regardless of magnetic concept, relies on sufficient confinement of the charged fusion product for plasma self-heating. As such, the confinement of fast ions and their impact on the bulk plasma are crucial issues. While well-studied in tokamak plasmas, relatively little is known in RFP plasmas about the dynamics of fast ions and the effects they cause as a large population. Spontaneously generated energetic ions (resulting from acceleration during magnetic reconnection events) in the RFP plasma are frequently observed, but detailed studies with a known source are now just beginning in MST with an intense 25 keV, 1 MW neutral beam injector. Fast ions are confined much better than thermal particles in the stochastic RFP magnetic field (over the entire range of collisionality of MST plasmas), and a substantial population develops during NB injection. There are several effects on the background plasma including enhanced toroidal rotation, electron heating and an altered current density profile. The abundant fast particles affect the plasma stability. Fast ions at the island of the core-most resonant tearing mode have a stabilizing effect, and up to 60% reduction in the magnetic fluctuation amplitude is observed during NBI. While purely classical modeling predicts a super-Alfvenic fast ion density up to 25% of the electron density, the onset of beam driven instabilities and resonant fast ion transport occur at a lower fast ion concentration, of approximately 10%. Indeed, an interesting physics problem has developed: at low concentrations, core-localized fast ions are nearly classically confined and the population increases rapidly; a critical level is reached at which the confinement of fast ions is severely degraded leading to a saturated fast ion density. Experiments with scanned injection power reveal the critical value by reducing the particle source to the point where beam-driven instabilities are marginally stable. Classical modeling is then valid to predict the fast ion pressure. At this apparent fast ion beta limit, energetic ions are well confined in MST at similar values of normalized gyro-radius (ρf/a), fast ion density (nf/ne) and pressure (βf) of a projected burning RFP plasma with 2.5GW of fusion power.

Work supported by USDOE.

Characterization: 1.0,1.2

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University of Texas

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR2013)
February 12-15, 2013
Fort Worth, Texas

EPR 2013