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(EX/P3-09) Performance and Stability Limits at Near-Unity Aspect Ratio in the Pegasus Toroidal Experiment

R. J. Fonck¹⁾, S. Diem¹⁾, G. Garstka¹⁾, M. Kissick¹⁾, B. Lewicki¹⁾, C. Ostrander¹⁾, P. Probert¹⁾, M. Reinke¹⁾, A. Sontag¹⁾, K. Tritz¹⁾, E. Unterberg^1)
 
1) University of Wisconsin, Madison, WI, USA

Abstract.  The Pegasus Toroidal Experiment is a mid-sized extremely-low aspect ratio (A) spherical torus (ST). It has the dual roles of exploring limits of ST behavior as A approaches 1 and studying the physics of ST plasmas in the tokamak-spheromak overlap regime. Major parameters are R = 0.25 - 0.45 m, A = 1.1 - 1.4, I_p $\leq$ 0.15MA, and B_t < 0.07T. High beta plasmas are produced at very low toroidal field by ohmic heating. Values of toroidal beta up to 25% have been obtained, and the operational space of beta vs I_p/aB_t is similar to that observed for NBI-heated START discharges. Achievable plasma current apparently is subject to a ``soft'' limit of I_p/I_tf $\leq$ 1. Access to higher-current plasmas appears to be restricted by the appearance of large internal MHD activity, including m/n=2/1 and 3/2 modes. Recent experiments have begun to access ideal stability limits, with disruptions observed as q₉₅ approaches 5, in agreement with numerical predictions.

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