Initial Experiments at High Normalized Current in the Pegasus Toroidal Experiment
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Initial experiments in the Pegasus ST exhibited an operational limit of I p ∼ I tf at A ∼ 1.15. With new power supplies, discharge control has greatly increased. Equilibria and stability modeling of the high I p/I tf regime has produced stable equilibria with I p/I tf up to 2 while values of I p/I tf approaching 3 show a potential upper bound on stability. A new operating regime has been accessed in recent experiments with the addition of electrostatic plasma sources. Using the sources as a pre-ionization technique, I p/I tf values of 1.5 have been accessed at very low toroidal fields. Using the sources as a means for non-inductive startup, values of I p/I tf up to 2.3 are attained. The MHD activity in these discharges is characteristically different than ohmic discharges, suggesting the new current sources are modifying the current profile to allow stable discharge evolution at minimal toroidal field.
KeywordsSpherical torus toroidal field utilization high normalized current magnetohydrodynamic stability
This work is funded by U.S. Department of Energy Grant DE-FG02-96ER54375.
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