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Studies in the high-field utilization regime at near-unity aspect ratio in the Pegasus Toroidal Experiment

Author: Raymond J Fonck
Requested Type: Consider for Invited
Submitted: 2009-12-04 16:35:34

Co-authors: J. Barr, M. Bongard, E. Hinson, A. Redd, D. Schlossberg, N. Woodruff

Contact Info:
University of Wisconsin
1500 Engineering Dr.
Madison, WI   53706
USA

Abstract Text:
A major thrust of the PEGASUS research program centers on exploring and exploiting the unique plasma characteristics in the tokamak-spheromak overlap regime at near-unity aspect ratio. This includes the attainment and study of very high-βT plasmas in the regime of high normalized current (IN > 10) or high toroidal field utilization factor (Ip/ITF > 1). Critical issues are macroscopic stability and current profile manipulation to explore both internal and external MHD limits in this unique A → 1 parameter regime. Early experiments showed ohmically initiated plasmas to be limited to Ip/ITF ≤ 1 due to the excitation of large 2/1 internal tearing modes in wide regions of minimal shear as q(0) ~ 2. These modes were transiently reduced with increasing shear near the resonant surface, but more control of the j(r) profile is needed to access the interesting Ip/ITF > 2 regime for a sustained period. To that end, new capabilities provide flexible control of the plasma evolution. These include: loop voltage programmability; a time-variable toroidal field system; extended poloidal field coil systems and divertor coils; and development of point-source helicity injection as a means of current and poloidal field generation without an ohmic solenoid. These capabilities are being exploited to further understand and manipulate the stability limits at near-unity aspect ratio. Specifically, the Ip/ITF > 1 regime has been attained through: rapid Bt rampdown after high Ip formation; strong ohmic drive with plasma gun pre-ionization to allow formation at very low Bt; and plasma initiation with very broad current profiles using helicity injection via strong current drive at the plasma periphery. These experiments have demonstrated access to the Ip/ITF > 2 regime, albeit at low Ip (< 0.05 MA) to date. The helicity-driven plasmas result in very low li ~ 0.2 – 0.3, broad current profiles that are MHD-quiescent for some time after gun termination. During that time, the current profiles relax from hollow, reversed shear profiles to the more peaked, near-zero shear profiles typical of ohmically driven low-A plasmas. As the profiles relax, the large n =1 modes reappear. Recent experiments at higher Ip formation via gun-helicity injection suggest that these modes can be mitigated for increasingly longer periods as the plasma temperature increases and resistivity decreases. Thus, the helicity-injection startup is being increased to the Ip ~ 0.3MA range to understand confinement during helicity injection, and to provide MHD-quiescent plasmas at Ip/ITF ≥ 2 as targets for auxiliary heating via High-Harmonic Fast-Wave Heating and/or Electron Bernstein Wave heating and current drive. Finally, operation at low li, and low BTF results in high values of jedge/B, and thus provides a unique opportunity for controlled experiments on the properties of peeling-ballooning modes at the plasma periphery.

Characterization: A2,E2

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