Non-inductive startup of the Pegasus spherical torus using localized washer-gun current sources
Abstract
Non-inductive formation of tokamak-like plasmas using localized washer-gun current sources has been demonstrated on the Pegasus Toroidal Experiment. The sources were located in the lower divertor region and aligned to inject current (Iinj) onto crossed toroidal and poloidal fields (BV) when biased relative to the upper divertor. At high BV, the injected current followed the helical field lines, and the current multiplication (M = I φ/Iinj) was determined by the toroidal transits of the vacuum field. At low BV (∼ 0.005 T), Iφ overwhelmed BV and relaxed into a tokamak-like configuration. Iinj < 4 kA produces discharges of Iφ > 50 kA. The tokamak-like nature of these relaxed plasmas was indicated by the independence of M from the helical windup of the vacuum field and increased decay time over that of helical plasmas Also, oxygen line ratios indicated increased heating of the plasmas, suggestive of improved confinement, and the relaxed plasmas conformed to tokamak equilibrium. The non-inductive current drive corresponded to helicity balance principles. The calculated effective loop voltage of the injectors closely matched the surface decay voltage of relaxed plasmas, and the current drive power efficiency closely matched the efficiency predicted assuming helicity balance. The relaxed plasmas exhibited a high toroidal field utilization factor, Ip/ITF < 2.3, IN > 12 MA/m-T. Application of ohmic induction during the decay phase resulted in Iφ greater than or equal to the sum of the corresponding purely non-inductive and purely inductive Iφ, indicating that the non-inductive plasmas were compatible with ohmic current drive.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2007
- Bibcode:
- 2007PhDT........30E