Plasma initiation in a tokamak with conducting superstructure: First results from Alcator C-MOD

摘要

Alcator C-MOD is a compact high field tokamak with capability for high current shaping, and closed and open diverters. Full tokamak operation was begun in May 1993 at 5T toroidal field, and plasmas lasting up to 0.4 seconds have been obtained so far. The tokamak's design includes a thick stainless-steel vacuum vessel that is toroidally continuous with resistance about 40 (mu)(Omega). The stainless-steel superstructure supporting the toroidal field coil is even more massive, and likewise has no toroidal breaks, primarily because structural strength would suffer. As a result, very large toroidal eddy currents are induced in the structures, especially during current initiation when the loop voltage is high. Future large experiments, such as ITER, are likely to adopt design approaches with comparable conducting structures. Despite the large eddy currents, which can reach 1 MA in Alcator C-MOD, reliable plasma initiation has been established by appropriate adjustment of the time evolution of the coil currents to cancel the passive contribution and obtain the desired field structure. Detailed diagnosis of the full magnetic field structure is necessary to do the adjustment; the field structure is then known in some detail, and its effects on the shape and stability during the current ramp have been studied. Flat-top plasma currents are presently up to 0.4 MA, and elongated and diverted plasmas have been obtained, as well as more circular limited configurations.