ATF flux surfaces and related plasma effects

Flux surfaces in the Advanced Toroidal Facility (ATF) were mapped using an electron beam which was incident on a fluorescent screen. Islands were found at r/a ≥ 0.6, indicating the existence of field errors. Failure of the island size to scale with magnetic field indicated that the islands were intrinsic to the coils. The source of the field errors was found to be uncompensated dipoles in the helical coil feeds. The electron temperature was observed to be very low in the vicinity of the islands. Modifications were made to the helical field buswork to eliminate the field errors, and the flux surfaces were again checked using an electron beam. Islands at r/a ≥ 0.6 were found to be greatly reduced in size, with the residual island at ι-bar = 1/2 scaling to 1 cm at B = 1 T. Initial experiments indicate that the plasma operating space has been extended since the buswork modifications. (author). 4 refs, 3 figs