Temperature behaviour during neutral beam injection in a torsatron (ATF) with field errors

Abstract

Target plasmas for neutral beam injection (NBI) in the Advanced Toroidal Facility (ATF) are formed using approximately 200 kW of 53 GHz electron cyclotron heating (ECH). During the early operational period of this device, field errors existed which produced large islands at the ι = 1/2 surface and several smaller, but non-negligible closely spaced islands at larger radii. The evolution of typical plasma parameters during NBI into plasmas produced in this perturbed magnetic configuration is discussed. The ion temperatures near the centre of the ECH target plasmas, as measured from Doppler broadening of impurity lines, are approximately 100 eV, and the central electron temperatures are close to 800 eV. NBI at powers up to 1.4 MW characteristically raises Tj(O) by factors of two to three for 50−80 ms, but thereafter the ion temperature decays. The electron temperature is not raised by NBI above its level in the ECH plasmas; it typically drops monotonically from the start of injection. Both the Ti and the Te profiles indicate the existence of a low temperature region from the ι = 1/2 surface (ρ = 0.6) to the plasma edge, apparently because of the island structure, and modelling suggests that the plasma collapse may be caused by the peculiar radiation profile which develops under these conditions. However, whether the characteristics of the temperature profiles and the collapse can truly be ascribed to the existence of the islands can be decided only after repair of the field errors.