Experimentally inferred ion thermal diffusivity profiles in the Doublet III tokamak: Comparison with neoclassical theory

Abstract

Measurements of ion temperature profiles, obtained on a single-shot basis, have been coupled with transport analysis to obtain spatial profiles of the ion thermal diffusivity χi for a series of 18 neutral-beam-heated discharges in the Doublet III tokamak. The expanded boundary divertor discharges examined in this study had the following range of parameters: Bt = 2.53 T, Ip = 0.35–0.89 MA, vertical elongation = 1.4–1.9, n̄e = (3.7–7.9) × 1013 cm−3, PT = 3.5–6.6 MW, and νi* = 0.03–0.22 , for r/a = 0.2–0.7. The experimentally inferred values of χi have been compared with the values of the ion thermal diffusivity predicted by neoclassical theory. On the basis of a detailed error analysis, the results for all discharges examined are the same: χi is approximately equal to near the plasma centre, but for r/a between about 0.2 and 0.8, χi is both larger than and almost certainly has a spatial variation different from that of . The differences between χi and are larger than can be explained by errors in the experimental measurements and indicate that there are processes operative for the loss of ion heat which are not included in the usual transport models for ion confinement.