Mode conversion electron heating in Alcator C-Mod: Theory and experiment

Abstract

Localized electron heating [full width at half maximum of Delta(r/a) approximate to 0.2] by mode converted ion Bernstein waves (IBW) has been observed in the Alcator C-Mod tokamak [I. H. Hutchinson , Phys. Plasmas 1, 1511 (1994)]. These experiments were performed in D(He-3) plasmas at high magnetic field (B-0 = 7.9 T), high-plasma density (n(e0) greater than or equal to 1.5 x 10(20) m(-3)), and for 0.05 less than or equal to n(He-3)/n(e) less than or equal to 0.30. Electron heating profiles of the mode converted IBW were measured using a break in slope analysis of the electron temperature versus time in the presence of rf (radio frequency) modulation. The peak position of electron heating was found to be well-correlated with He-3 concentration, in agreement with the predictions of cold plasma theory. Recently, a toroidal full-wave ion cyclotron range of frequencies (ICRF) code TORIC [M. Brambilla, Nucl. Fusion 38, 1805 (1998)] was modified to include the effects of IBW electron Landau damping at (k(perpendicular to)rho(i))(2) much greater than 1, This model was used in combination with a 1D (one-dimensional) integral wave equation code METS [D. N. Smithe et al., Radio Frequency Power in Plasmas, AIP Conf. Proc. 403 (1997), p. 367] to analyze these experiments. Model predictions were found to be in qualitative and in some instances quantitative agreement with experimental measurements. A model for mode conversion current drive (MCCD) has also been developed which combines a toroidal full wave code with an adjoint evaluation of the ICRF current drive efficiency. Predictions for off-axis MCCD in C-Mod have been made using this model and will be described.