A comparison of measured and calculated toroidal Alfvén eigenmode damping rates in Alcator C-Mod

Abstract

Measured damping rates of stable toroidal Alfvén eigenmodes (TAEs) have been compared with damping rates calculated with the NOVA-K code for a number of experimental conditions on Alcator C-Mod to test the NOVA-K damping model. Very low amplitude ( 10−7 at the wall) stable TAEs are excited in Ohmic plasmas with a pair of active MHD antennas inside C-Mod. By sweeping the excited frequency of the antennas through the expected TAE frequency, the frequency width of a mode resonance can be measured with poloidal field pick-up coils on outboard limiters to determine the damping rate of that mode. The calculated equilibrium and main plasma parameters at the time of the resonance are input into the NOVA-K code to then calculate the expected damping rate for the measured toroidal mode number and compare with the experimental value. The calculations include continuum damping, Landau damping on both the electrons and the background and impurity ions and collisional damping on the electrons. Comparisons have been made for inner wall limited and diverted cases over a range of ITER-relevant moderate toroidal mode numbers (4 ⩽ |n | ⩽ 9). Good agreement between the experiment and theoretical calculations can be obtained with reasonable q profiles for the plasma conditions in which perturbative NOVA models are applicable, but the results are found to depend very sensitively on the assumed q profile.