Sawtooth period changes with mode conversion current drive on Alcator C-Mod

Significant changes in the sawtooth period have been observed on the Alcator C-Mod tokamak during phased ion cyclotron range of frequencies (ICRF) operation in the mode conversion regime. As the mode conversion layer was swept outwards through the q = 1 surface in D(3He) plasmas, the sawtooth period was found to increase and then decrease for counter-current drive phasing. For co-current drive and heating phasings, it was observed to decrease and then increase. With 2 MW ICRF power, the period varied from 3 to 12 ms. The observed evolution is consistent with localized current drive by mode converted waves in the vicinity of the q = 1 surface. Simulations with the full wave code TORIC indicate that the electron heating and current drive are due to mode converted ion cyclotron waves. The observed evolution for symmetric (heating) phasing is difficult to attribute to localized heating, since temperature profile stiffness prohibits large changes in the resistivity gradient at the q = 1 surface. An alternative explanation is found in TORIC simulations, which predict co-current drive for symmetric phasing due to a strong up-down asymmetry in the ICW wave field.

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