The effect of energetic asymmetrically distributed ions on the stability of the internal kink mode in tokamaks is analyzed. Circulating ions which intersect the resonant surface due to finite radial excursion contribute to the mode either in the region of favorable or unfavorable curvature depending on the sign of $v_{\parallel }$. Internal kink mode stabilization for predominantly cocirculating ion populations is consistent with the observation of long sawteeth using tangential coinjection of neutral beams in JT-60U [G. J. Kramer et al., Nucl. Fusion 40, 1383 (2000)]. Off-axis neutral beam heating emerges as a possible means of assisting sawtooth control in future fusion grade experiments.

• Received 8 October 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.185003