Differential Rotation to Prevent 2/1 Tearing Modes Driven by 3-Wave Coupling

Presented here is the first demonstration of the prevention via increased plasma differential rotation of disruptive neoclassical tearing modes (NTMs) seeded by nonlinear three-wave coupling. As TMs degrade confinement and can lead to disruptions, stabilization strategies are crucial to the successful operation of future devices. In ITER-relevant scenarios on DIII-D, rotationally coupled m/n=1/1 and 3/2 modes have been observed to drive 2/1 islands through three-wave coupling [1]. The frequency of the 2/1 daughter mode is set by the parent frequencies. When the driven mode frequency matches the local plasma rotation frequency, for e.g. at low differential rotation, the driven 2/1 island can grow into a disruptive NTM. Using neutral beam torque as an actuator to scan the differential rotation, these experiments demonstrate that a sufficiently large frequency mismatch prevents destabilization of disruptive 2/1 NTMs by this mechanism. This work implicates differential rotation as a useful actuator in the prevention of NTMs from nonlinear three-wave coupling and can help inform the development of stabilization strategies and scenarios. 

Work supported by US DOE under DE-FC02-04ER54698, DE-FG02-86ER53218 and DE-AC52-07NA27344.

[1] Bardoczi et al. Phys. Rev. Lett. 127, 055002.