Stabilization of Sawteeth with Additional Heating in the JET Tokamak

Experiments in the JET tokamak with additional heating power (ion cyclotron resonance heating and/or neutral beam injection) above 5 MW show that the plasma can undergo a transition to a new regime. In this regime, the sawtooth instability is suppressed for periods up to 1.6 s and the level of long-wavelength, coherent MHD activity is very low. An improvement in the global energy confinement time of up to 20% is observed. Possible mechanisms for the stabilization of the $m=1$ instability and the implications for the near-ignition regime are discussed.