Impact of strongly driven fishbones and Alfvén Eigenmodes on fast ion losses

Abstract

The instabilities responsible for ion cyclotron resonance heating (ICRH) accelerated fast ion losses in plasmas with monotonic profile of the safety factor and long period sawteeth, as well as the range of energies at which the losses occur, were identified on the JET tokamak. The temporal evolution of fast ion losses with resolution on energy and pitch angle was measured using the scintillator probe, which allows us to determine the orbits of the fast ions that reach the detector. On the other hand, the orbits of the fast ions in resonance with Alfvénic instabilities were identified using the CASTOR-K code, so it is possible to identify which instabilities were responsible for the losses collected in the detector. Two different phases of losses, with different characteristics, were identified. The first phase occurs just after a sawtooth crash, when toroidal Alfvén eigenmodes (TAEs) and high frequency fishbones are unstable. In this phase, TAEs are responsible for most of the measured losses. Losses caused by fishbones, if any, are not significant. The second phase initiates when tornado modes (TAE inside the q = 1 radius) become unstable: the peak loss signal increases by a factor of 2 to 5, the average energy of the ions that reach the detector decreases and its pitch angle increases. In this phase, the strong drive of the tornado modes by the ICRH accelerated ions initiates the losses.