Pitch-angle redistribution of beam ion losses caused by Alfvén instabilities in sub-cyclotron frequency range on the Mega Amp Spherical Tokamak Upgrade
Dedicated experiments were conducted on Alfvénic instabilities in the ion sub-cyclotron frequency range excited by energetic beam ions in the MAST-Upgrade tokamak. Losses of the beam ions associated with plasma instabilities were measured with a scintillator-type fast ion lost detector (FILD), which provided a 2D image of the losses as functions of their pitch-angle and energy. The data acquisition of the FILD allowed, for the first time, detection of the beam ion losses up to half the beam ion cyclotron frequency range. This made it possible to assess the losses due to global Alfvén eigenmodes (GAEs), excited via beam ion cyclotron resonances. The Doppler-shifted cyclotron resonance interaction with counter-propagating GAE results mainly in the redistribution of the beam ions in pitch-angle. Linear modelling indicate that the pitch-angle is caused by a wave-particle energy transfer, leading to changes in the pitch-angle that are several times larger than the net energy lost by the fast ions. The results concerning the pitch-angle redistribution may be relevant to heating and current drive efficiency with neutral beam injection in future fusion devices and of interest to the pitch-angle scattering in space plasmas.