Fast Ion Dynamics in Magnetically Confined Plasma Measured by Collective Thomson Scattering

Magnetically confined fusion plasmas are mostly heated by small populations of energetic ions. In current devices these fast ions are mainly generated by auxiliary heating, while in ITER fusion generated alpha particles will dominate. A multitude of MHD phenomena, some of them driven by the fast ions, can redistribute or eject the energetic ions prematurely, affecting fusion performance and potentially damaging walls. Theory and modeling of fast ion dynamics in fluctuating or turbulent plasmas is challenging and needs guidance from - and bench marking against - measurements of the fast ion dynamics. Collective Thomson Scattering (CTS) can provide such measurements of the confined fast ions. Here we present CTS measurements of the TEXTOR tokamak plasma which show fast ions responding to sawteeth and display slowdown evolution of the fast ion velocity distribution after switch off of neutral beam heating. The toroidal rotation velocity of the bulk ions is inferred from the measurements. Plans for an ITER fast ion CTS are also briefly discussed.