ABSTRACT:
Future tokamaks such as ITER will produce self-heated, or burning, plasmas by confining fusion-produced energetic ions as they transfer their energy to the background plasma. Energetic ions are produced through both fusion and auxiliary heating in existing tokamaks where experiments and modeling show that these ions take advantage of many transport mechanisms. The most extreme transport channels produce losses by expelling energetic ions from the plasma. These losses are measured by a Fast Ion Loss Detector (FILD) in the DIII-D tokamak and the data are used to better understand the interactions between the energetic ion population and the bulk plasma.
The FILD is a magnetic spectrometer that measures the energy and pitch angle of energetic ions that reach its position along the outer wall. This information allows for the reconstruction of ion trajectories and provides stringent constraints for transport simulations that intend to describe, and eventually predict, energetic ion transport in reactors. During rf-heating, lost energetic ions are traced back to the antenna demonstrating the presence of undesirable energy absorption in the far plasma edge. Loss fluxes are also observed at frequencies corresponding to MHD modes (Alfven eigenmodes) driven by the energetic ions.
Modeling of coherent losses shows that sensitive ion loss boundary dependencies determine whether the Alfven eigenmodes (AEs) produce limited radial transport or send ions into the tokamak walls. The discrete positions of eight neutral beams in DIII-D provide unique orbit geometries in which beam ions interact with a single AE before being lost, essentially causing the ions to act as probes that measure the AE amplitudes and spatial structure. Future experiments seek to apply these new diagnostic methods to other phenomena including resonant magnetic perturbations that are often applied as a means of controlling edge instabilities in high-performance tokamak plasmas.