The proposed fast ion collective Thomson scattering (CTS) diagnostic system for ITER provides the unique capability of measuring the temporally and spatially resolved velocity distribution of the confined fast ions and fusion alpha particles in a burning ITER plasma. The present paper describes the status of the iteration toward the detailed design of the ITER fast ion CTS diagnostic and explains in detail a number of essential considerations and challenges.

The diagnostic consists of two separate receiving systems. One system measures the fast ion velocity component in the direction near perpendicular, and the other measures the component near parallel to the magnetic field. Each system has a high-power probe beam at an operating frequency of 60 GHz and a receiver unit. In order to prevent neutron damage to moveable parts, the geometry of the probes and receivers is fixed. An array of receivers in each receiving unit ensures simultaneous measurements in multiple scattering volumes. The latter receiving system (resolving the parallel component) is located on the high field side (HFS) of the plasma, and this constitutes a significant challenge. This HFS receiving unit has been central in the studies, and new HFS receiver mock-up measurements are presented as well as neutron flux calculations of the influence of the increased slot height.