Alpha-particle velocity-space diagnostic in ITER

We discuss alpha-particle velocity-space diagnostic in ITER based oil the planned collective Thomson scattering (CTS) and gamma-ray spectrometry (GRS) systems as well as ASCOT simulations of the alpha-particle distribution function. GRS is sensitive to alpha-particles with energies E greater than or similar to 1.7 MeV at all pitches p, and CTS for E greater than or similar to 0.3 MeV and |p| less than or similar to 0.9. The remaining velocity space is not observed. GRS and CTS view the plasma (almost) perpendicularly to the magnetic field. Hence we cannot determine the sign of the pitch of the alpha-particles and cannot distinguish co- and counter-going alpha-particles with the currently planned alpha-particle diagnostics. Therefore we can only infer the sign-insensitive 2D distribution function f{E,|p|) by velocity-space tomography for E greater than or similar to 1.7 MeV. This is a serious limitation, since co- and counter-going alpha-particle populations are expected to have different birth rates and neoclassical transport as well as different anomalous transport due to interaction with modes such as Alfven eigenmodes. We propose the installation of an oblique GRS system on ITER to allow us to diagnostically track such anisotropy effects and to infer the full, sign-sensitive f(E,p) for E greater than or similar to 1.7 MeV. alpha-particles with E less than or similar to 1.7 MeV are diagnosed by CTS only, which does not allow velocity-space tomography on its own. Nevertheless, we show that measurements of the alpha-particle energy spectrum, which is an ITER measurement requirement, are now feasible for E greater than or similar to 0.3 MeV using a velocity-space tomography formalism assuming isotropy in velocity space.