Fast alpha particle diagnostics using knock-on ion tails

A new method of measuring the fast confined alpha particle distribution in a reacting plasma is proposed. The presence of alpha particles in a DT plasma will create high energy tails in the deuterium and tritium ion energy distributions. A 3.5 MeV alpha can transfer 3.4 MeV to a tritium ion in a single elastic scattering interaction. Calculations of the size of these knock-on tails in tokamaks such as TFTR, JET and ITER show that it may be possible to measure these tails and provide information on the fast confined alphas. The knock-on tail ions will produce DT neutrons with energies up to 20.6 MeV, so that DT neutron spectroscopy can be used to monitor the alpha population. Neutron spectroscopy looks especially attractive for ITER. A collimated array of threshold neutron activation detectors could be used to measure the confined alpha density profile. Tests of this diagnostic can also be done on TFTR and JET. Existing high energy neutral particle analysers may allow observation of the ion tails directly via passive and/or active charge exchange