FAST ALPHA-PARTICLE DIAGNOSTICS USING KNOCK-ON ION TAILS

A new method of measuring the fast confined alpha panicle 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 t ritium ion in a single elastic scattering interaction. Calculations of the size of these knock-on tails in tokamaks such as TFTR, JET and IT ER show that it may be possible to measure these tails and provide inf ormation on the fast confined alphas. The knock-on tail ions will prod uce DT neutrons with energies up to 20.6 MeV, so that DT neutron spect roscopy can be used to monitor the alpha population. Neutron spectrosc opy looks especially attractive for ITER. A collimated array of thresh old neutron activation detectors could be used to measure the confined alpha density profile. Tests of this diagnostic can also be done on T FTR and JET. Existing high energy neutral particle analysers may allow observation of the ion tails directly via passive and/or active charg e exchange.