Observation of the alpha particle 'knock-on' neutron emission from magnetically confined DT fusion plasmas

Suprathermal fuel ions from cr-particle knock-on collisions in fusion DT plasmas are predicted to cause a weak feature in the neutron spectrum of d + t --> alpha + n. The knock-an feature has been searched for in the neutron emission of high (>1 MW) fusion-power plasmas produced at JET and was found using a magnetic proton recoil type neutron spectrometer of high performance. Measurement and predictions agree both in absolute amplitude and in plasma-parameter dependence, supporting the interpretation and model. Moreover, the results provide input to projecting ct-particle diagnostics for future self-heated fusion plasmas.