Suprathermal fuel ions from cr-particle knock-on collisions in fusion DT pl
asmas 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 performan
ce. Measurement and predictions agree both in absolute amplitude and in pla
sma-parameter dependence, supporting the interpretation and model. Moreover
, the results provide input to projecting ct-particle diagnostics for futur
e self-heated fusion plasmas.