To evaluate the efficiency of alpha particle heating in the DT phase of JET, measurement of the energy distribution of the fast alpha particle population is necessary. The fast ion and alpha particle diagnostic at JET will use collective Thomson scattering of high power microwave radiation to determine the alpha particle velocity distribution between their birth energy (3.5MeV) and ~0.5MeV with a typical spatial resolution <0.1m and time resolution ~0.1s. The diagnostic uses a high power long pulse 140GHz gyrotron as the source of probing radiation and a heterodyne receiver system to measure the spectrum of scattered radiation.