Alpha particle studies during JET DT experiments

The 1997 DT experiment (DTE1) at the Joint European Torus included studies of the behaviour of alpha particles in high temperature plasmas. Alpha particle heating was clearly observed in a series of 10 MW hot ion H modes by scanning the DT mixture from 0% T to 93% T. Maxima in central temperature and energy content were obtained which corresponded with the maximum in fusion yield. Alfven eigenmodes (AEs) have been detected in JET, driven by NBI or ICRH fast ions. However, in agreement with theory, no AE activity was observed in DT plasmas which could be attributed to alpha particle drive, except in the afterglow of some optimized shear pulses. Ion cyclotron emission (ICE) was detected at harmonics of the alpha particle cyclotron frequency at the outer edge of the plasma. The ICE is interpreted as being close to magnetoacoustic cyclotron instability, driven by inverted alpha distributions at the plasma edge. The high energy neutral particle spectra showed features which are ascribed to a mixture of alphas, neutralized by helium-like impurities, and deuterons born from elastic collisions with alpha particles and neutralized by hydrogen-like impurities. The results of all these studies are consistent with classical alpha particle trapping and slowing down. Future DT experiments will aim to increase alpha particle pressure so that interactions with plasma instabilities can be studied. The measurement of knock-on neutral triton spectra offers an unambiguous way to determine confined alpha densities in these future experiments.