Modelling of alpha-particle behaviour in H-mode plasmas with trace tritium at JET

Recently gamma(gamma)-ray emission measurements have been performed for the first time in trace tritium experiments in the Joint European Torus (JET) [Kiptily et al 2004 Phys. Rev. Lett. 93 115001]. The decay in the gamma-ray emission has been measured after a short duration tritium neutral beam injection (NBI) into deuterium plasmas. The measured gamma -ray emission is produced in the reaction between the a-particles and intrinsic beryllium impurity, and therefore it carries information about the a-particle behaviour. The interpretation of the gamma-ray emission decay after the tritium NBI and short single gas puff into H-mode plasmas, based on the modelling of the a-particle evolution with the TRANSP code, is given here. It is shown that the a-particle confinement may determine the decay time of the gamma-ray emission during the very short post-NBI phase (similar to 150-200ms) unresolved with present diagnostics (time resolution of gamma-ray emission measurements is 250 Ins). The later gamma-ray decay correlates with the confinement of thermalized tritium (i.e. with the decay of the a-particle source). An upper estimate of the a-particle confinement time is determined, but this estimate exceeds the a-particle slowing down time giving a large uncertainty in the estimation of anomalous (non-collisional) losses in these plasmas. The a-particle behaviour is analysed also in discharges with tritium gas puffs with the goal of finding a scenario where the a-particle production and losses could be clearly distinguished. It is found that a large time delay between the decay of the a-particle source and its products occurs in discharges performed at high plasma density clue to the large transient orbit losses of a-particles. These scenarios are proposed for testing a-particle orbit losses in future deuterium-tritium experiments.