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

Abstract

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