Overview of transport, fast particle and heating and current drive physics using tritium in JET plasmas

Full text: The JET Trace Tritium Experimental (TTE) campaign used trace tritium (T) plasmas (nT/nD<5%) to investigate thermal fuel-ion transport, fast particle dynamics and heating and current drive physics. Precisely timed tritium gas puffs, and short pulse (<500 ms) Tritium Neutral Beam Injection (TNBI) were used. Transport coefficients were derived by fitting 14 MeV and 2.5 MeV neutron profiles' evolution. Rho*, nu*, beta and q95 were varied in pairs of sawtoothing ELMy H-Mode discharges, comparing fuel-particle and energy transport coefficient scalings. Particle transport data are consistent with gyro-Bohm scaling, similar to energy transport. Particle confinement increases with nu* and declines with beta, but energy confinement degrades weakly with nu* and is independent of beta. High edge diffusion during ELMs clearly affects the neutron emission. TTE experiments compared three fuelling methods in otherwise similar Internal Transport Barrier (ITB) plasmas: T gas-puffs; T wall-recycling; and TNBI. T gas-puff data showed reduced triton diffusion at all ITB locations. Thermalised TNBI ions deposited inside the ITB radius, demonstrated ITB effects on central particle confinement. In hybrid plasmas (qmin∼1, low positive shear, no sawteeth), T particle confinement time (τpT*) improved by ∼50% as triangularity was varied from 0.2-0.46 at constant energy confinement (HβN/q952 ∼ 0.42). At low delta, τpT * scaled with Ip. Fast-ion confinement in Current-Hole (CH) plasmas, with minimal central toroidal current density was tested by injection of TNBI into JET CH plasmas. Gamma-rays from nuclear reactions between fusion alphas (generated by TNBI) and beryllium impurities were measured in majority-deuterium plasmas after TNBI turn-off. Gamma-ray emission rate decay times measured the fast fusion-alpha population evolution. These decay times for JET plasmas are consistent with classical alpha slowing down times for high plasma currents (Ip >2MA) and monotonic q-profiles. In CH discharges the gamma-ray emission decay times are a factor 5 below classical values based on total Ip, indicating alpha confinement degrades due to orbit losses as predicted by a 3-D Fokker Planck numerical code. In TTE, ICRF at 23 MHz was coupled to T-minority ions, producing energetic tritons with tail temperatures 80-120keV, derived from neutron energy spectra. (author)