Confined Trapped-Alpha Behaviour in TFTR Deuterium-Tritium Plasmas
Authors: S.S. Medley, R.V. Budny, H.H. Duong, R.K. Fisher, M.P. Petrov, N.N. Gorelenkov, M.H. Redi, A.L. Roquemore, and R.B. White
Confined trapped-alpha energy spectra and differential radial density profiles in TFTR D-T plasmas are obtained with the Pellet Charge-eXchange (PCX) diagnostic which measures high energy (Ealpha = 0.5 - 3.5 MeV), trapped alphas (v||/v = - 0.048) at a single time slice [(delta)t ~ 1 msec) with a spatial resolution of (delta)r ~ 5 cm. Tritons produced in D-D plasmas and RF-driven ion tails (H, 3He or T) were also observed and energetic tritium ion tail measurements will be discussed.
PCX alpha and triton energy spectra extending up to their birth energies were measured in the core of MHD-quiescent discharges where the expected classical slowing down and pitch angle scattering effects are not complicated by stochastic ripple diffusion and sawtooth activity. Both the shape of the measured alpha and triton energy distributions and their density ratios are in good agreement with TRANSP predictions, indicating that the PCX measurements are consistent with classical thermalization of the fusion-generated alphas and tritons. From calculations, these results set an upper limit on possible anomalous radial diffusion for trapped alphas of Dalpha less than or equal 0.01 m2sec-1. Outside the core, where the trapped alphas are influenced by stochastic ripple diffusion effects, the PCX measurements are consistent with the functional dependence of the Goldston-White-Boozer stochastic ripple threshold on the alpha energy and the q-profile.
In the presence of strong sawtooth activity, the PCX diagnostic observes significant redistribution of the alpha signal radial profile wherein alphas are depleted in the core and redistributed to well outside the q = 1 radius, but apparently not beyond the energy-dependent stochastic ripple loss boundary. The helical electric field produced during the sawtooth crash plays an essential role in modeling the sawtooth redistribution data. In sawtooth-free discharge scenarios with reversed shear operation, the PCX diagnostic also observes radial profiles of the alpha signal that are significantly broader than those for supershots. ORBIT modeling of reversed shear and monotonic shear discharges are in agreement with the q-dependent alpha profiles observed. Redistribution of trapped alpha particles in the presence of core localized TAE activity was observed and modeling of the PCX measurements based on a synergism involving the alpha-TAE resonance and the effect of stochastic ripple diffusion is in progress.