Confined trapped alpha behaviour in TFTR deuterium-tritium plasmas

Abstract

Confined trapped alpha energy spectra and differentialradial density profiles in TFTR D-T plasmas were obtained with thepellet charge exchange (PCX) diagnostic, which measures high energy(Eα = 0.5-3.5 MeV) trapped alphas (v||/v = -0.048) at a single time slice (Δt ≈ 1 ms) with a spatialresolution of Δr ≈ 5 cm. Tritons produced in D-Dplasmas and RF driven ion tails (H, 3He or T) were also observedand energetic tritium ion tail measurements are discussed. PCX alphaand triton energy spectra extending up to their birth energies weremeasured in the core of MHD quiescent discharges where the expectedclassical slowing down and pitch angle scattering effects are notcomplicated by stochastic ripple diffusion and sawtooth activity.Both the shape of the measured alpha and triton energy distributionsand their density ratios are in good agreement with TRANSPpredictions, indicating that the PCX measurements are consistent withclassical thermalization of the fusion generated alphas and tritons.From calculations, these results set an upper limit on possibleanomalous radial diffusion for trapped alphas of Dα ⩽ 0.01 m2·s-1. Outside the core, where the trappedalphas are influenced by stochastic ripple diffusion effects, the PCXmeasurements are consistent with the functional dependence of theGoldston-White-Boozer stochastic ripple threshold on the alphaenergy and the q profile. In the presence of strong sawtoothactivity, the PCX diagnostic observes significant redistribution ofthe alpha signal radial profile wherein alphas are depleted in thecore and redistributed to well outside the q = 1 radius, butapparently not beyond the energy dependent stochastic ripple lossboundary. The helical electric field produced during the sawtoothcrash plays an essential role in modelling the sawtoothredistribution data. In sawtooth free discharge scenarios withreversed shear operation, the PCX diagnostic also observes radialprofiles of the alpha signal that are significantly broader thanthose for supershots. ORBIT modelling of reversed shear andmonotonic shear discharges is in agreement with the q dependentalpha profiles observed. Redistribution of trapped alpha particles inthe presence of core localized toroidal Alfvén eigenmode (TAE)activity was observed and modelling of the PCX measurements based ona synergism involving the α-TAE resonance and the effect ofstochastic ripple diffusion is in progress.