Energetic particle transport and alpha driven instabilities in advanced confinement DT plasmas on TFTR

Abstract

The article reviews the physics of fusion alpha particles andenergetic neutral beam ions studied in the final phase of TFTRoperation, with an emphasis on observations in reversed magnetic shear(RS) and enhanced reversed shear (ERS) DT plasmas. Energy resolvedmeasurements of the radial profiles of confined, trapped alphas in RSplasmas exhibit reduced core alpha density with increasing alphaenergy, in contrast to plasmas with normal monotonic shear. Themeasured profiles are consistent with predictions of increased alphaloss due to stochastic ripple diffusion and increased first orbit lossin RS plasmas. In experiments in which a short tritium beam pulse isinjected into a deuterium RS plasma, the measured DT neutron emissionis lower than standard predictions assuming first orbit loss andstochastic ripple diffusion of the beam ions. A microwavereflectometer measured the spatial localization of low toroidal modenumber (n), alpha driven toroidal Alfvén eigenmodes (TAEs) in DTRS discharges. Although the observed ballooning character of the n = 4mode is consistent with predictions of a kinetic MHD stability code,the observed antiballooning nature of the n = 2 mode isnot. Furthermore, the modelling does not show the observed strongdependence of mode frequency on n. These alpha driven TAEs do notcause measurable alpha loss in TFTR. Other Alfvén frequency modeswith n = 2-4 seen in both DT and DD ERS and RS discharges arelocalized to the weak magnetic shear region near qmin. In 10-20% ofDT discharges, normal low n MHD activity causes alpha loss at levelsabove the first orbit loss rate.