Energetic particle transport and alpha-driven instabilities in advanced confinement D-T plasmas on TFTR

This paper reviews the physics of fusion alpha particles and energetic neutral beam ions studied in the final phase of TFTR operation, with an emphasis on observations in Reversed magnetic Shear (RS) and Enhanced Reversed Shear (ERS) D-T plasmas. Energy-resolved measurements of the radial profiles of confined, trapped alphas in RS plasmas exhibit reduced core alpha density with increasing alpha energy, in contrast to plasmas with normal monotonic shear. The measured profiles are consistent with predictions of increased alpha loss due to stochastic ripple diffusion and increased first-orbit loss in RS plasmas. In experiments in which a short tritium beam pulse is injected into a deuterium RS plasma, the measured D-T neutron emission is lower than standard predictions assuming first orbit loss and stochastic ripple diffusion of the beam ions. A microwave reflectometer measured the spatial localization of low-toroidal mode number (n), alpha-driven Toroidal Alfven Eigenmodes (TAEs) in D-T RS discharges. Although the observed ballooning character of the n=4 mode is consistent with predictions of a kinetic-MHD stability code, the observed anti-ballooning nature of the n=2 mode is not. Furthermore, the modeling does not show the observed strong dependence of mode frequency on n. These alpha-driven TAEs do not cause measurable alpha loss in TFTR. Other Alfven frequency modes with n=2-4 seen in both D-T and D-D ERS and RS discharges are localized to the weak magnetic shear region near qmin. In 10-20% of D-T discharges, normal low-n MHD activity causes alpha loss at levels above the first orbit loss rate. (author)