The Use of Pellet Diagnostics for the Study of Thermonuclear α Particles in the TFTR Tokamak

An α particle diagnostic technique based on impurity-pellet injection into tokamak plasma is described. This technique has been developed in collaboration with the Princeton Plasma Physics Laboratory, General Atomic, and Ioffe Institute for Physics and Technology and has been used in the TFTR tokamak. When lithium or boron pellets are injected into tokamak plasma, some fraction of α particles are neutralized in the ablation cloud due to two-step single electron capture or double electron capture. Outward fluxes of neutralized α particles in the MeV energy range are analyzed by the Gemma-2 high-energy analyzer, which was developed in the Ioffe Institute for Physics and Technology. This diagnostic has been used in DT experiments in the TFTR tokamak and provided measurements of the local energy spectra and radial distributions of α particles in the 0.5-3.5 MeV energy range. Results of the measurements of energy spectra and radial distributions of confined α particles in the TFTR, influence of sawtooth oscillations on the α particle distribution, and evidence for the radial redistribution of α particles in the presence of the toroidal Alfvén modes induced by α particles are presented. Possible ways of implementing α particle diagnostics with the use of pellets in the ITER are also discussed.