Initial operation of the alpha charge exchange diagnostic using impurity pellet injection into deuterium plasmas on TFTR

In a collaboration between General Atomics, the A.F. Ioffe Physical-Technical Institute, and the Princeton Plasma Physics Laboratory, the feasibility of using impurity pellet injection to measure the energy distribution of the fast-confined alpha particles in a reacting plasma is being investigated on TFTR. A small fraction of the alpha particles incident on the pellet ablation cloud will be converted to helium neutrals. In the case of lithium pellets, both double charge exchange (He2+ + Li+ → He0 + Li3+) and sequential single charge exchange (He2+ + Li+ → He+ + Li2+ followed by He+ + Li+ → He0 + Li2+) reactions will result in helium neutrals whose energy is essentially unchanged by the charge transfer process. Interactions with Li+ are of special interest because lithium pellets are used to produce beneficial wall conditioning effects on TFTR, the cross sections are relatively large (compared with B or C), and a large spatial region of the cloud is expected to be dominated by this helium-like ionization state of lithium. By measuring the resultant helium neutrals escaping from the plasma using a mass and energy resolving charge exchange analyzer, this technique offers a direct measurement of the energy distribution of the incident high-energy alpha particles. The final goal of the TFTR experiments, of course, is to measure the energy distribution of the 3.5 MeV alpha particles during DT operation. (author) 2 refs., 4 figs