Operation of the TFTR pellet charge exchange diagnostic in the pulse counting mode during H+ radio frequency minority heating

The pellet charge exchange technique on TFTR has been used primarily to obt ain active charge exchange measurements using a high-energy (0.5-4.0 MeV) n eutral particle analyzer (NPA) in conjunction with impurity pellet injectio n (Li and B) with the scintillator-photomultiplier detector system operated in the current mode. While passive measurements using pulse counting were also obtained using this instrumentation, operation in this mode was very r estrictive with pulse counting rates limited to less than similar to 10 kHz in the absence of any significant neutron and gamma induced background sig nal. An upgrade to a specialized pulse counting capability which was develo ped by the Ioffe Institute was implemented which consisted of CsI(Tl) scint illators having features designed to minimize signals induced by background neutron and gamma rays and 16-channel pulse height analysis electronics on each of the eight NPA energy channels. Passive measurements of rf-driven e nergetic hydrogen minority ions which served to verify operation of the pul se counting mode are reported. It is shown that in the passive mode the mai n donors for the neutralization of H+ ions in this energy range are C5+ ion s. The measured effective H+ tail temperatures range from 0.15 MeV at a rf power of 2 MW to 0.35 MeV at 6 MW. (C) 1999 American Institute of Physics. [S0034-6748(99)64801-8].