Neutralization of TFTR neutral beams

Long pulse ion sources have recently been operated on both the injection beamlines and test stand at TFTR. Operation has shown that these ion sources can be run in a very high gas efficiency mode. Such operation unfortunately leads to under-neutralization of the extracted beam with the unpleasant side effects of increased power loading on the ion dumps, decreased neutral power, and decreased full-energy neutral fraction injected into TFTR. This effect has been studied with a set of instruments that include waterflow calorimetry, thermocouple arrays, ionization gauges, and various beam species diagnostics (optical multichannel analyzer, Rutherford backscattering, implantation and a charge exchange analyzer). As the gas efficiency is increased, the line integrated gas density of the neutralizer falls below that predicted by room temperature gas and molecular flow. This effect is evident in the reduced bulk neutralization of the beam as well as increased power on the ion dump and downward-shifting profile on the full-energy ion dump. Similar difficulties have been encountered on the JET beamlines