Vacuum pumping of multimegawatt neutral beam injection lines

Neutral beam injection lines employed for heating large tokamak and mirror plasmas have a unique gas pumping requirement, which is to maintain a high pressure (∼10−3 Torr) in the gas cell neutralizer and a low pressure (∼10−6 Torr) downstream after the neutralizer. Large hydrogen gas inputs (a few tens of Torr l/s) needed for operating an ion source and also the packing of multiple ion sources in a single beamline indeed make the beamline pumping a technological challenge, requiring a pumping speed greater than 106 l/s. We have witnessed, as beamlines get bigger, that beamline pumping techniques have shifted from conventional to gettering, and then to cryocondensation pumping. Cryopumping techniques are now employed in almost every beamline for high speed pumping; cryopanel design varies widely since the panels are an integral part of a beamline. A brief review is given of the beamline pumping technology, covering such topics as gas loading characteristics, vacuum components, cryopumping techniques, gette...