Vacuum system of the neutral beam injectors at the stellarator TJ-II

In the fusion plasma experiment TJ-II, two neutral beam injectors provide 2 MW heating power to the plasma. Beam losses occur at different stages due to collisions of beam ions or neutrals with surrounding H-2 molecules. Efficient use of the injected gas is of utmost importance, in order to minimize losses while ensuring optimum neutralization. In the design phase of the vacuum system, extensive use of the computer code OPTIMUS has been made. Calculations show that in order to keep reionization losses at an acceptable level, the maximum allowed pressure during the beam pulse is 10(-4) mbar, which requires an installed pumping speed of 350,000 l/s. Titanium getter pumps have been chosen as the primary vacuum system for these injectors. Base pressures in the range of 5 x 10(-9) mbar are attainable with titanium pump operation. (C) 2002 Elsevier Science Ltd. All rights reserved.