Vacuum Cryopumping a Heavy Ion Source in a High Voltage Terminal

Two coldfingers differentially cryopump the heavy ion source of the SuperHilac 3-MV injector. One coldfinger near the source freezes most of the source gas load and provides adequate clean vacuum for source operation. The second coldfinger near the beam entrance into the high voltage column produces high vacuum to reduce ion beam loss through charge exchange and accidental electrical discharges. The 20°K temperature is produced by a small refrigerator in the terminal, and is transmitted to the coldfingers by hydrogen-charged heat pipes. The effective system pumping speed for air at the source measures 180 l/s. While running beams of argon ions, pressures are 8 x 10-5 mm Hg at the source and 5 x 10-6 at the column entrance. The integral of pressure with distance along the terminal beam path is 1.8 x 10-3 mm Hg-cm.