Economical helium bath cryopump: design and testing

Abstract

A small UHV helium bath cryopump with low cryoliquid consumption has been designed, manufactured and tested. The cryopump is designed to keep UHV in electron optical devices without generation of disturbing electromagnetic fields or vibrations. The outer volume of its shell is 15 l, the filling volumes of liquid helium (LHe) and liquid nitrogen (LN 2) are 3.0 l and 3.4 l, respectively. Operating times between cryoliquid fillings are 35 days for LHe and 6 days for LN 2. A pump speed of 25 l/s was measured for the He gas. A lowest pressure of 1.5×10 −7 Pa was achieved in a test chamber and the pumping tests indicated that an even lower pressure would be achieved in a well-degassed chamber. Good agreement between the calculated and measured values both of the pump speed and the cryoliquid evaporation has been found. The system of chevron-type baffles was designed and checked by Monte Carlo simulations of molecular flow and radiative heat transfer. The pump prototype was successfully used to evacuate an operating cryostat of a NMR spectrometer.