High vacuum cryocondensation pumping of the ITER cryostat

Prior to cool-down of the ITER magnet system, the enveloping cryostat has to be evacuated to a low enough pressure to reduce gas convection and conduction heat loads to the magnet structures to an acceptable level (/spl sim/-0.1 mPa). In order to standardize cryopump designs to the greatest extent practical, the design of the cryostat high vacuum (HV) cryopump used for this evacuation is almost identical to that of the cryosorption pumps used for the torus primary pumping system, but without the cryosorbent (cryocondensation pumping). Additionally, the pump location has been moved from that described in the 2001 Final Design Report (under the cryostat lid) and is now at the lower port level, closer to the cryo-supply manifolds, thereby facilitating rationalization of the cryodistribution system. On account of the large area of warm vacuum-facing surfaces within the cryostat, including epoxy, the cryocondensation pumpdown is dominated by outgassing (helium within the cryostat vacuum envelope, resulting from small leaks in the liquid helium magnet coolant piping, is pumped by separate cryosorption refrigerator pumps). The pumpdown performance is assessed from a closed solution of the time-dependent gas balance, wherein the transient pressure is given by a power series of the elapsed time, the coefficients being functions of the effective pumping speed, 1 hour outgassing rate, and logarithmic decay index. It is shown that the series solution is convergent in the time domain of practical interest. We present the evolution of the cryostat pressure for various outgassing characteristics and demonstrate that the pumping provision is adequate, even under adverse outgassing scenarios.