Design and operation of the in-vessel cryopump for the new upper divertor in ASDEX Upgrade

Abstract

To study alternative divertor configurations in the ASDEX Upgrade (AUG) tokamak a new upper divertor, Div-IIo, with two in-vessel coils and a cryopump (CP) will be installed in 2022. For efficient density control in the new divertor configuration during plasma discharges, the CP has to guarantee an effective pumping speed for deuterium (D2) of at least Seff = 15 m3/s. The CP will be installed behind the inner divertor with a toroidal pumping gap of width 30 mm between the inner and outer divertor plates. It consists of 7 modules with a major radius of R = 1.2 m and a total active length of 5.4 m.To investigate reactor relevant D2-He gas mixtures during upper single null discharges, the liquid helium cryo-panels are coated with activated charcoal as cryosorbent.For quick regeneration of the charcoal from the accumulated gases, heating wires on the cryo-panels are installed.To reduce the pumping speed during high density discharges, an in-vessel cryo valve between two modules is installed. The pumping speed can be reduced by 35%.The cryo-panels operate with saturated liquid helium (LHe) at a temperature of 4.4 K. The thermal radiation shield consisting of reflector and chevron-baffle operate with saturated liquid nitrogen (LN2) at 80 K. To supply the new upper CP with LHe and LN2 the existing LHe-LN2-supply of the lower CP will be used. Therefore, an additional distributor box and a multi-channel transfer line to the upper CP have been installed in 2019.