Hydrogen frosting scenarios with the ASDEX upgrade in-vessel cryo pump

The ASDEX upgrade vacuum vessel is equipped with 14 turbo molecular pumps (TMPs) and a cryo pump (CP). The CP is cooled with boiling helium (He). To obtain similar conditions with the CP for hydrogen (H2) and deuterium (D2) pumping a lower helium panel temperature (THP) is required in the case of H2. This is achieved by reducing the He boiling pressure (pb) and thus the He boiling temperature (Tb) via pump down of the He cryostat. The requirements are predicted by computations and verified experimentally for this case. By raising the cryostat pressure above ambient, the prediction of the H2 end pressure (peH2) of the CP could be compared with measurements in the range 5×10−7<peH2 (mbar)<5×10−4. The beneficial effect of argon frosting for helium pumping is only shown experimentally.