Assessment of the ITER dwell pump-down

The ITER vacuum vessel has to be pumped down to a terminal pressure of ≤0.5 mPa during the dwell periods between plasma pulses, prior to pre-fill, in order to allow for reliable breakdown and plasma initiation. The main gas load during dwell pumping is hydrogen isotopes, implanted as energetic particles during the previous DT plasma discharge, outgassing from the beryllium armour of the plasma facing components. Previous experimental and theoretical studies indicate a power law decay in the initial outgassing rate with respect to time. Based on these previous studies, we derive an outgassing equation and use it to predict the time dependent pressure during ITER dwell pumping. The results indicate that for the shortest available pumping time, corresponding to the 1400 s dwell period between successive 400 s burn plasma pulses at the maximum repetition rate, the terminal dwell pressure requirements are satisfied.