Note on the Efficiency of Differential Pumping in the LHC Polarized Gas Target (PGT)

For a sufficient density of a window-less gas target based on a storage cell, a substantial gas flow rate is required, exceeding that of UHV vacuum systems. Therefore, differential pumping has to be applied in order to limit the gas flow into neighbouring sections. Applying analytical formulae valid in the molecular regime, the on-axis flow – which can not be collimated – from the downstream end of the target tube towards the VELO vessel is estimated. It has been shown that the existing vacuum systems can cope with the flow.