= Differential pumping
<Differential pumping> maintains different <gas> <pressures> in connected regions by restricting the connecting <gas> flow and pumping each region separately. A small aperture or narrow tube <limits> the <vacuum conductance> between stages.
In the <molecular flow> regime, a connection with conductance $C$ carries throughput $Q=C(p_1-p_2)$. A downstream stage pumped at effective speed $S$ has $Sp_2=C(p_1-p_2)$ in <steady state>, giving $p_2/p_1=C/(S+C)$. This lets a source operate at a higher <pressure> while an adjacent <particle beam> travels in a better <vacuum>.<Vacuum Technology for Ion Sources by Paolo Chiggiato>[molecular-flow conductance and vacuum-electrical analogy]{ref}
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