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 $S{p}_2=C(p_1-p_2)$ 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.^[ref]