Pumping Arrangement

A differentially pumped system comprises a plurality of pressure chambers; and a pumping arrangement ( 100 ) attached thereto for evacuating the chambers, the pumping arrangement comprising first and second compound pumps ( 102, 104 ) each comprising at least a first inlet ( 120 ); ( 124 ), a second inlet ( 122 ); ( 126 ), a first pumping section ( 110 ) and a second pumping section ( 112 ) downstream from the first pumping section, the sections being arranged such that fluid entering the pump from the first inlet passes through the first and second pumping sections and fluid entering the pump from the second inlet passes through, of said sections, only the second section, wherein the first inlet ( 120 ) of the first pump ( 102 ) is attached to an outlet from a first, relatively low, pressure chamber ( 10 ), the second inlet ( 122 ) of the first pump ( 102 ) and the first inlet ( 124 ) of the second pump ( 104 ) are attached to an outlet or respective outlets from a second, common medium pressure chamber ( 16 ), and the second inlet ( 126 ) of the second pump ( 104 ) is attached to an outlet from a third, relatively high pressure chamber ( 14 ).