Differential pumping via core of annular supersonic jet

An aperture (5) connects a first chamber (1) with a second chamber (2), and is surrounded by an annular nozzle (7) formed by inner and outer walls (6, 8), which connects the first chamber (1) with a third chamber (3). A supersonic annular gas jet (9) is ejected by the annular nozzle (7) into the first chamber (1), creating a Venturi pumping action at the core of the jet in the vicinity of the aperture (5). The second chamber (2) may thus be maintained at a substantially lower pressure than the first chamber (1). Inner wall (6) and outer wall (8) may be relatively movable for varying gas flow, and the first chamber (1) may include baffles or skimmers to modify gas flow, e.g., to create a high density molecular beam. An electron or ion beam (4) may be transferred from the second chamber (2) to the first chamber (1), e.g., as part of an environmental scanning electron microscope.