A charged particle propagation apparatus comprising: a generator comprising a vacuum chamber with a gun therein for discharging a charged particle beam from within the vacuum chamber and out of the vacuum chamber through a beam exit disposed in a wall of the vacuum chamber; a higher pressure region adjoining the vacuum chamber at the beam exit that is maintainable at a pressure greater than a pressure of the vacuum chamber; a plasma interface disposed at the beam exit comprising a plasma channel, wherein the plasma channel: is aligned with the beam exit; has a first end and a second end; has at least three electrode plates disposed between the first end and the second end, and characterised in that the plasma channel has a plurality of insulators, each insulator being disposed between a pair of adjacent electrode plates in the plasma channel, each electrode plate and insulator having a central aperture coaxially aligned with the plasma channel which the charged particle beam may pass there through, and a control system adapted to apply a sequence of electrical currents to the electrode plates causing each plate to behave selectively as either an anode or cathode thereby causing a plurality of plasmas to move concurrently from the first end to the second end of the plasma channel thereby pumping down the beam exit, and, in use, the charged particle beam is propagated from the vacuum chamber through each plasma in the plasma channel and into the higher pressure region. a generator comprising a vacuum chamber with a gun therein for discharging a charged particle beam from within the vacuum chamber and out of the vacuum chamber through a beam exit disposed in a wall of the vacuum chamber; a higher pressure region adjoining the vacuum chamber at the beam exit that is maintainable at a pressure greater than a pressure of the vacuum chamber; a plasma interface disposed at the beam exit comprising a plasma channel, wherein the plasma channel: is aligned with the beam exit; has a first end and a second end; has at least three electrode plates disposed between the first end and the second end, and characterised in that the plasma channel has a plurality of insulators, each insulator being disposed between a pair of adjacent electrode plates in the plasma channel, each electrode plate and insulator having a central aperture coaxially aligned with the plasma channel which the charged particle beam may pass there through, is aligned with the beam exit; has a first end and a second end; has at least three electrode plates disposed between the first end and the second end, and characterised in that the plasma channel has a plurality of insulators, each insulator being disposed between a pair of adjacent electrode plates in the plasma channel, each electrode plate and insulator having a central aperture coaxially aligned with the plasma channel which the charged particle beam may pass there through, and a control system adapted to apply a sequence of electrical currents to the electrode plates causing each plate to behave selectively as either an anode or cathode thereby causing a plurality of plasmas to move concurrently from the first end to the second end of the plasma channel thereby pumping down the beam exit, and, in use, the charged particle beam is propagated from the vacuum chamber through each plasma in the plasma channel and into the higher pressure region.