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 . <EOS>