A charged particle propagation apparatus has a generator including a vacuum chamber with a gun therein for discharging a charged particle beam through a beam exit. A higher pressure region adjoins the vacuum chamber at the beam exit and is maintainable at a pressure greater than a pressure of the vacuum chamber. A plasma interface located at the beam exit includes a plasma channel having at least three electrode plates disposed between its first end and its second end. A control system is adapted to apply a sequence of electrical currents to the electrode plates, which cause at least one plasma to move 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 the, or each, plasma into the higher pressure region.