A closed drift ion source is disclosed. The ion source has an open end 1 and a central axis 150 around which are arranged outer magnetic pole piece 3, inner magnetic pole piece 5, anode 2, shield 6 and back magnetic shunt 17. In one embodiment the anode 2 and inner magnetic pole piece 5 are annular. Permanent magnets 7 are located behind the shield 6 and in contact with outer magnetic pole piece 3 and the back magnetic shunt 17. As a result the magnetic field lines pass through the magnetic pole pieces and a mirror magnetic field is set up in the discharge region between them. The inner magnetic pole piece 5 is hollow which facilitates production of the mirror magnetic field.