The positive ions formed in the ion chamber are drawn out by a small electrostatic field between the larger repeller plate (positively charged) and the first accelerating slit (negatively charged).
A strong electrostatic field between the 1st and 2nd accelerating slits of 400-4000 V, accelerates the ions of various masses to their final velocities. The ions emerge from the final focusing slit
(0 volts) as a collimated ion beam with velocities and Kinetic Energies as