Ion source with substantially planar design

An ion source comprising: a conductive cathode comprising an inner cathode portion (5a) and a portion of outer cathode (5b) in a given plane; a hollow ion emission (22) at least partially formed between the part of inner cathode and the portion of outer cathode, which includes an inner periphery defined by the periphery of the inner cathode (5a) and an outer periphery defined the inner periphery of the outer cathode portion (5b); an anode (25) located between the inner cathode portion (5a) and the outer cathode portion (5b) seen perpendicularly from above and / or below the given plane; characterized in that no part of the portion of inner cathode (5a) protrudes above the anode (25), and no part of the portion of outer cathode (5b) protrudes above the anode (25) seen perpendicularly from above and / or below the given plane.