OPTIMIZATION OF A 2-STAGES ELECTROSTATIC REFLEX ION-SOURCE

In our argon, oxygen, or nitrogen broad beam ion source, 80 mm in diam eter, which is either an electrostatic reflex ion source, or a magneti c and electrostatic reflex ion source, the hot filament has been repla ced by a carefully optimized microwave plasma (MP) cathode under the e lectron cyclotron resonance condition. Different geometries of antenna e in which the microwave transverse electric field is more or less fav ored have been tested. A major improvement was obtained when a dc pola rization of these antennae was superimposed onto the microwave field, allowing a fine tuning via the sheath thickness adjustment. Registered cathode currents under various conditions of pressure of neutrals, mi crowave power, and polarization voltage have shown a gain of a factor of 3 relative to the previous case where the single disk antenna was a t a floating potential. Best results with argon gas at a pressure of 3 x 10(-2) mbar were obtained with a spear-head shaped antenna fed at a microwave power of 50 W and polarized at 26 V. Extracted ion beam cur rent densities as high as 5 mA/cm2 resulted from these different MP ca thode improvements. Moreover, we have shown that the internal electron ic tuning of the source can advantageously replace the traditional ext ernal tuning.