Negative surface ionization: Intense halogen-ion source

Abstract

A surface negative-ionization source was created in order to produce intense negative-ion beams. The advantages of an ionizer functioning in transmission were shown, which led to the theoretical determination of ionizer-pore useful dimensions. It was thus shown that, contrary to the case with positive ions, the pore diameter necessary for a satisfactory transmission-ionization efficiency is highly dependent on the electron affinity of the element to be ionized. A lanthanum hexaboride ionizer was then chosen since it gives a good ionization of halogens and especially of iodine. Experimental study showed that the maximum densities of the extracted-ion current are comparable to those obtained by the same process for positive Cs+ ions ionized on tungsten. Finally, the influence of the ionizer-pore diameter on ion-current characteristics according to temperature and flow was highlighted. These characteristics confirm the hypothesis established for the working of transmission ionizers.