High Current Ion Sources for Nuclear Investigations

Abstract

Since the capillary ion source described by Tuve, Dahl and Van Atta was a decided advance over previous types, it seemed desirable to investigate its possibilities further. The essential modification in the present design lies in the elimination of the auxiliary electrode for drawing ions out of the arc. This simplifies construction and greatly reduces the difficulties usually experienced in focusing ion beams. Currents as high as 4.2 milliamperes have been converged into a beam 12 mm wide with a focusing voltage of 10 kv. Possible explanations are presented for the observed dependence of the total ion current on arc current density, condition of the walls, and design of the outlet. Relative ease of starting and stability of the arc are discussed for various designs of tubes. When the arc was operated in hydrogen, the mass spectrograph showed a preponderance of diatomic and triatomic ions with proton fractions ranging from 5 to 20 percent of the total ion current. Ions other than those of hydrogen were not present in fractions exceeding 1 percent of the total. Evidence is presented for the breaking up of the heavy ions into protons and neutral particles on collision at high speed with gas molecules. Upper and lower limits are given for the efficiencies of these processes.