Optimization of the plasma-sputter-type negative ion source

A study is being made to increase the beam current of the negative ions extracted from a plasma-sputter-type ion source operated with Cs. The result of the mass analysis showed that polyatomic carbon negative ions such as C/sub 3//sup -/, C/sub 4//sup -/ and C/sub 5//sup -/ were extracted from the source with C/sup -/ and C/sub 2//sup o/ns. The percentages of the sum of C/sup -/ and C/sub 2//sup -/ currents to the total current increased by the Cs supply from 40% to 90%. To investigate the loss of negative carbon ions in the ion source plasma, the dependence of the ion beam currents on the gas pressure was measured. The C/sub 2//sup -/ ion beam current took the maximum at 0.06 Pa ion source pressure with Ar plasma. The effective cross sections for electron detachment of C/sub 2//sup -/ in Ar, Ne and He plasmas with electron density of 10/sup 11/-10/sup 12/ cm/sup -3/ and electron temperature of 2-6 eV were measured. They were 1.5/spl plusmn/0.1/spl times/10/sup -15/ cm/sup 2/, 1.8/spl plusmn/0.1/spl times/10/sup -15/ cm/sup 2/ and 1.4/spl plusmn/0.1/spl times/10/sup -15/ cm/sup 2/, for Ar, Ne and He respectively.