Impact due to Impurity Contamination upon Cs Consumption of a Negative Hydrogen Ion Source

Positive hydrogen ions in a plasma of negative hydrogen (H-) ion source cannot sputter out Cs on the plasma grid surface with the energy they acquire in the plasma. Oxygen ions (O+) can exist in the source as impurities and have enough momentum to sputter out Cs on the Cs/Mo surface. The Cs sputtering yields for Cs/Mo surface by oxygen ions were calculated with the ACAT (Atomic Collision in Amorphous Target) code to obtain the sputtering yield near the threshold energy. The results showed that the O+ impact with more than 10 eV should cause substantial amount of Cs sputtering yield above 10(-6) from the plasma electrode provided the surface binding energy of Cs was only 0.8 V. The yield did not exceed 10(-6) below 50 eV O+ incident energy when the Cs binding energy was as high as 3.0 eV. Sputtering yields of Cs on Mo against tungsten ions were also calculated.