Enhancement of Ar8+ current extracted from RIKEN 18 GHz electron cyclotron resonance ion source by moving the plasma electrode toward the resonance zone

We have successfully increased the beam intensity of Ar8+ ions by moving the plasma electrode of RIKEN 18 GHz electron cyclotron resonance ion source 1.0 cm toward the resonance zone. At this electrode position, 1.25 mA of Ar8+ was obtained at an extraction voltage of 13.5 kV an microwave power of 650 W. We observed that the beam intensity increased approximately 50% by moving the plasma electrode at the same extraction voltage. We also found that the beam intensity of the lower charged states, such as 9+, 8+, is strongly enhanced, while the beam intensity of the higher charged states of Ar ions (11+, 12+) is markedly decreased. (author)