Effect of a biased electrode on operation of electron cyclotron resonance ion source using liquid He free superconduction solenoid coils

We have constructed a liquid He free SC-ECRIS and successfully extracted intense beams of various heavy ions. To improve its performance, we installed a negatively biased electrode in the plasma chamber and observed its effect on the beam intensity systematically. We measured the beam intensity and the current of biased electrode as a function of both the applied bias voltage and the electrode position. Using the negatively biased electrode, the beam intensity of highly charged Xe ions was strongly enhanced. Furthermore, both the beam intensity and the current of biased electrode oscillated strongly. The frequency became higher when increasing the magnitude of bias voltage. This result shows that the biased electrode causes a certain instability in the electron cyclotron resonance plasma, so that the beam intensity is enhanced. In this contribution, we present the results of our experiment and the discussions about possible mechanisms of such instability.