Isotope effects in an electron cyclotron resonance ion source in mixtures of N-15/N-14

Isotope effects in an electron cyclotron resonance ion source are investiga ted in mixtures of N-15 and N-14. The ratio of the extracted currents I (N- 15(q+))/I (N-14(q+)) increases with increasing charge state q. This so-call ed 'isotope anomaly' proves to be dependent on both the mixture ratio of th e gases as well as the microwave power fed into the discharge. It is shown that the often-utilized 'ion cooling' model cannot solely explain the obser ved isotope effects as long as the temperature is assumed to be equal for a ll ion species. Since we observed the occurrence of low-frequency noise in correlation with the isotope anomaly, we suggest that the ions are addition ally heated by ion Landau damping. Due to the mass dependence of this proce ss, the lighter ion species will be heated predominantly. This results in a n enhanced transport of lighter ions out of the plasma and a better confine ment for the heavier component, which, consequently, may be more effectivel y ionized to higher charges states.