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

Abstract

Isotope effects in an electron cyclotron resonance ion source areinvestigated in mixtures of 15N and 14N. The ratio of the extractedcurrents I(15Nq + )/I(14Nq + ) increases with increasingcharge state q. This so-called `isotope anomaly' proves to be dependent onboth the mixture ratio of the gases as well as the microwave power fed intothe discharge. It is shown that the often-utilized `ion cooling' model cannotsolely explain the observed isotope effects as long as the temperature isassumed to be equal for all ion species. Since we observed the occurrence oflow-frequency noise in correlation with the isotope anomaly, we suggest thatthe ions are additionally heated by ion Landau damping. Due to the massdependence of this process, the lighter ion species will be heatedpredominantly. This results in an enhanced transport of lighter ions out ofthe plasma and a better confinement for the heavier component, which,consequently, may be more effectively ionized to higher charges states.