Experimental investigation of electrostatic capture of 1+ions in charge breeder electron cyclotron resonance ion source plasma

We present comprehensive experimental data demonstrating that the capture process of the 1 + ions in charge breeder electron cyclotron resonance ion sources (ECRIS) is dominated by electrostatic deceleration by the ambipolar plasma potential, not by cumulative small-angle scattering of the incident ions in ion-ion collisions as postulated previously. To achieve this we varied the plasma potential of an ECRIS charge breeder by adjusting the microwave power applied to sustain the helium discharge and measured the corresponding shift in the optimum injection energy of the 1 + ions. The experiment was repeated with sodium, potassium, and cesium ions. It is shown that the optimum injection energy does not depend on the incident ion mass, which contradicts the collisional drag model. Conversely, the optimum injection energy (in eV) shifts in unison with the plasma potential (in V), which provides strong evidence for the electrostatic deceleration hypothesis.