The longitudinal energy spread of ion beams extracted from an electron cyclotron resonance ion source

We present a study of factors affecting the energy spread of ion beams extracted from a Charge Breeder Electron Cyclotron Resonance Ion Source (CB-ECRIS). The comprehensive simulations, supported by experiments with a Retarding Field Analyser (RFA), reveal that the longitudinal and transverse energy spread of the extracted beams are strongly affected by the electrostatic focusing effects, namely the extraction geometry and plasma beam boundary, to the extent that the electrostatic effects dominate over the magnetic field induced rotation of the beam or the effect of plasma potential and ion temperature. The dominance of the electrostatic focusing effect over the magnetic field induced rotation complicates parametric studies of the transverse emittance as a function of the magnetic field strength, and comparison of emittance values obtained with different ion sources having different extraction designs. Our results demonstrate that the full ion beam energy spread, relevant for the downstream accelerator, can be measured with the RFA only when all ions are collected. On the contrary, studying the effect of plasma properties (plasma potential and ion temperature) on the longitudinal energy spread requires heavy collimation of the beam accepting only ions near the symmetry axis of the beam for which the electrostatic and magnetic effects are suppressed. As the extraction system of the CB-ECRIS is similar to a conventional ECRIS, the conclusions of the study can be generalised to apply for all high charge state ECR ion sources. Finally, we present the results of systematic plasma potential measurements of the Phoenix-type CB-ECRIS at LPSC, varying the source potential, the microwave power and the axial magnetic field srength. It was observed that the plasma potential increases with the extraction magnetic field and the microwave power.