2025

Tarvainen, O; Toivanen, V; Timonen, O; Kronholm, R; Koivisto, H

We review the current understanding of high charge state ion confinement and ion temperature in Electron Cyclotron Resonance Ion Sources (ECRIS). Experiments probing the ion confinement time of various charge states strongly favour a confinement scheme where the high charge state ions are trapped in a local dip of the ambipolar plasma potential. The electrostatic confinement permits ion confinement times of 10 ms order-of-magnitude. The dwelling time of the ions, undergoing stepwise ionisation from neutrals to high charge states, is long enough for the energy transfer from the plasma electrons to heat the ions to 10–15 eV while the energy exchange in ion–ion collisions results in all charge states having essentially the same temperature. We then describe a technique, using the ion temperatures obtained through optical emission spectroscopy and afterglow transient beam currents, to estimate the magnitude of the potential dip. In our example, measured with a 14 GHz ECRIS the value of the potential dip is 1.3–1.9 V. We demonstrate that the temporal characteristics of the afterglow transient occurring in 1 ms scale can be estimated by assuming that the afterglow peak of high charge state ion currents is caused by a change of the ion confinement scheme from electrostatic trapping to random walk diffusion resulting in order-of-magnitude reduction of the ion confinement time.