The effect of gas mixing on the afterglow transient of beams extracted from an electron cyclotron resonance ion source

Abstract Gas mixing and afterglow operation are common methods to improve high charge state ion beam performance of Electron Cyclotron Resonance (ECR) ion sources. In gas mixing a light gas species is introduced into the ion source plasma to enhance the ion currents of a heavier main element. The benefits of this technique are commonly attributed to the “cooling” of the heavier element ions through collisions with those of the lighter element, leading to longer ion confinement times and enhanced high charge state ion production. In afterglow operation the RF power heating the plasma is pulsed, and as a result, an intense short burst of high charge state ions is observed following the RF switch-off, with a current that can be several times higher than the steady-state ion current. This effect is attributed to the change of the ion confinement scheme from electrostatic trapping of the ions by an ambipolar potential dip to diffusive losses. This work studies the influence of gas mixing on the properties of high charge state ion beams extracted from an ECR ion source operated in the afterglow mode. The focus is on the temporal behaviour and magnitude of the extracted ion currents during the steady-state phase of the beam pulse and the afterglow current transient. It is shown that gas mixing improves the peak afterglow current and the total high charge state ion charge extracted during the afterglow but suppresses the afterglow to steady-state current ratio compared to operation without gas mixing.