The present invention relates to ion sources (14) comprising a cathode (20) and a counter-cathode (44) that are suitable for ion implanters (10). Typically, the ion source is held under vacuum and produces ions using a plasma generated within an arc chamber (16). Plasma ions are extracted from the arc chamber and subsequently implanted in a semiconductor wafer (12). The ion source according to the present invention further comprises a cathode (40) arranged to emit electrons into the arc chamber; an electrode (44) positioned in the arc chamber such that electrons emitted by the cathode are incident thereon; one or more voltage potential sources (76) arranged to bias the electrode; and a voltage potential adjuster (82) operable to switch between the voltage potential source biasing the electrode positively thereby to act as an anode and the voltage potential source biasing the electrode negatively thereby to act as a counter-cathode.