A device for generating negative ions comprises: a) an ionizer (14) including a heatable ionizer surface; b) a heater (60) for heating said ionizer whereby positive ions (30) are generated at said ionizer surface (14e); c) a target (34) including a material for generating negative ions when said positive ions impigne on said material; wherein d) said ionizer is arranged opposite the target; e) said target is electrically negatively biased in respect to said ionizer; f) said ionizer comprises an aperture (22) through which said generated negative ions are extracted from said target to generate a beam (50) of negative ions; and wherein g) said ionizer surface (14e) is planar.