The present invention relates to a guide tube for an ion beam in an ion implanter located adjacent a semiconductor wafer. Such guide tubes are provided to confine charged particles used for wafer neutralisation during implantation. According to the invention, a guide tube comprises an axis, open ends to receive an ion beam along said axis, a tube wall substantially parallel with said axis, and atleast one opening through the tube wall forming a gas conduction passage from inside to outside the guide tube, said passage having a length aligned at an acute angle to said guide tube axis and a minimum dimension transverse to said length such that a line of sight through the passage perpendicular to said guide tube axis is substantially occluded.