The invention provides an ion source comprising first and second cathode pole pieces spaced apart from one another to form a cavity therebetween, an edge of the first cathode pole piece being spaced apart from an edge of the second cathode pole piece to define an elongate cathode gap between the respective edges of the pole pieces, the elongate cathode gap having a longitudinal axis; at least one magnet arranged for magnetising the first and second cathode pole pieces with opposite magnetic polarities; an elongate anode located in the cavity, the anode being spaced apart from the first and second cathode pole pieces and having a longitudinal axis, the longitudinal axis of the elongate anode and the longitudinal axis of the elongate cathode gap substantially coplanar; a first electrical connection which extends from outside the cavity to the anode; and a gas feed conduit which extends from outside the cavity to inside the cavity for introducing a gas into the cavity.