2a is hotter than the interior of the upper bottle 17 one of the main features of the valving system of the present invention is that the valve itself and the charge materials path from charge to arc are located in the above mentioned constantly increasing thermal gradient that is to say once the vapor leaves the charge it will never condense on any surface there is no clogging of the valve because the valve assembly is located between the charge 13 and the heater 21 this feature allows the ion source to be used for low temperature solid compounds or elements as well as charges existing in the liquid state since all contact surfaces at the orifice are graphite the valve is essentially self lubricated graphite construction allows the valve to be operated at elevated temperatures without damage or malfunction in addition there is essentially no detrimental chemical action on the system from halide charges there is no organic polymer packing that is usually encountered in valving systems which is subject to deterioration by heat and vacuum conditions the push rod 16 for valve control enters the bottle through the upper portion of the graphite charge bottle 17 this feature serves both as a packing seal and a lubricated dry feedthrough access the extraneous heat reaching the charge is minimized by the water cooled pads and cover 9 11 25 surrounding the charge bottles there is no charge oven which is present on the standard graphite source another feature of the present ion source is that the arc plate 2 not only directs the charge vapor into the arc discharge but being located immediately behind the arc serves to ground out normally existing equipotential lines surrounding the arc column when.