Abstract: Electron beam is focussed in a focussing column in a high vacuum environment and emitted downward through upper, middle, and lower orifices. A second stage vacuum housing concentrically surrounds a lower portion of the beam focussing column and separates the upper and middle orifices. A first stage vacuum housing concentrically surrounds the second stage vacuum housing and separates the middle and lower orifices. An outer wall surrounding the second housing tapers downward to the lower orifice, allowing it to be positioned as closely as desired to a workpiece regardless of surface topography. The apparatus comprises an orifice housing having an internal cavity which receives the lower part of the beam focussing column, an inner wall which defines the outer boundary of the second housing, and a support spool borne against by the outer wall, which is fixed to the bottom of the orifice housing and fixes the other components relative to each other in the housing.