Cathode units with a carbon fiber field emitter

We have developed a carbon fiber field emitter (CFFE) with a diameter of 7 mm and a bullet shape at the top.We found that a micro-pit on the top of the fiber can focus the emitted electron beam, so that the beam is inside a small solid angle around the axis of symmetry. Our investigations show that due to this effect the fiber emitter with optimized dimensions of the micro-pit has good emission stability, good reproducibility for different cathode samples and high electron optical brightness. The CFFE with micro-pit on the top has been successfully used for 14 years in the ‘‘Cwikscan-50A’’ electron microscope instead of the conventional tungsten field emitter