Characterization of carbon fiber flocked cathode materials

Summary form only given. The paper studies the emission characteristics of new cathode materials made of carbon fiber flock that have the potential to improve electron gun performance. Relativistic electron beams with high current densities and low temperatures are a prerequisite for the successful operation of free electron lasers and other sources or coherent radiation that use electron beams. If high radiation powers are also needed, high currents, high current densities and high brightness are necessary. These requirements become more and more stringent the shorter the radiation wavelength. We examined three different carbon fiber flocks and measured emittance using a pepper-pot technique. Our anode was 0.38 mm thick stainless steel and had a single 0.51 mm pinhole. A 0.038 mm Al foil coated on the downstream side with a 0.076 mm coating of ZnS was used as a scintillator. All three flocked cathodes had fiber diameters of 7 microns with lengths of 0.020", 0.050", and 0.100" respectively. Voltages ranged from 80-350 kilovolts. Anode-cathode gap distances varied from 0.3 to 1.2 cm. Current during the voltage pulse was also measured. Current densities as high as 130 A/cm/sup 2/ were attained. Turn on fields were obtained from the associated V-I characteristics. The results were compared to those obtained for a standard POCO graphite cathode. The carbon fiber flocked cathodes had substantially lower turn on fields, reduced emittance, and subsequently much greater brightness.