Light Emission From CsI-Coated Carbon Velvet Cathodes Under Varied Conditions

Many high-power electron devices utilize cold-cathode diodes to generate intense electron beams. These cold cathodes have the advantage of being capable of supplying several kiloamperes of current spread over a large cross section without the need for auxiliary components such as a heater supply. However, they suffer from many known problems such as nonuniform emission that can result in small areas of high current density on the anode and, thus, excessive anode heating. As a consequence, outgassing and vaporization of bulk material frequently leads to premature impedance collapse. Hence, minimizing nonuniform anode heating due to beam nonuniformity is paramount. As previously demonstrated, the use of a CsI-coated carbon velvet cathode improved beam uniformity, reduced outgassing, and mitigated early impedance collapse. To quantify the uniformity, temporal and spatially resolved images of the cathode plasma were taken for a CsI-coated carbon fiber cathode, operated at an average current density of ~150 A/cm² under various conditions, i.e., without a field shaping ring, before and after discharge cleaning, and with a field shaping ring. All cathodes were operated in a sealed tube with a small integrated sputter ion pump to restore vacuum levels to 10^-9 torr levels between subsequent shots.