Design of a simple annular electron beam source and its operating characteristics in single and repetitive shot modes

In this work, design and performance of an annular carbon fiber cathode are presented. Measurements on this cathode were performed in a single high-voltage pulse generator (600 kV, 50 ns, and 50 Ω) and a repetitive one (350 kV, <10 ns, 180 Ω, and 100 Hz), respectively. In a single pulse regime, emphasis was placed on the uniformity of electron beam extracted from this cathode. It was found that this cathode could deliver uniform electron beams with current densities exceeding kA/cm2. Cesium iodide (CsI) coating eliminated hot spots on the cathode surface, significantly improving the uniformity of electron emission. Under repetitively pulsed operation, this cathode exhibited a good shot-to-shot reproducibility at the pressure of 1.5x10-4 Torr, suggesting an ability of surviving even in poor vacuum. However, once the base pressure rose up to 3.76x10-4 Torr, the cathode performance gradually degraded as the pulse shot proceeded. Besides, some possible explanations for these experimental results are presented. These results show that given proper diode design, carbon fiber with CsI coating has great promise as electron emitter producing high-current electron beams.