Fig. 1. Vacuum gap model of coaxial structure

Fig. 2. Surface electron distribution of cathode rod

Fig. 3. Structure diagram of experimental device

Fig. 4. Electric field distribution at diode voltage 400 kV

Fig. 5. Maximum electric field on the cathode rod surface varies with the diode voltage

Fig. 6. Cathode rod with epoxy coating

Fig. 7. Experimental device

Fig. 8. Voltage and current waveforms before breakdown

Fig. 9. Breakdown voltage and current waveforms

Fig. 10. E_s and E_bi of cathode rod (R_z＝0.8 µm) with ER film and AC film

Fig. 11. E_s and E_bi of cathode rod (R_z＝0.2 µm) with PI film and PEEK film