The Anode Rod-Pinch Diode (RPD) is considered the most ideal diode for flash X-ray machines within the 1-4 MV voltage range due to its simple structure and stable focusing characteristics. In this study, based on the experimentally measured diode voltage and its geometric structure, a detailed analysis of the physical processes in the three stages of RPD operation—namely, the Space-Charge-Limited (SCL) stage, the Weak Pinch (WP) stage, and the Magnetic Insulation (ML) stage—was conducted. The analysis results indicate that during the unipolar SCL stage, the rapid decrease in diode impedance is primarily caused by the increase in voltage, which expands the emission area of the cathode disk. In the bipolar SCL and WP stages, the gradual increase in anode ion current enhances the space-charge-limited current from the cathode, further reducing the diode impedance. During the ML stage, the generation rate of anode ions exceeds the rate at which positive ions are emitted from the anode to the cathode under bipolar space-charge-limited conditions. As the electric field on the anode surface approaches zero, excess positive and negative charges accumulate near the anode, forming a plasma. The diffusion of this plasma reduces the anode-cathode (AK) gap, leading to a further decrease in diode impedance.
 

anode rod-pinch diode,flash X-ray machine,particle simulation,plasma