Investigation of cathode plasma evolution characteristics in high-current electron beam diodes

In high-current relativistic electron beam diodes, the operational state is influenced by the generation and expansion of electrode plasma. Studying the characteristics and evolution of this plasma is essential for improving diode performance and stabilizing impedance. However, previous studies have typically simplified plasma dynamics as purely axial, neglecting its radial expansion along the cathode surface. This simplification, largely due to the transient nature of the plasma and the complex electromagnetic environment, has resulted in theoretical models that exhibit poor agreement with experimental observations. To address this limitation, we extend the conventional axial diffusion model for cathode plasma by incorporating radial expansion velocity, thereby proposing a revised plasma diffusion model applicable to planar diodes and an improved formulation for the electrode gap closure velocity. Experimental validation under ∼100 kV/1 kA nanosecond pulsed conditions demonstrates good agreement between theory and measurement when the radial plasma velocity is approximately 1.85 times the axial velocity, corresponding to an axial gap closure velocity of ∼35 cm/μs. These findings enhance the plasma evolution model for high-current electron beam diodes and provide a useful basis for device design and optimization.