Space Charge Effects on Short-Pulse Electron Beam Dynamics

Space charge effects on the propagation of short-pulse electron beams can significantly alter density profiles, energy distributions, and phase-space distributions. This study combines a one-dimensional (1D) multiple-sheet model and two-dimensional (2D) XOOPIC particle-in-cell (PIC) simulations to analyse the evolution of short electron pulses in a vacuum diode and drift space, for square-top, trapezoidal, and Gaussian pulses with fixed total charge. The effects of charge density, pulse duration, injection energy, and gap size are explored. In both the diode and drift space, a metric is introduced to quantify pulse distortion. For a given total charge, all pulse profiles exhibit similar levels of distortion, although Gaussian pulses show slightly greater broadening due to their extended tails. Distortion increases with charge density and decreases with pulse duration. The maximum current density that can be transported across the diode is well predicted by the short-pulse Child–Langmuir (CL) law, with excellent agreement between analytical and PIC results.

Although electrons are emitted with identical initial energy, space-charge-induced interactions lead to energy exchange between the front and rear of the pulse, producing a broadened and non-uniform energy distribution. These distortions degrade beam quality and present important design challenges for high-performance beam transport systems. To address this, critical current thresholds are derived and validated through PIC simulations, providing guidance for optimizing system performance.