Space Charge Effects on Short-Pulse Electron Beam Dynamics in a Classical Vacuum Diode

Space charge effects pose significant challenges to the advancement of electron beam-based technologies. In this study, we investigate the influence of space charge effects on the evolution of short-pulse beam profiles in a vacuum diode using a 1-D multiple-sheet model and 2-D particle-in-cell (PIC) simulations. The effects of different initial profiles (square-top, trapezoidal, or Gaussian), charge densities, and pulse widths are analyzed. We examine the current density limit as the pulselength decreases and the resulting distortion of the beam as it traverses the gap. It is found that for the same total charge, square-top, trapezoidal, and Gaussian pulses undergo similar degree of distortion, where Gaussian pulses show slightly larger distortion (especially at longer pulselength and smaller charge density) due to its wider spread in shape. The distortion becomes more significant for shorter pulselength. For larger charges in the pulse, the tail of the pulse travels through the gap at a decelerated rate. A shorter pulse duration also leads to a larger beam energy spread for all three pulse profiles, with the peak electron densities found at low and high energy ends of the distribution. For longer pulses, the density peaks are at the center of the energy distribution. In addition, the maximum current density that can be transported across the diode follows the short-pulse Child-Langmuir (CL) law, regardless of the initial pulse shape. The results from the multiple-sheet model are in good agreement with PIC simulations.