Design of a radio-frequency photoinjector for generating relativistic vortex electron beams

Abstract

We present the design and an analysis of the classical and quantum beam dynamics for an S-band radio-frequency (RF) photoinjector dedicated to the generation of relativistic vortex electron beams carrying a quantized orbital angular momentum (OAM). The field-map-based tracking simulations were performed in the ultra-low bunch charge regime ($Q=0.63$ pC), where space-charge effects are strongly suppressed. We demonstrate that optimizing the injection phase and solenoidal focusing preserves the intrinsic thermal limit of the photoemission source, yielding a stable, emittance-compensated beam with a final kinetic energy of 4.9 MeV and a normalized transverse emittance of $56\pi \mu \mathrm{m}\cdot \mathrm{mrad}$. Furthermore, our quantum-mechanical analysis of single-electron Laguerre-Gaussian wave packets inside the designed accelerating fields reveals that rapid energy gain suppresses transverse quantum spreading by nearly three orders of magnitude compared to free-space propagation. This suppression protects the characteristic OAM-dependent spatial structure of the wave packet. These results establish the beam-dynamics operating regime and vacuum conditions required for subsequent experiments on the generation and transport of relativistic vortex electron beams.