Classical and quantum beam dynamics simulation of the RF photoinjector test bench

TL;DR

This work combines classical and quantum beam-dynamics simulations to assess a dedicated S-band RF photoinjector test bench at JINR for generating high-quality, low-emittance electron beams and preserving intrinsic orbital angular momentum. Using ASTRA with Dowell–Schmerge photoemission, the authors optimize injection phase and employ an emittance-compensation solenoid to achieve a final normalized emittance of $3.2\pi\ \mathrm{mm\,mrad}$ at $Q=0.63\ \mathrm{pC}$, while demonstrating robust operation despite RF-induced phase-space distortions. In parallel, they model single-electron Laguerre-Gaussian states with OAM up to $\ell=64\hbar$ and show that acceleration in both DC and RF fields substantially suppresses transverse quantum spreading, preserving the radial OAM structure up to multi-MeV energies. The results indicate the test bench can support forthcoming experimental studies of relativistic vortex electrons, with commissioning underway and plans to incorporate collective effects in future work. $\varphi_0 \approx 180^{\circ}$ and a gradient of $45\ \mathrm{MV/m}$ are key operational parameters, and the final emittance target is $3.2\pi\ \mathrm{mm\,mrad}$ for $Q=0.63\ \mathrm{pC}$.

Abstract

We present beam-dynamics simulations for an S-band RF photoinjector test bench under development at the Joint Institute for Nuclear Research. The facility is intended to produce high-quality electron beams and to enable future generation of relativistic vortex electrons carrying quantized orbital angular momentum (OAM). Simulations of a 1.5-cell RF photogun operating at the currently available accelerating gradient of 45 MV/m demonstrate stable bunch formation at low bunch charge (Q = 0.63 pC), where space-charge effects are weak and the transverse emittance is primarily determined by RF-induced correlations. Optimization of the injection phase and the cathode-region solenoid field results in a robust emittance-compensated regime, yielding a final normalized emittance of 3.2 pi mm mrad. To evaluate the prospects for accelerating vortex electron beams, we further model the quantum evolution of single-electron Laguerre-Gaussian wave packets. The simulations show that acceleration to the multi-MeV energy range significantly suppresses free-space wave-packet spreading and preserves the initial OAM structure, indicating that the proposed test bench provides suitable conditions for forthcoming experimental studies of relativistic vortex electrons.

Figures (18)

• Figure 1: Assembled RF photoinjector test bench during commissioning. The main components are indicated: RF gun, emittance-compensation solenoid, focusing solenoid, steering magnet, RF waveguide, UV laser transport line, vacuum mirror, and beamline.
• Figure 2: Internal geometry of the S-band RF photogun. The design consists of 1.5 accelerating cells operating in the $\pi$-mode at 2856 MHz. The central aperture serves both for laser injection and electron beam extraction.
• Figure 3: Simulated axial distribution of the accelerating electric field along the beam propagation axis $z$, obtained using eigenmode analysis in CST Microwave Studio.
• Figure 4: Mechanical layout and positioning of the emittance compensation solenoid along the beamline.
• Figure 5: Measured axial magnetic field distribution of the emittance compensation solenoid. The measurement was carried out in 5 mm steps along the $z$-axis using a magnetic field meter with a coil current of 20 A.