Performance improvement of an X-ray ionization beam position monitor for soft X-ray free-electron lasers

An X-ray ionization beam position monitor (XIBPM) has been developed and evaluated for beam position diagnostics at a soft X-ray free-electron laser (FEL). The redesign was guided by electrostatic simulations and focused on mitigating electric-field distortions near the microchannel plate (MCP) and improving the internal electrode geometry and structural stability. The introduction of a conductive shielding cover effectively suppressed electric-field distortion from the MCP lead, substantially reducing displacement errors and improving measurement stability. A mesh-free electrode structure was also investigated, reducing photo-ion loss and increasing photo-ion transmission; however, under high-intensity FEL operation, the enhanced signal amplitude occasionally led to saturation effects. Beam position measurements were performed at photon energies of 500-1100 eV in both pink-beam and mono-beam modes. Under optimized conditions, a beam position resolution of approximately 24-25 µm was achieved for pulse-by-pulse analysis and improved to approximately 10-12 µm when averaging over ten pulses. The transverse ion profiles were found to be significantly broader than the actual photon beam size, reflecting intrinsic ion-transport dynamics and detector point-spread effects that dominate over the intrinsic photon beam size. These results demonstrate that the shielding cover represents the most significant improvement for accurate and stable beam position monitoring, while the electrode configuration can be selected flexibly depending on the operating regime. The improved XIBPM provides a practical, non-invasive solution for beam position diagnostics in soft X-ray FEL beamlines.