An electro-optical bunch profile monitor for FCC-ee

TL;DR

This paper presents a non‑destructive, single‑shot longitudinal bunch profile monitor for FCC‑ee based on an in‑vacuum electro‑optic (EO) approach. It advances a single‑pass prism‑based crystal holder to avoid upstream signal overlap and reduce impedance, enabling measurements of both long and short bunches across FCC‑ee modes. CST wakefield simulations and KARA measurements establish the modeling framework and feasibility, while a CLEAR prototype confirms the concept's viability; ongoing work will optimize for FCC‑ee integration and multi‑bunch operation, with potential impact on precise bunch diagnostics and accelerator performance. All mathematical relations are expressed with appropriate notation, such as the phase retardation $\\Gamma$ and the detected intensity $I_{\\text{det}}(\\theta,\\phi,\\Gamma)$, to support rigorous quantification and reproducibility.

Abstract

The Future Circular Lepton Collider (FCC-ee) presents challenges for a longitudinal bunch profile monitor due to its wide range of bunch lengths and charge densities across its four distinct operational modes. For commissioning, monitoring the top-up injection, and energy calibration, the FCC-ee requires non-destructive, single-shot measurements of the bunch length and profile. This contribution proposes an in-vacuum electro-optical (EO) longitudinal bunch profile monitor for single-shot measurements at high repetition rates, building on the successful EO monitor at the Karlsruhe Research Accelerator (KARA) at the Karlsruhe Institute of Technology. A novel single-pass conceptual design for the in-vacuum holder of the electro-optical crystal is presented, utilizing prisms instead of a mirror to guide the laser through the crystal, which additionally allows measurements of the long bunches foreseen for FCC-ee operation mode at the Z-pole energy. A first prototype has been constructed and tested at the in-air test stand of the CERN Linear Electron Accelerator for Research (CLEAR). Results from the prototype tests are presented, demonstrating the proof of principle for the single-pass prism-based EO monitor design for FCC-ee.

Figures (17)

• Figure 1: Schematic of electro-optical spectral decoding (EOSD) and electro-optical sampling (EOS) at KARA. The laser is guided in a 35m long fiber from a laboratory to the vacuum chamber of KARA, through the crystal, a polarizer and back to the laboratory for the analysis and data acquisition. The fiber can either be connected to a spectrometer for single-shot bunch profile measurements with EOSD, or to a photodiode (PD) for an EOS scan of the averaged Coulomb- and wakefield of the electron bunch.
• Figure 2: Simulation of the total phase retardation $\Gamma$ at KARA in red, with the upstream component in green (dashed) and downstream component in yellow (dash-dotted). $t_\text{o}$ highlights the time of the Coulomb peak from the bunch overlapping with the laser. The later peaks of the phase retardation originate in the wakefield. Around the time of the overlap $t_{\text{o}}$, the unwanted upstream signal has a low amplitude.
• Figure 3: Comparison of the simulated modulation $M$ of the laser pulse (blue) and the modulation of a simulated eos measurement (orange) including effects of the laser pulse length, the pd and the lock-in amplifier.
• Figure 4: Comparison of the Fourier-transformed simulated modulation $M$ (blue) and a simulated measurement (orange), corresponding to of the time-domain plots in Fig. \ref{['fig:ch2:eos_lockin']}. It shows the low-pass filtering of the simulated data acquisition of measurements in comparison to the unfiltered simulated modulation.
• Figure 5: KARA EOS scan with a lock-in amplifier in blue, which corresponds to the measured laser intensity with the pd reissig2024. The dashed orange line indicates an exponential fit, highlighting the general signal amplitude drift over time. The delay scan started on the right and took 15 to complete.