Designing a Femtosecond-Resolution Bunch Length Monitor Using Coherent Transition Radiation Images

Ultrashort bunch length measurements are crucial for characterizing electron beams in short-pulse accelerators, including novel accelerators like EuPRAXIA and those used for free-electron lasers (FELs). This work provides an overview of the design process and the current status of a single-shot bunch length monitor prototype based on a broadband spatial imaging system for coherent transition radiation (CTR), which was recently installed at the MAX IV short-pulse facility (SPF). The THz-based imaging system was designed using optical system simulation software for full bunch simulation. CTR images were captured experimentally, followed by image analysis for comparison with reference bunch length data from the transverse deflecting cavity (TDC). This paper presents the conceptualization and design choices for the optical system of the bunch length monitor, the current experimental set-up, the installation details, and preliminary positive observations confirming the potential of this method as a novel approach to bunch length monitoring using spatial CTR images and a scalar technique, with potential for future bunch profile measurements.