Coherent Smith-Purcell radiation for minimally invasive bunch length measurement at the subpicosecond time scale

We have designed a tool to measure the bunch length of an electron beam in a minimally invasive way by means of coherent Smith-Purcell radiation (SPR). The technique has been employed successfully at a test apparatus for the Mainz Energy-recovery Superconducting Accelerator MESA, demonstrating it is possible to determine the bunch length while losing less than 0.6% of the electron beam. The impact of the space charge on the bunch length can be reduced while tuning the longitudinal bunch preparation system during a live measurement at beam currents up to 1 mA. Doing so, RMS bunch lengths of70μmcan be achieved in a typical operating mode of the low-energy beam transport system of MESA. In addition to the bunch length measurements, typical properties of the generated SPR are demonstrated.