Design and performance study of an improved cavity bunch length monitor based on an optimized offline test scheme

Abstract

Cavity bunch length monitors (CBLMs) used monopole modes within resonant cavities to obtain the root mean square (rms) electron bunch length. This paper reports the design and performance analysis of a CBLM to be used in FELiChEM, an infrared free-electron laser (FEL) facility in the National Synchrotron Radiation Laboratory (NSRL). Instead of traditional CBLMs which consist of two cavities resonating at TM010 mode, the higher-order mode, TM020 mode, is utilized to increase the working frequency and improve the resolution. To study the performance of this monitor, an improved offline test scheme using a thin wire to simulate the beam current is proposed and exploited. The influences of the beam transverse offset and the bunch transverse distribution on the bunch length measurement are studied based on the offline test platform, and an error correction methods is put forward. Simulation and experimental results show that the CBLM can measure the bunch length at the picosecond level with very high accuracy. The resolution can reach 50 fs when the signal-to-noise ratio (SNR) is greater than 70 dB.