Independently from electron beam based procedures, photon beam based diagnostics is an alternative way for alignment and commissioning of the numerous undulator cells in a high-gain short-wavelength free-electron laser (FEL). In this paper, using the seed laser modulated electron beam and the undulator fine tuning technique, a coherent photon beam based diagnostic was proposed for seeded FEL, and some preliminary experimental results at Shanghai deep ultraviolet FEL test facility were presented. It demonstrates that the spatial distribution of the coherent harmonic radiation from one individual or two consecutive undulator segments can be used to optimize the electron beam trajectory, to verify the undulator magnetic gap, and to adjust the phase match between two undulator segments.
@article{arxiv.1207.2616,
title = {Coherent photon beam based diagnostics for a seeded extreme ultraviolet free-electron laser},
author = {Chao Feng and Haixiao Deng and Manzhou Zhang and Shunqiang Tian and Meng Zhang and Bo Liu and Zhimin Dai},
journal= {arXiv preprint arXiv:1207.2616},
year = {2014}
}