Use of Coherent Transition Radiation to Set Up the APS RF Thermionic Gun to Produce High-Brightness Beams for SASE FEL Experiments

摘要

We describe use of the Advanced Photon Source (APS) rf thermionic gun, alphamagnet beamline, and linac to produce a stable high-brightness beam in excessof 100 amperes peak current with normalized emittance of 10 pi mm-mrad. Toobtain peak currents greater than 100 amperes, the rf gun system must be tunedto produce a FWHM bunch length on the order of 350 fs. Bunch lengths this shortare measured using coherent transition radiation (CTR) produced when the rf gunbeam, accelerated to 40 MeV, strikes a metal foil. The CTR is detected using aGolay detector attached to one arm of a Michelson interferometer. The alphamagnet current and gun rf phase are adjusted so as to maximize the CTR signalat the Golay detector, which corresponds to the minimum bunch length. Theinterferometer is used to measure the autocorrelation of the CTR radiation. Theminimum phase approximation is used to derive the bunch profile from theautocorrelation. The high-brightness beam is accelerated to 217 MeV and used toproduce SASE in five APS undulators installed in the Low- Energy Undulator TestLine (LEUTL) experiment hall. Initial optical measurements showed a gain lengthof 1.3 m at 530 nm. * Work supported by U. S. Department of Energy, Office ofBasic Energy Sciences, under Contract No. W-31-109-ENG-38.