Author/Authors :
Lumpkin، نويسنده , , A.H. and Dejus، نويسنده , , R. and Lewellen، نويسنده , , J.W. and Berg، نويسنده , , W. and Biedron، نويسنده , , S. and Borland، نويسنده , , Sheldon M. and Chae، نويسنده , , Y. and Erdmann، نويسنده , , M. and Huang، نويسنده , , Z. and Kim، نويسنده , , K.-J. and Li، نويسنده , , Y. and Milton، نويسنده , , S.V. and Moog، نويسنده , , E. and Rule، نويسنده , , D.W. and Sajaev، نويسنده , , V. and Yang، نويسنده , , B.X.، نويسنده ,
Abstract :
We report the initial, comprehensive set of z-dependent measurements of electron-beam microbunching using coherent optical transition radiation (COTR) in a saturated self-amplified spontaneous emission (SASE) free-electron laser (FEL) experiment. In this case the FEL was operated near 530 nm using an enhanced facility including a bunch-compressed photocathode gun electron beam, linac, and 21.6 m of undulator length. The longitudinal microbunching was tracked by inserting a metal foil and mirror after each of the nine 2.4-m-long undulators and measuring the visible COTR spectra, intensity, angular, distribution, and spot size. We observed for the first time the z-dependent transition of the COTR spectra from simple lines to complex structure/sidebands near saturation. We also observed the change in the microbunching fraction after saturation, multiple fringes in the COTR interferogram that are consistent with involvement of a smaller core of the e-beam transverse distribution, and the second harmonic content of the microbunching. The results will be compared to relevant calculations using GENESIS and/or GINGER.
Keywords :
SASE , FEL , electron beam , Microbunching , Coherent transition radiation
