Abstract
Observations of strongly enhanced optical transition radiation (OTR) following significant bunch compression of photoinjector beams by a chicane have been reported during the commissioning of the Linac Coherent Light Source accelerator and recently at the Advanced Photon Source (APS) linac. These localized transverse spatial features involve signal enhancements of nearly a factor of 10 and 100 in the APS case at the 150-MeV and 375-MeV OTR stations, respectively. They are consistent with a coherent process seeded by noise and may be evidence of a longitudinal space charge microbunching instability which leads to coherent OTR emissions. Additionally, we suggest that localized transverse structure in the previous self-amplified spontaneous emission (SASE) free-electron laser (FEL) data at APS in the visible regime as reported at FEL02 may be attributed to such beam structure entering the FEL undulators and inducing the SASE startup at those ``prebunched'' structures. Separate beam structures 120 microns apart in $x$ and 2.9 nm apart in wavelength were reported. The details of these observations and operational parameters will be presented.
Highlights
The self-amplified spontaneous emission (SASE) free-electron laser (FEL) experiments [1] at the Advanced Photon Source (APS) included a series of observations on z-dependent microbunching as measured by visible light coherent optical transition radiation (COTR) [2,3,4]
The measurements were performed at the APS facility which includes an injector complex with two thermionic cathode (TC) rf guns for injecting an S-band linac that typically accelerates the beam to 325 MeV, the particle accumulator ring (PAR), the booster synchrotron that ramps the energy from 0.325 to 7 GeV in 220 ms, a booster-to-storage-ring transport line, and the 7-GeV storage ring (SR)
We have connected for the first time the previously reported transverse dependencies on SASE-induced microbunching (SIM) COTR reported at FEL02 with broadband LSCinduced microbunching (LSCIM) effects as identified in investigations described over the past year
Summary
The self-amplified spontaneous emission (SASE) free-electron laser (FEL) experiments [1] at the Advanced Photon Source (APS) included a series of observations on z-dependent microbunching as measured by visible light coherent optical transition radiation (COTR) [2,3,4]. This SASE-induced microbunching (SIM) exhibited clear spectral effects at the fundamental and second harmonic of the FEL wavelength. ‘‘unexpected’’ enhancements of the signals in the visible light-IR OTR monitors (CCD cameras) were observed after compression in a chicane bunch compressor at Linac Coherent Light Source (LCLS) [10] and APS [11]
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