Abstract
The Advanced Photon Source (APS) injector complex includes an option for rf photocathode (PC) gun beam injection into the 450-MeV S-band linac. At the 150-MeV point, a four-dipole chicane was used to compress the micropulse bunch length from a few ps to sub-0.5 ps (FWHM). Noticeable enhancements of the optical transition radiation (OTR) signal sampled after the APS chicane were then observed as has been reported in the Linac Coherent Light Source (LCLS) injector commissioning. A far-infrared (FIR) coherent transition radiation detector and interferometer were used to monitor the bunch compression process and correlate the appearance of localized spikes of OTR signal (5 to 10 times brighter than adjacent areas) within the beam-image footprint. We have performed spectral-dependency measurements at 375 MeV with a series of bandpass filters centered in 50-nm increments from 400 to 700 nm and with an imaging spectrometer and observed a broadband enhancement in these spikes. Mitigation concepts of the observed coherent OTR, which exhibits an intensity enhancement in the red part of the visible spectrum as compared to incoherent OTR, are described.
Highlights
The generation of ultrabright electron beams for various accelerator-based applications often involves compression of a photocathode (PC) rf gun beam using a four-magnet chicane configuration [1,2,3]
A second issue became more apparent during the commissioning of the Linac Coherent Light Source (LCLS) injector in 2007, when unexpected enhancements of the signals in the visible light optical transition radiation (OTR) monitors occurred after compression in a chicane bunch compressor [9]
If we consider transport efficiency during the phase scan, photon number enhancements, intensity fluctuations, charge dependence, and spectral structure observed, we can discriminate among longitudinal space charge (LSC) microbunching, a longitudinal spike, multiple submicron spikes, and CSRinduced transverse clumping
Summary
The generation of ultrabright electron beams for various accelerator-based applications often involves compression of a photocathode (PC) rf gun beam using a four-magnet chicane configuration [1,2,3]. A second issue became more apparent during the commissioning of the LCLS injector in 2007, when unexpected enhancements of the signals in the visible light optical transition radiation (OTR) monitors occurred after compression in a chicane bunch compressor [9]. These signals were attributed to a LSC-induced microbunching (LSCIM) that started from noise with a concomitant generation of coherent OTR (COTR) at the Al screens [8]. A set of mitigation options for the COTR will be described which complement the expected effects of a laser heater [7]
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