A. H. Lumpkin, N. D. Arnold, W. J. Berg, M. Borland, U. Happek*, J. W. Lewellen, and N. S. Sereno
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois *Physics Department, University of Georgia, Athens, Georgia
The interest in generating beams of sufficient brightness to support self-amplified spontaneous emission (SASE) free-electron laser (FEL) experiments continues to drive beam characterization techniques at the Advanced Photon Source (APS). In this case we have developed an EPICS-compatible version of a bunch length monitor based on the autocorrelation of coherent transition radiation (CTR). A compact Michelson interferometer was used with a Golay detector as the far infrared (FIR)-mm wave detector. The S-band rf thermionic gun with alpha magnet was used to inject a 40-ns-long macropulse into the APS linac for acceleration to 40 MeV at the diagnostics station. Optimization of the gun's performance by appropriate adjustments of the rf power to the gun cavity, the alpha magnet current, and a scraper installed on the low energy particle trajectory was guided by the observed strength of the CTR signal. The processing of the data to obtain the bunch profile was done using a series of application scripts. The bunch profile was calculated using the minimal phase technique of Lai and Sievers. Bunch lengths of 400 to 500 fs (FWHM) were measured in the core of the beam, which corresponded to about 100-A peak current in each micropulse. The dependence of the CTR signal on the square of the beam charge for the beam core was demonstrated. Results of the optimization will be reported as well as the first use of the beam accelerated to 217 MeV for visible wavelength SASE FEL gain experiments.
