• A.H. Lumpkin, A.S. Johnson, J. Ruan, J.K. Santucci, Y.-E. Sun, R. Thurman-Keup
  Fermilab, Batavia
• P. Piot
  Northern Illinois University, DeKalb, Illinois

The A0 photoinjector (A0PI) facility at Fermilab has an ongoing proof-of-principle experiment to demonstrate the exchange of the transverse horizontal and longitudinal emittances[1]. This experiment relies on measurements of the transverse emittances and longitudinal emittance upstream and downstream of an emittance-exchanger beamline. At several locations along the accelerator beamline, YAG:Ce powder scintillator screens are used to determine beam size, divergence, and energy spread when used in an electron beam spectrometer. The screens have ~5-micron grain size and are deposited on a metal substrate as provided by DESY[2]. We have recently performed direct comparisons of beam image and slit image sizes using both the OTR screens and the YAG:Ce screens. For micropulse charges of 250 pC and with beam energies of 15 MeV, we systematically observed larger beam image sizes with the YAG:Ce screens than with the OTR screens. We deduced a YAG:Ce screen spatial resolution limit of σ=140 to 180 microns. Results at both the beam profile stations at different drift lengths from the slits and in the electron spectrometer will be reported as well as the needed corrections to the emittances.

[1] T.W. Koeth et al., Proceedings of PAC09, Vancouver, Canada.
[2] The YAG:Ce samples were manufactured at DESY and the associated specifications were provided by Klaus Floettmann, DESY.

• A.H. Lumpkin
  Fermilab, Batavia
• Y.L. Li, S.J. Pasky, N. Sereno
  ANL, Argonne

The observations of the strong enhancements of the optical transition radiation (OTR) signal observed after bunch compression of photocathode beams in the Advanced Photon Source (APS) linac chicane and at the Linac Coherent Light Source (LCLS) continue to be of interest. Spectral-dependence measurements of the coherent OTR (COTR) were done at APS initially at the 375-MeV station using a series of band pass filters inserted before the CCD camera, but recent tests with an Oriel spectrometer with ICCD readout have extended those studies. In this case we used a microchannel plate image intensifier with a GaAs photocathode to explore the NIR regime out to 880 nm. We report for the first time an ~40% intensity modulation of ~20-nm period throughout the observed broadband COTR spectra for some localized spatial structures. This effect was observed at full electron beam compression as determined from the CTR monitor located after the chicane, and it persisted with two different grating dispersions and for different spectrometer offsets. In addition, the observation of a ring-like COTR image structure in x-y space for a mismatched beam at 355 MeV will be presented and discussed.