• C.R. Prokop, P. Piot
  Northern Illinois University, DeKalb, Illinois
• M.C. Lin, P. Stoltz
  Tech-X, Boulder, Colorado

Terahertz (THz) radiation occupies a very large portion of the electromagnetic spectrum and has generated much recent interest due to its ability to penetrate deep into many organic materials without the damage associated with ionizing radiation such as X-rays. One path for generating copious amount of tunable narrow-band THz radiation is based on the Smith-Purcell free-electron laser (SPFEL) effect first proposed by Walsh. In this paper we present the design and start-to-end simulation of a compact SPFEL. The device is based on a low energy (20-40 keV) electron accelerator capable of producing sheet electron beams needed to enhance the SPFEL interaction. The beam tracking simulations are carried with a quasistatic particle-in-cell program (Astra from DESY) while the beam dynamics and electromagnetics of the SPFEL interaction is modeled using a finite-difference time-domain electromagnetic solver (VORPAL from Tech-X Corporation).

• 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.