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

Terahertz (THz) radiation has generated much recent interest due to its ability to penetrate deep into many organic materials without the damage associated with ionizing radiations. The generation of copious amounts of narrow-band THz radiation using a Smith-Purcell FEL operating as a backward wave oscillator is being pursued by several groups. In this paper we present start-to-end simulations of a Smith-Purcell FEL operating in the superradiant regime. Our concept incorporates a double grating configuration to efficiently bunch the electron beam, followed by a single grating to produce Smith-Purcell radiation. We demonstrate the capabilities and performances of the device, including initial beam properties (emittance and energy spread), with the help of numerical simulations using the conformal finite-difference time-domain electromagnetic solver VORPAL.

• B.M. Cowan, M.C. Lin, B.T. Schwartz
  Tech-X, Boulder, Colorado
• R.L. Byer, C. McGuinness
  Stanford University, Stanford, California
• E.R. Colby, R.J. England, R.J. Noble, J.E. Spencer
  SLAC, Menlo Park, California

Photonic crystal waveguides are promising candidates for laser-driven accelerator structures because of their ability to confine a speed-of-light mode in an all-dielectric structure. Because of the difference between the group velocity of the waveguide mode and the particle bunch velocity, fields must be coupled into the accelerating waveguide at frequent intervals. Therefore efficient, compact couplers are critical to overall accelerator efficiency. We present designs and simulations of high-efficiency coupling to the accelerating mode in a three-dimensional photonic crystal waveguide from a waveguide adjoining it at 90 degrees. We discuss details of the computation, including an optimization routine to modify the geometric parameters of the coupler for maximum efficiency, the resulting transmission, and estimates of the fabrication tolerance for these devices. We include some background on the accelerator structure and photonic crystal-based optical acceleration in general.