• C.A. Brau, H.L. Andrews, C.H. Boulware, J.D. Jarvis
  Vanderbilt University, Nashville, Tennessee

Funding: Medical Free Electron Laser Program of the Department of Defense under grant number F49620-01-1-0429.

In a SP-FEL, the electrons interact with an evanescent mode of the grating whose frequency is below the lowest frequency for SP radiation [1] and which travels along the grating with no losses except from dissipation. At low electron energy, the group velocity is negative and the SP-FEL operates on an absolute instability; no optical resonator is required. Due to the finite conductivity of the grating surface, dissipative losses attenuate the evanescent wave [2]. Computations for a lamellar grating show that attenuation is important at frequencies above 1 THz, and dominates when the group velocity is small. Due to the interaction with the evanescent wave, the electrons are bunched at the evanescent wave frequency. The superradiant emission from periodic bunches is characterized by spectral and angular narrowing at harmonics of the bunching frequency. Experiments are in progress to demonstrate these effects using a 40-keV electron beam photoemitted from a needle cathode in 5-ns pulses. The grating is 15 mm long, with a 250-micron period. We expect lasing at a wavelength near 1 mm, which will allow us to observe superradiant emission near 330 microns (third harmonic of the evanescent wave) on the second order of the SP radiation.

[1] H. L. Andrews and C. A. Brau, Phys. Rev. ST-AB 7, 070701 (2004). [2] H. L. Andrews, et al., Phys. Rev. ST-AB (in press).