A. Bakhtyari, J. H. Brownell, J. E. Walsh; M. Kimmit
Dept. of Physics and Astronomy, Dartmouth College, Hanover, NH 03755; University of Essex
The incoherent spontaneous emission, or in the classical limit shot noise, produced by electrons skimming over a diffraction grating produces not only a radiation wake field but leaves behind a component of its wake field in the form of a bound surface wave. These latter components of the field have phase velocities below the speed of light. It is through coupling with the velocity synchronous component of this bound wake field that a portion of the kinetic energy of the electron is converted to radiation. In response to the wake of earlier electrons a stimulated component of the field may also develop. This is in effect a variant of the SASE process. In a recent series of experiments emission in both the spontaneous or shot noise limit and an above threshold coherent emission have been observed. The apparatus used for these experiments was a modified scanning electron microscope and the radiation observed was in the FIR region of the spectrum. Below the threshold for coherent emission the spontaneous power varies linearly with the beam current. Above threshold the current dependence is very steep and the theory for this region is still under development. In the incoherent-coherent transition region evidence of pre threshold beating of sub threshold field components can be observed. Progress on the theory and the experiment will be discussed.
