Enhancement of frequency-tunability for a Smith-Purcell free-electron laser in the millimeter-wave region

The frequency-tunability for a Smith-Purcell free-electron laser in the millimeter wave region is enhanced with the aid of a quasi-periodic distributed Bragg reflector (DBR) made of metallic waveguide gratings with rectangular grooves. For a quasi-periodic DBR, the grating parameters are adjusted by means of genetic algorithms to get the desired reflection coefficient, thus compensating for the frequency dependence of the overall output characteristics. For a typical numerical example, flat output characteristics have been obtained over 10 % bandwidth around the center frequency 121 GHz.