Enhancement of harmonic generation using a two section undulator

Enhancement of the 2nd and 3rd harmonic of the wavelength of a Free-Electro n Laser (FEL) has been measured when a single electron beam is crossing a t wo-section undulator. To produce the harmonic radiation enhancement, the un dulator is arranged so that the resonance wavelength of the 2nd undulator ( downstream) matches a harmonic of the Ist undulator (upstream). Both the fu ndamental and the harmonic optical fields evolve in the same optical cavity and are coupled out with different extraction efficiency, through a hole i n one of the cavity mirrors. We present measurements that show that the opt ical power at the 2nd and 3rd harmonic can be enhanced, by about one order of magnitude, in two configurations: when the resonance wavelength of the 2 nd undulator matches the harmonic of Ist one (harmonic configuation), or wh en the gap of the 2nd undulator is slightly larger than first one (step-tap ered configuration). We examine the dependence of the harmonic power on the gap of the 2nd undulator, This fundamental/harmonic mode of operation of t he FEL may have useful applications in the production of coherent X-ray and VUV radiation, a spectral range where high reflectivity optical cavity mir rors are difficult or impossible to manufacture. (C) 1999 Elsevier Science B.V. All rights reserved.