Coherent XUV harmonic production

In the past few years many synchrotron light sources have been developed. They provide high brilliance sources in the V.U.V. (lambdaapprox. =100 about 1000 A) and X-rays (lambda<100 A) spectral ranges. However an increase of several order of magnitudes in spectral brilliance would be obtained by coherent production of photons in these spectral ranges. This is the reason of the interest given to the development of Free electron lasers (FEL /SUB s/ ) working on storage rings. To date one Storage ring FEL has been operated at Orsay at lambda not =6500 A. In the future one hopes to make FEL working in the V.U.V. Since the optical gain decreases and since it becomes more and more difficult to make good mirrors as the wavelength decreases it will be very difficult to reach the XUV region. An alternative to the ''true'' FEL oscillator is the harmonic generation technique. In this case the electron beam passing through an undulator or ''Optical Klystron'' does not act as an amplifier, like in the FEL case, but as a non linear medium producing harmonics of an external laser. There is no threshold condition like in the case of an FEL (i.e. optical gain > cavity losses). An appreciable production of coherent light is expected in the XUV spectral range when focusing on the beam an external tunable powerfull laser working in the near UV and visible ranges. Such a laser is well within present technology