VISA: Announcement of Experimental Evidence for Saturation

On behalf of the VISA collaboration, we are very pleased to announce that VISA has recently demonstrated a gain of 2×10⁶, and an exponential growth rate (intensity gain length) of 18.5 cm. There is also evidence of saturation of the FEL intensity at about 3.6 m from the undulator entrance. The experiment was done using a 71 MeV high brightness electron beam, with a peak current of 80 to 90 A, a normalized projected emittance of about 1.5 mm mrad, at a radiation wavelength of 830 nm. The results indicate that the gain depends on the slice emittance, and that the slice emittance is substantially smaller than the projected emittance.

Spectral measurements of the radiation at saturation show large second and third harmonics, in addition to the fundamental. The third harmonic is many orders of magnitude larger than the corresponding spontaneous intensity. Additional measurements will be done during the next few months to obtain more data and more information on the spectral and angular characteristics of the FEL radiation.

The VISA (Visible to Iinfrared SASE Ampl ifier) experiment was designed, built, installed and carried out by a BNL-LLNL-SLAC-UCLA collaboration. The experiment is being done using the Accelerator Test Facility at the National Synchrotron Light Source, Broo khaven National Laboratory.

Claudio Pellegrini, UCLA, Spokeperson for the VISA collaboration
Max Cornacchia, SLAC
Karl Van Bibber, LLNL
X.-J. Wang, BNL

Friday, March 30, 2001