The VISA experiment, conducted at the BNL Accelerator Test Facility (ATF), has studied the properties of SASE FEL process in a saturating system. The experiment utilized a high brightness electron beam and a strong focusing undulator. Saturated gain greater than 108, with a power gain length below 18 cm, was obtained at 840 nm. Measurements of FEL gain, spectral and angular properties of SASE radiation are reported, and the results are compared to theory and to start-to-end simulations of the system. FEL performance was found critically dependent on the compression of the electron beam. /home/httpd/docserver/releases/20080613005430/public/paper/file/209/murokh_2002_0375.pdf murokh_2002_0375.pdf 24 209 ultra-short gain length, VIA, ATF, BNL, SASE FEL 24 2002 PACS 375 2007-10-19T15:49:57-07:00 1-20 2002