The Super-ACO FEL operation with shorter positron bunches

The recent installation of a 500 MHz RF cavity on the storage ring Super-AGO allows the positron bunch to be reduced by a factor of 3. The first tests on the positron beam started at the end of March this year: besides the bunch length reduction observed on a double sweep streak camera, a vertical unstability usually appears and could be damped under given circumstances (current, voltage on the harmonic cavity, value of the chromaticity...). The Free Electron Laser (FEL) was nevertheless successfully operated with a higher gain at 350 nm: the laser threshold at low current is 7 mA instead of 25 mA with the same set of mirrors, the laser tunability is enhanced by a factor of 2, and the pulse duration is shortened by a factor of 2. Besides, the laser establishment stabilizes the vertical beam instability, in a complex dynamical process involving transverse to longitudinal coupling. Stability analysis will be carried out in more details. (C) 1998 Elsevier Science B.V. All rights reserved.