An advanced UV optical cavity for the European FEL project

A European collaboration(1) is constructing a short wavelength FEL for the ELETTRA storage ring. The optical cavity has been designed and constructed at Daresbury Laboratory for delivery to Sincrotrone Trieste in Autumn 1999, following commissioning tests over the Summer. Initial FEL operation will be at 350 nm but subsequently down to 200 nm or less and mirrors will be 40 mm diameter. The 32 m optical cavity is controllable to 0.01 mu rad in mir ror pitch and yaw using digital piezo translators. A novel feature is the s imultaneous presence of three remotely interchangeable mirrors to extend th e tuning range and also to interchange damaged mirrors immediately. In addi tion, a transfer arm and load-lock arrangement will permit a mirror to be w ithdrawn from the chamber and replaced without disruption to the UHV system . The FEL is designed to operate at high power (1-10 W) and multi-watt spon taneous emission is also present: power loading has been investigated by FE A analysis and has necessitated specification of a water cooling system on the mirror holders, a considerable challenge made more difficult by the mir ror translation complexity. Full design details, construction description a nd test results will be presented in the paper. (C) 2000 Published by Elsev ier Science B.V. All rights reserved.