Nuclear Instruments and Methods in Physics Research A246 (1986) 63-66 63 North-Holland, Amsterdam OPTICAL CAVITY OF THE ADONE FEL EXPERIMENT M. AMBROSIO, G.C. BARBARINO, M. CASTELLANO, N. CAVALLO, F. CEVENNI and M.R. MASULLO Instituto Nazionale di Fisica Nucleare, Sezione di Napoli, Napoli, Italy P. PATTERI and M. PREGER lnstituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, Frascati (Roma), Italy A detailed description of the parameter choice for the LELA optical cavity is presented. Particular attention has been devoted to the alignement problems solved by means of a remote control system. First results on mirror damage due to UV radiation will be also reported. 1. Introduction The LELA (Laser ad Elettroni Liberi in Adone) is a feasibility experiment designed in order to study prob- lems involving the interaction between radiation and the recirculating electron beam of a storage ring (Adone of LNF) [1] using a transverse electromagnetic undula- tor of 20 periods. The main characteristic of the experi- ment are summarized in table 1. After the installation of the undulator, accurate spontaneous radiation [1] and optical gain [2-4] meas- urements have been worked out. Now an optical cavity of about 17.5 m has been built by prolongating the vacuum vessel of the Adone storage ring. Due to the very small peak gain (3 X 10 -4) the cavity has been designed to operate in high vacuum condition avoiding Table 1 Main characteristic of the experiment Machine Energy E = 550-625 MeV Circumference L c = 104.96 m Bunch to bunch distance A t = 117 ns Fractional energy spread ap = 2.3 × 10 -4 Bunch length a t = 350-1000 ps RF frequency fz = 51 MHz Number of bunches 3 Undulator Undulator period )'w = 11.6 cm Number of periods N = 20 Length (with clamps) L w = 2.412 m Maximum magnetic field on axis B0 = 4.95 kG Field parameter K = 4.825 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) the use of any intracavity window which could increase the losses. In this paper a brief description of the problems encountered in this construction is presented. Sect. 2 deals with the undulator features while the mechanical design characteristics are summarized in sect. 3. The remote control using piezoelectric (PZT) pushers and the preliminary experimental tests performed in order to obtain a good alignment of the cavity are described in sect. 4. In the last section some preliminary results about the mirror damage caused by the UV components of the spontaneous radiation are reported. 2. Characteristics of the LELA undulator The electromagnetic undulator installed in a straight section of the Adone storage ring has 20 periods. It has been designed to have the smallest period ()~w = 11.6 cm) to reach short wavelengths and to give a maximum vertical magnetic field on axis By = 4.95 kG. The corresponding field parameter K has, in such a way, a value large enough to provide the spontaneous radiation spectrum with an extremely rich presence of harmonics in the UV range. This fact is one of the major problems involved in the radiation damage of the cavity mirrors as it can be seen in sect. 5. Accurate measurements of the angular and spectral distribution of the spontaneous radiation from the un- dulator were performed in the first stage of the experi- ment [1]. The obtained results fit very well the calcu- lated distribution showing no distortion due to a "not ideal" magnetic field. In fig. 1 one of the measured radiation spectra is shown. l(b). WIGGLERS AND UNDULATORS