Superfine adjusting of resonant cavity length for free-electron laser

ABSTRACTThe importance,fundamental principles and methods of tuningoptical cavity for free-electron laser are described. An adju-stable mechanical device controlled by a microcomputer is usedfor adjusting with a fine resolution,and an electrostrictivedevice controlled by the same microcomputer is used for adjustingwith super-fine resolution. The fine adjusting range is 6cm andthe resolution is better than 0.5i&m. The super-fine adjustingrange is 10 pm,and the resolution is better than 0.1 sin. 1 . INTRODUCTION The resonant cavity for free—electron laser (called FEL below)contains two mirror,an upstream mirror and a downstream mirror,anundulator and a vacuum pipe with several optical windows (seeFig.1). Several groups of magnets in the undulator and the tran-smission channels for monitoring electron beam occupy large spaceso that two mirrors must be kept at several meters distance fromeach other. The chracteristic of FEL requires that resonant fre—quency of the optical cavity is synchronized with repetition rateof micro-pulses in electron beam in order to obtain laser output.In addition,thermal effect and mechanical disturbance can causevariation of optical cavity length. For these reasons, longitu-dinal tuning cavity length is a important technology to ensureFEL opration.Tuning cavity is devided into two steps: (1) After two mirrorsof cavity are installed on the basis of the theoretical calcula-tion value of the cavity length,an adjustable mechanical devicecontrolled by a microcomputer is used for a fine tuning to makecavity length approach optimum value in order to enter the opera-tion range of FEL.