FEL cavity length measurement with an external laser

The nature of linac-driven free electron laser (FEL) operation requires that the length of the laser optical resonator be matched to the repetition rate of the accelerator in order for the returning optical pulse to overlap the newly arriving electron bunch. The range of cavity lengths over which oscillation will occur is limited in typical FEL designs to a few tens of microns, while the total resonator length is often many meters. The task of correctly setting the length of a new FEL resonator is therefore quite difficult, particularly if the cavity geometry contains more than a single line segment. We have demonstrated that such an absolute length measurement with micron accuracy over many meters in any cavity configuration is possible by establishing resonances in the cavity with an external laser of known repetition frequency. This technique was successfully applied in the development of two new FELs at the Stanford FEL Center. We will discuss the advantages of this method over conventional measurement schemes.