Observation and analysis of interferences on undulator coherent radiation stored in an optical cavity

Abstract

We measured the interferences of the coherent radiation, produced by a train of electron bunches in an undulator and stored in an optical cavity. The device used here is a free-electron laser, whose optical cavity stores the radiation pulse for a multipass amplification process. The amplification process is avoided by detuning the optical cavity. This is done by increasing the cavity length, producing a desynchronism between the radiation pulses and electron bunches. Then, the output intensity is dependent on the cavity lengthening and exhibits periodical interferences. An analytical analysis makes a description of this effect. In practice, it allows a direct and simple measurement of the position of the ``zero detuning,'' corresponding to the perfect synchronism of the light pulse and electron bunch in the free-electron laser process.