First demonstration of a free-electron laser driven by electrons from a laser-irradiated photocathode

Abstract

We report the results from the first operation of a free electron laser (FEL) driven by an electron beam from a laser-irradiated photocathode. The Rocketdyne/Stanford FEL achieved sustained oscillations, lasting in excess of three hours, driven by photoelectrons accelerated by the Stanford Mark III radiofrequency linac. A LaB6 cathode, irradiated by a tripled Nd: Yag mode-locked drive laser was the source of photoelectrons. The drive laser, operating at 95.2 MHz, was phase-locked to the 30th subharmonic of the S-band linac. Peak currents in excess of 125 A were observed and delivered to the Rocketdyne 2 m undulator which was operated as a stand-alone oscillator. Sustainable small-signal gain of 100% per pass was observed over a 2 h time period with periodic observation of small-signal gain as high as 150% per pass. Preliminary estimates of the electron-beam brightness deliverable to the undulator range from 3.5 × 1011 to 5.0 × 1011 A/(rad m)2.