Electron pulse shaping in the FELIX RF accelerator

The FELIX free-electron laser uses short pulses of relativistic electrons p roduced by an RF accelerator. The design target for the duration of these e lectron bunches was around 3 ps. In experiments we observed that the bunche s emit coherently enhanced spontaneous emission (CSE) when they travel thro ugh an undulator. It was demonstrated that the power level of the CSE criti cally depends on the settings of the accelerator. In this article we seek t o explain these observations by studying the length and shape of the electr on bunches as a function of the settings of the accelerator. A particle-tra cking model was used to simulate the acceleration and transport processes. These include bunch compression in a 14-cell travelling wave buncher cavity , acceleration in a travelling wave linear accelerator, and passage through a (dispersive) chicane structure. The effect of the phase setting of the RF accelerating field with respect t o the arrival time of the electron bunch in each accelerator structure was studied. The parameter range of the simulations is related to that of an ac tual free-electron laser experiment using these bunches. We find that, for specific settings of the accelerating system, electron pulses with a length of 350 mu m FWHM (1 ps) are produced. The charge in the bunch rises steepl y within a distance of 25 mu m This bunch shape explains the high level of coherently enhanced spontaneous emission observed in the FELIX laser. (C) 1 999 Elsevier Science B.V. All rights reserved.