THE FRESH-BUNCH TECHNIQUE IN FELS

The "fresh bunch" technique is being proposed as a method of increasing the gain and power of FEL amplifiers in which the length of the optical radiation pulse is shorter than the length of the electron bunch. In a multi-stage FEL, electron beam energy spread is increased by the FEL interaction in the early stages. In the "fresh bunch" technique, the low energy spread of the electron beam is recovered by shifting the radiation pulse to an undisturbed part of the electron bunch, thus improving the gain and trapping fraction in later stages. A test case for the application of the "fresh bunch" method is demonstrated by numerical simulation. In this particular example we examine a subharmonically seeded VUV free-electron laser. We begin with the generation of harmonic radiation, which takes place over one part of the electron bunch. Then the radiation is shifted by means of a strong dispersive section to a fresh part of the bunch for exponential amplification and tapered wiggler amplification. By starting over with a new ensemble of electrons, the energy spread introduced by the bunching in the fundamental is removed, leading to an increased gain. Furthermore, it is possible to use a much stronger seed in the fundamental without incurring the penalty of a large energy spread later on. We note that more than a single application of the "fresh bunch" method may be done in a single FEL multiplier-amplifier. Thus X-ray wavelengths may be reached by successive multiplication in a chain of FEL amplifiers starting from a tunable seed laser.