Giant pulse generation using gain modulator

Abstract

Using fast steering magnet allowed us to redistribute average power of a storage ring free electron laser in series of short pulses with high peak power. Theory of operation and experimental results are presented. Good agreement was found between measured and predicted parameters. THEORY OF OPERATIONAverage power of the free electron laser (FEL), installed on a storage ring, is limited by an induced energy spread caused by an interaction of an electron beam with optical pulse. This is a well-known Renieri limit [1,2]. Maximal average power is limited either by degradation of the FEL gain or by electron beam losses when energy spread exceeds storage ring acceptance. In most cases the first mechanism prevails and FEL demonstrates spiking behavior, well known in the conventional lasers [3].To reach higher peak power it is possible to switch to a giant pulse mode, when all available in an active media power is released in single macropulse, followed by a relatively long relaxation period.For the storage ring FEL conventional approach, utilizing modulation of optical cavity losses, is not suitable due to the insertion losses and high vacuum requirements. The active media -the electron beam -has negligible inertia and is much easier to manipulate. To stop lasing one can break synchronism between electron and optical bunches by modulating RF frequency (i.e. revolution frequency) or by moving the electron beam from the axis of the optical cavity. We prefer the latter method, because it allows faster handling of the electron beam and do not excite inevitable synchrotron oscillations. It also makes FEL pulse more reproducible with a higher peak power.The gain modulator operates in the following manner: • electron beam is shifted from the lasing orbit and is cooling off towards natural energy spread; • when energy spread reduced to the desirable value, the electron beam is adiabatically moved to the lasing position; • giant pulse is generated causing increase in the energy spread; • cycle is repeated.Frequencies of the betatron oscillations for the Duke storage ring [4] are 0.13·f rev horizontal and 0.185·f rev vertical, where f rev =2.79 MHz is a revolution frequency. Therefore the electron beam can be moved from one orbit to another without exciting of the betatron oscillations in few microseconds.