Simulations ofthe 100kW TJNAF FEL using a step-tapered undulator

The Thomas Jefferson National Accelerator Facility (TJNAF) free electron laser (FEL) can be upgraded to operate at 100kW average power in the near future using a configuration that recirculates the electron beam to recover energy. It is important to extract the maximum energy from the electron beam in a pass through the undulator while inducing the minimum amount of exhaust energy spread. A larger energy extraction reduces the requirement for a large recirculating current, while a smaller exhaust energy spread allows the intense electron beam to be recirculated without damaging components. To improve FEL performance, we explore the use of the step-tapered undulator, which alters the resonance condition halfway through the undulator. Short pulses complicate the desired interaction. Comparisons are made to the conventional periodic and linearly-tapered undulators.The authors are grateful for the support of the Office of Naval Research, Thomas Jefferson National Accelerator Facility, and contributions of Dave Douglas of TJNAF

Simulations of high-power free electron lasers with strongly focused electron and optical beams

The article of record as published may be located at http://dx.doi.org/10.1016/S0168-9002(03)00834-9A high-power free electron laser (FEL) is being designed in collaboration with Jefferson Laboratory, University of Maryland and Advanced Energy Systems, using short Rayleigh-length resonators to increase the spot size at the mirrors and hence avoid mirror damage. A short Rayleigh length implies a very small optical mode waist in the center of the cavity. It may be desirable to strongly focus the electron beam as well, to improve overlap with the intense optical fields in the interaction region. Three-dimensional simulations are used to study the effects of varying the electron beam radius and angular spread to enhance FEL gain and efficiency. The effects of off-axis shifting and tilting of the electron beam are also studied