Optimization of the Parameters of a Storage Ring for a High Power XUV Free Electron Laser

Optimization of the Parameters of a Storage Ring for a High Power XUV Free Electron LaserA. Jackson, J. Bisognano, S. Chattopadhyay, M. Cornacchia, A. Garren, K. Halbach,K.J. Kim, H. Lancaster, J. Peterson, M. S. ZismanLawrence Berkeley Laboratory#1 Cyclotron Rd. MS 47 -112Berkeley, California 94720C. Pellegrini, G. VignolaBrookhaven National LaboratoryUpton, L.I., New York 11973AbstractIn this paper we describe the operation of an XUV high gain FEL operating within abypass of an electron storage ring, and discuss the implications on storage ring optimiza-tion imposed by FEL requirements. It transpires that, in the parameter regime of inter-est, collective effects within the beam play an important role. For example, intrabeamscattering dilutes the transverse emittance of the beam and the microwave instabilityincreases the momentum spread. Both phenomena reduce the effectiveness of the FEL. Acomputer code, ZAP, has been written which, for a given lattice design, takes all sucheffects into consideration and produces a figure of merit for FEL operation for thatmachine. We show the results of ZAP for several storage ring designs, all optimized forFEL operation, and present a design example of a facility capable of producing coherentradiation at 400 A with tens of megawatts of peak power.1. IntroductionThere has recently been remarkable progress in demonstrating the generation of coherentradiation through Free Electron Laser (FEL) interaction in the infrared and microwaveregion (Ref. 1). With electron beams of suitable quality, the technique could be extendedto wavelengths shorter than 1000 A.With present day technology, there are two promising approaches to the vacuumultraviolet (XUV) FEL. One is based on cavity formation by end mirrors (Refs. 2 and 3),the other through the development of high gain in a single pass device. The former "FELoscillator" is currently restricted to longer wavelengths because high reflectivitymirrors (although rapidly evolving through multilayer technology) are not yet available(Ref. 2). In the second approach, which we call the High Gain FEL, the interactionbetween the electron beam and the undulator occurs in a single pass, and no mirrors arerequired.The most promising source of electrons with the characteristics required for FELoperation is an electron storage ring. The mode of operation is to deflect the circula-ting electron bunch into a special bypass containing the FEL undulator, as shown schemati-cally in Figure 1. The beam, which is severely disrupted in the FEL interaction is thenreinjected into the storage ring, where its equilibrium characteristics are restoredthrough the process of radiation damping. After one damping time (50 -100 ms), the beam isready to be switched back into the FEL bypass and the process is repeated. References 4and 5 give a more detailed description of FEL bypass operation.In this paper we show how the evolution of the optical pulse in the FEL is determined bycertain characteristics of the electron pulse, in particular, the charge density and mo-mentum spread. We show how these requirements lead to conflicting demands on the storagering design, and how these conflicts have been assessed in a systematic fashion throughthe development of a new computer code, ZAP.This novel, systematic approach has been used to choose between candidate storage ringlattices, all of which were optimized for high gain FEL operation. Based on our study wepresent a design example that is capable of producing coherent radiation at 400 A withtens of megawatts peak power.