Technical Design and Optimization Study for the FERMI@Elettra FELPhotoinjector

Lidia, Steven M. ; Penco, Giuseppe ; Trovo' ; , Mauro

Lawrence Berkeley National Laboratory

关键词: Photoinjector;    Space Charge;    Geometry;    Beam Transport;    43 Particle Accelerators;   

DOI  :  10.2172/886082
RP-ID  :  LBNL--60725
RP-ID  :  DE-AC02-05CH11231
RP-ID  :  886082

美国|英语

来源: UNT Digital Library

【 摘 要 】

The FERMI {at} Elettra FEL project will provide a novel, x-ray free electron laser user facility at Sincrotrone Trieste based on seeded and cascade FEL techniques. The electron beam source and injector systems play a crucial role in the success of the facility by providing the highest quality electron beams to the linac and FEL undulators. This Technical Note examines the critical technology components that make up the injector system, and demonstrates optimum beam dynamics solutions to achieve the required high quality electron beams. Section 2 provides an overview of the various systems and subsystems that comprise the photoinjector. The different operating modes of the injector are described as they pertain to the different linac configurations driven by the FEL and experimental design. For each mode, the required electron beam parameters are given. Sections 3 and 4 describe the critical beamline elements in the injector complex: the photocathode and drive laser, and the RF gun. The required drive laser parameters are given at the end of Section 3. Additional details on the design of the photoinjector drive laser systems are presented in a separate Technical Note. Design considerations for the RF gun are extensively presented in Section 4. There, we describe the variation of the cavity geometry to optimize the efficiency of the energy transfer to the electron beam. A study of the power coupling into the various cavity modes that interact within the bandwidth of the RF drive pulse is presented, followed by a study of the transient cavity response under several models and, finally, the effects on extracted beam quality. Section 5 describes the initial design for the low energy, off-axis diagnostic beamline. Beam dynamics simulations using ASTRA, elegant, and MAD are presented. Section 6 presents the optimization studies for the beam dynamics in the various operating modes. The optimized baseline configurations for the beamline and incident drive laser pulse are presented, supported by simulation results from space-charge tracking codes. Optimization of the beam transport through the downstream linac to the FEL undulator entrance requires significant deviations from the canonical ''flat-top'' temporal laser pulse distribution at the photocathode. The physics of nonlinear electron current emission are examined to determine the optimum temporal profile of the drive laser in order to produce the required linear current ramp at the injector exit. Parametric sensitivity studies are performed around the baseline configurations, and jitter studies are presented that analyze the stability of the solutions.