Single-pass High-gain Free Electron Laser Electron Beam Diagnostics

Consensus reached in the last few years that fourth generation light source will most likely be a X-ray or a UV coherent source based on single-pass high-gain free electron laser (FEL), such as Self Amplified Spontaneous Emission (SASE), or seeded high-gain harmonic -generation (HGHG) free electron lasers. High -gain (> 10) required for single-pass FEL puts great constrain on the quality of electron beam, and demands detailed distribution information of electron beam in six -dimension. The typical accelerator system for single -pass FEL consists of a photocathode RF gun injection system, a linac and magnetic bunch compressors, and a long undulator. The major challenges in beam diagnostics for single -pass FEL are to characterize the pico-seconds high-brightness electron beam in six -dimension produced by photocathode RF gun injector, and impro ve the stability and reliability of the photocathode RF gun injection system. Characterization of short electron bunch (∼100 fs) produced by the compressors, and co-align the electron beam with FEL radiation inside long undulator are also critical for FEL performance. We will discuss many diagnostic techniques developed at the Brookhaven Accelerator Test Facility (ATF) for the photocathode RF gun injection system, present several techniques for femto-seconds bunch length measurement, especially RF kicker cavity. Tomography technique for both transverse and longitudinal phase space measurements is discussed. Concept of multiple alignment -laser stations is present for beam alignment in the long undulator.