Chirped Electron Bunch Energy Compensation For An X-Ray Light Source

Short electron beam pulses (subpicosecond) are central to many of the next generation light source initiatives, like X-Ray Free Electron Lasers. These FELs are based on linear accelerators which produce at the output of the last compressor an electron beam with a residual chirp to compensate for wakefield effects through the rest of the accelerating stage. It is required that this small but significant energy spread be compensated using a specially designed device. We propose to use a dielectric layered waveguide with an adjustable aperture (~2 mm) to be installed, for example, at the output of the last compressor, or at the end of the linac. This short device (~20cm) would allow the compensation of the remaining energy spread (~5 MeV chirp) after longitudinal bunch compression, using its self-wakefield-- Cherenkov radiation generated by the bunch passing through the dielectric loaded waveguide. In Phase I of this proposal Euclid Techlabs demonstrated energy chirp compensation by means of a quartz- loaded 0.95 THz cylindrical waveguide: the 200 keV correlated energy spread of a 60 MeV beam at the BNL Accelerator Test Facility (ATF) was reduced by a factor of three to the spectrometer resolution limit. The energy modulation of a long chirped bunch was demonstrated. A paper based on these results was accepted by Physical Review Letters. We have also developed tools to simulate how the beam energy is affected by its self-wake. This code was benchmarked against experimental results at the ATF. Finally, a tunable energy chirp correction structure was developed for further testing in Phase II. In Phase II we will build and test an optimized energy chirp compensating structure using the ATF beam. We have also designed a quartz structure for energy chirp compensation of the FACET (SLAC) beam which will be available for experiments in 2013 2014.The experience and knowledge gained in Phase I will help further optimization of the experimental design and ensure the success of the project. Commercial Applications and Other Benefits: The techniques we propose in this project can reduce the energy chirp of an electron beam produced with existing technologies to make it suitable to drive the next generation of X-Ray FELs. Ultrashort x-ray pulses are a powerful tool for addressing grand challenges in science, e.g. control of materials and processes at the level of electrons, design and perfect atomic- and energy-efficient synthesis of new forms of matter with tailored properties, understanding of the remarkable properties of the matter emerging from complex correlations of atomic and electronic constituents, etc.