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| WG423 | Transverse to Longitudinal Emittance Exchange to Improve Performance of High-Gain X-Ray Free Electron Laser | |
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The ability to generate small transverse emittance is a limiting factor for the performance of high-gain free electron lasers for X-rays. Noting that beams from an RF photocathode gun can have energy spread much smaller than that required for an X-ray FEL, we present a method to produce a normalized transverse emittance 0.1 mm·mrad, an order of magnitude smaller than the state-of-the-art. The method consists of producing a pancake-shaped beam of emittance (1, 1, 0.1) mm·mrad in the (x-,y-,z-)direction, applying the flat beam technique [1] to obtain (10, 0.1, 0.1) mm·mrad, and then exchanging the x-emittance with the longitudinal(z)-emittance, finally obtaining (0.1, 0.1, 10) mm·mrad. We show that the space charge effect does not degrade the small longitudinal emittance of the pancake-shaped beam. We found that the optical scheme studied previously [2] for an approximate longitudinal-transverse exchange is not adequate for the present case due to the large emittance ratio. However, we found a new scheme giving rise to an exact exchange necessary for the method. Results of preliminary simulation confirm the analytical theory.
[1] R. Brinkmann, Ya Derbenev, and K. Floettmann, Phys. Rev. ST Acc. Beams 4, 053501 (2001) |
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