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Electron Bunch Compression with Dynamical Nonlinearity Correction for a Compact FEL

Electron Bunch Compression with Dynamical Nonlinearity Correction for a Compact FEL . Toru Hara, Kazuaki Togawa , Hitoshi Tanaka XFEL/RIKEN. Contents. Compact FEL for widespread distribution of light sources Conventional nonlinearity correction scheme using a high-harmonic cavity

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Electron Bunch Compression with Dynamical Nonlinearity Correction for a Compact FEL

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  1. Electron Bunch Compression with Dynamical Nonlinearity Correction for a Compact FEL Toru Hara, Kazuaki Togawa, Hitoshi Tanaka XFEL/RIKEN

  2. Contents • Compact FELfor widespread distribution of light sources • Conventional nonlinearity correction scheme using a high-harmonic cavity • Nonlinearity correction using a frequency up-conversion • Application to XFEL/SPring-8 • Summary

  3. Compact SR sources Three big facilities of 3rd generation • Medium size new 3rd generation • competitive brilliance by using short-period undulators • - feasible cost for a single country

  4. Compact FEL source Big facilities of several kilometers. Less than 1 km, but still large. High-gradient accelerator requires a high rf frequency. How to compensate nonlinearity in bunch compression? • Downsizing needs • short-period undulators • low-emittance injector • high-gradient accelerator

  5. Bunch compression 4-bend chicane bunch compressor Zero bunch length can be obtained in a linear transformation. • Nonlinearities in real sysmtem • limit the attainable peak current. • - sinusoidal rf curvature • high-order momentum • compaction factors of BCs • - space chage, wakefields, CSR···

  6. Nonlinearity correction with a high-harmonic cavity Second-order nonlinearities of the rf field and the BC have the same sign. An additional device needed. The electron beam is decelerated in a correction cavity, so the frequency of the correction field should be higher than main acceleration field.

  7. Nonlinearity correction with a high-harmonic cavity T. Limberg et al., “Optimized bunch compression system for the future European XFEL”, PAC’05, p.1236 (2005). R. Akre et al., Phys. Rev. ST Accl. Beams 11, 030703 (2008). SCSS X-FEL Conceptual Design Report (2005).

  8. Nonlinearity correction for a compact FEL A compact FEL uses high-frequency rflinac, C-band or X-band. An extremely high-frequency correction cavity, X-band or K-band, encounters a technological difficulty. Use bunch compressor as a frequency up-converter. Bunch compression by CB

  9. Nonlinearity correction for a compact FEL • A conventional correction scheme implicitly includes the effect of frequency up-conversion. • By directly using this effect, a correction cavity can be operated at the same frequency as the main linac. • Instead, a sub-harmonic accelerator is installed before the correction cavity. (d) A simple model composed of a single BC. Linearization of the energy chirp at the end of the main linac (d).

  10. Analytical expressions up to second-order for linearization Initial energy chirp before the BC is a longitudinal electron position with respect to a reference particle in the bunch. Bunch compression in the BC This should be zero. Energy chirp after the main linac

  11. Correction parameters L-band C-band Correction cavity phase -180 deg. R56 is varied to change CB. Constant for L-band compensation. C-band compensation decreases for a large CB.

  12. Dynamical evolution of energy chirp (a) (b) (c) (d) For a multi-stage bunch compression system Space charge, CSR and wake fields also become nonlinear sources. If CSR in BC and wake fields of C-band are included, parameter optimization with a particle tracking is necessary. Correction cavity voltage and phase are 5 MV and -180 deg. CB is 6.3. Initial beam current 50 A (uniform). ELEGANT is used for a particle tracking. Correction cavity voltage 5 MV -> 4 MV. CB 6.3 -> 7.8.

  13. Application to XFEL/SPring-8 Accelerator tunnel XFEL/SPring-8 building RF frequency is gradually increased according to the bunch length compression. Beam commissioning will be started in 2011. Klystron gallery

  14. Nonlinearity correction in XFEL/SPring-8 Space charge, CSR and wakefields add additional nonlinearity. Initial emittance 0.6 pmm-mrad is not degraded in the acceleration and bunch compression to 8 GeV. PARMELA and ELEGANT are used for particle tracking.

  15. Summary • A nonlinearity correction scheme directly using a frequency up-conversion due to bunch compression is shown. • Derived analytical expression and simulations confirm the validity of the correction scheme. • The correction scheme without a high-harmonic cavity is important for a compact FEL.

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