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Ali Nassiri and Geoff Waldschmidt Accelerator System Division Advanced Photon Source

A Brief Report on the Status of Rf Deflecting Cavity Design for the Generation of Ultra-Short X-Ray pulses at APS. Ali Nassiri and Geoff Waldschmidt Accelerator System Division Advanced Photon Source. ICFA Mini-Workshop on “Frontiers of Short Bunches in Storage Rings”

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Ali Nassiri and Geoff Waldschmidt Accelerator System Division Advanced Photon Source

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  1. A Brief Report on the Status of Rf Deflecting Cavity Design for the Generation of Ultra-Short X-Ray pulses at APS Ali Nassiri and Geoff Waldschmidt Accelerator System Division Advanced Photon Source ICFA Mini-Workshop on “Frontiers of Short Bunches in Storage Rings” Laboratori Nazionali di Frascati, 7-9 November 2005

  2. Acknowledgements Special thanks to Kenji Hosoyama (KEK), Derun Li and J. Shi ( LBNL), and Tim Koeth (Fermilab) for many productive and useful discussions. A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  3. Feasibility study group* Undulator radiation & x-ray optics L. Assoufid R. Dejus D. Mills S. Shastri RF K. Harkay D. Horan R. Kustom A. Nassiri G. Pile G. Waldschmidt M. White Beam dynamics M. Borland Y.-C. Chae L. Emery W. Guo K.-J. Kim S. Milton V. Sajaev B. Yang A. Zholents, LBNL * All affiliated with APS except where noted A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  4. Outline • Introduction • SC vs. RT option • Crab cavity modeling • Summary A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  5. Parameters / Constraints: What hV is Required? Can get the same compression as long as h*V is constant V=6, h=4 V=4, h=6 Higher V and lower h: more linear chirp and less need for slits V=6, h=8 Higher h and lower V: smaller maximum deflection and less lifetime impact Cavity design and rf source issues h=7, V<6 MV? Higher h and maximum V: shortest pulse, acceptable lifetime Beam dynamics simulation study: h ≥ 4 (1.4 GHz) V ≤ 6 MV (lifetime) M. Borland, APS ps Workshop, May 2005 A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  6. Parasitic modes (squashed geometry) TM010 Accelerating mode TM110h/TE111h TM011 frequency TM110v APS crabbing mode TE111v • Vertical crabbing mode (APS): horiz axis “squashed” • Maximize mode separation for optimized damping • HOMs above beam pipe cutoff, propogate out • Lower-order mode (TM010) may strongly couple to beam; freq. below cutoff, adopt KEKB coaxial line strategy (for SC) • Multiple cells produce multiplicity of parasitic modes (issue for SC) • Orbit displacement causes beam loading in crabbing mode; adopt KEKB criterion of y = ±1 mm (for orbit distortions ± 0.1 mm) • Generator power increased to compensate; de-Q to decrease sensitivity A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  7. RT vs. SC rf • RF sources • for SC option are available with minimal reconfiguration • for RT are non-typical and modification is required (1 kHz) • Cavity fill time vs. susceptibility to phase noise • Long for SC cavity; makes it less susceptible • Short for RT structure; makes it more susceptible • Need to compensate frequency detuning • Due to pulse heating for RT case • From microphonics for SC case A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  8. 9 Cells SW Deflecting Structure • Pulsed heating < 100 deg. C • BMAX < 200 kA/m for 5 μs pulse (surface) • Limited available power ≤ 5 MW • EMAX < 100 MV/m (surface) V. Dolgashev, SLAC, APS seminar, June 2005 A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  9. SC RF Cavity Study for APS • Single-cell vs. multiple-cell SC cavity configurations • Orbit displacement causes beam loading in crabbing mode; adopt KEKB criterion of y = ±1 mm (for orbit distortions ± 0.1 mm) Superconducting Deflecting Cavity Design Parameters A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  10. Damping Parasitic Modes f < fc A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  11. Coaxial transmission lines Rejection filter not shown LOM Damping • Damping load is placed outside of cryomodule. • Ridge waveguide and coaxial transmission lines transport LOM / HOM to loads • Efficiency of deQing was simulated by creating the TM010 mode with an axial antenna. • Stability condition for LOM achieved when Q < 12,900 for 100 mA beam current. • Unloaded Q of LOM was 4.34e9. • Coaxial beam pipe damper with four coaxial transmission lines, damped the LOM to a loaded Q of 1130. Rejectionfilter Coaxialtransmissionline Excitationantenna A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  12. Deflecting mode filter Waveguide to damper load Single-Cell Deflecting Cavity: Rejection Filter • Deflecting mode creates surface currents along the coaxial beam pipe damper, but does not propagate power. • When a resistive element is added, there is substantial coupling of power into the damping material. • A radial deflecting mode filter rejects at ~ -10 dB. • Performance improvement pursued as well as physical size reduction. A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  13. Design A Configuration • Ten single-cell cavities with KEK-type coaxial beam pipe damper and rejection filter • Ion pump/valves/bellow assembly will need at least 0.4m on both sides of the cavity assembly. • The total space required by the following physical arrangement is ~ 2.6 m. • Beam impedance considerations may require different cavity configuration • Upstream/Downstream location of coaxial beam pipe damper may be significant • Downstream location may increase beam impedance excessively • Configuration change would require additional space Input Coupler Coaxial Damper Rejection Filter Coaxial Beam Pipe A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  14. Issues with KEK-Type Layout @ 2.8 GHz • Alignment of coaxial beam pipe dampers (CBD) will be difficult. • Thickness of (CBD) as modeled is 4mm which includes the cooling channel. Rigidity and mechanical stability and cooling capabilities are questionable • Rejection filter may be difficult to implement efficiently. • Results of stress analysis of cavity performed by KEK required stiffening of KEK cavity - tuning by deformation was abandoned. • CBD also functions as tuner in KEK design. This will require a separate adjustable CBD for each cavity. • CBD tuner will require more space and increase complexity • KEK locates CBD on the upstream side of the cavity due to possible impedance issues – will require more space. A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  15. Design B with Waveguide Dampers: Monopole Modes • Waveguide dampers are placed near cavity to intercept leakage fields of the LOM*+ • LOM couples to waveguide and is strongly damped Qext= 500. • Other monopole modes also couple to TE10 waveguide mode and are strongly damped. Power Flow and Efield vector plot of LOM * A. Nassiri, APS/ANL + D. Li, LBL A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  16. Design B with Waveguide Dampers: Dipole Modes • Coaxial input coupler considered to permit variable coupling. • Deflecting dipole mode couples to waveguide as TE20 mode and is rejected by > 30 dB in current configuration due to waveguide cutoff frequency. • “Degenerate” deflecting mode couples to TE10 waveguide mode and is strongly damped. • Asymmetric cavity may no longer be necessary depending on HOM spectrum. A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  17. Input Coupler Waveguide Damper Coaxial Damper Design B Configuration • Ten single-cell cavities with waveguide damper. • The total space required by ten single-cell cavities in the following physical arrangement is ~ 2.4 m assuming ion pump/valves/bellow assembly installed on both ends. • Additional dampers may be required based on full HOM analysis A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  18. R&D Plan • Feasibility study completed • SC rf technology chosen • Finalize RF system design, refine simulations • Observe assembly and testing of KEKB crab cavities in 2005, 2006 • Model impedance effects (parasitic modes, head-tail) • Conduct proof of principle tests (beam dynamics, x-ray optics) • Chirp beam using synchrobetatron coupling (transient) (W. Guo) • Install 1 MV RT S-band structure, quarter betatron tune (M. Borland, W. Guo, A. Nassiri) (AIP) • Install warm model of SC rf cavity (passive), parasitic mode damping (K. Harkay, A. Nassiri) (AIP) A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

  19. Summary • We believe x-ray pulse lengths ≤ 1 ps achievable at APS • SC RF chosen as baseline after study of technology options • Recent simulation results on LOM and HOM damping are encouraging. • Input coupler design is underway • Beam impedance calculation may have appreciable effect on final design • Proof of principle R&D is underway: beam/photon dynamics • Operational system possibly ≤ 4 yrs from project start A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

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