Argonne National Laboratory is managed by The University of Chicago for the U.S. Department of Energy 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

ICFA – FSBSR05 2 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 Acknowledgements Special thanks to Kenji Hosoyama (KEK), Derun Li and J. Shi ( LBNL), and Tim Koeth (Fermilab) for many productive and useful discussions.

ICFA – FSBSR05 3 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 Feasibility study group* Beam dynamics M. Borland Y.-C. Chae L. Emery W. Guo K.-J. Kim S. Milton V. Sajaev B. Yang A. Zholents, LBNL RF K. Harkay D. Horan R. Kustom A. Nassiri G. Pile G. Waldschmidt M. White Undulator radiation & x-ray optics L. Assoufid R. Dejus D. Mills S. Shastri * All affiliated with APS except where noted

ICFA – FSBSR05 4 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 Outline Introduction SC vs. RT option Crab cavity modeling Summary

ICFA – FSBSR05 5 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 Can get the same compression as long as h*V is constant Higher h and lower V: smaller maximum deflection and less lifetime impact Higher V and lower h: more linear chirp and less need for slits Higher h and maximum V: shortest pulse, acceptable lifetime Parameters / Constraints: What hV is Required? M. Borland, APS ps Workshop, May 2005 Cavity design and rf source issues h=7, V<6 MV? V=4, h=6 V=6, h=4 V=6, h=8 Beam dynamics simulation study: h ≥ 4 (1.4 GHz) V ≤ 6 MV (lifetime)

ICFA – FSBSR05 6 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 Parasitic modes (squashed geometry) Vertical crabbing mode (APS): horiz axis “squashed” Maximize mode separation for optimized damping HOMs above beam pipe cutoff, propogate out Lower-order mode (TM 010 ) 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 frequency TM010 Accelerating mode TM110v APS crabbing mode TM110h/TE111h TM011 TE111v

ICFA – FSBSR05 7 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 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

ICFA – FSBSR05 8 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November Cells SW Deflecting Structure V. Dolgashev, SLAC, APS seminar, June 2005  Limited available power ≤ 5 MW  E MAX < 100 MV/m (surface)  Pulsed heating < 100 deg. C  B MAX < 200 kA/m for 5 μs pulse (surface)

ICFA – FSBSR05 9 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 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) Frequency2.81 GHz Deflecting Voltage6 MV QoQo 1 x 10 9 B MAX < 100 mT RF loss at 2 K< 100 W HOM: R 100 mA< 2.5 MV/m HOM: R s *f 100 mA < 0.8 M  - GHz Available length2.5 m Superconducting Deflecting Cavity Design Parameters

ICFA – FSBSR05 10 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 Damping Parasitic Modes f < fc

ICFA – FSBSR05 11 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 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 TM 010 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 Rejection filter not shown Coaxial transmission lines Excitation antenna Rejection filter Coaxial transmission line

ICFA – FSBSR05 12 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 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. Deflecting mode filter Waveguide to damper load

ICFA – FSBSR05 13 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 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 Rejection Filter Coaxial Beam Pipe Coaxial Damper

ICFA – FSBSR05 14 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 Issues with KEK-Type 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.

ICFA – FSBSR05 15 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 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 Q ext = 500. Other monopole modes also couple to TE 10 waveguide mode and are strongly damped. Power Flow and Efield vector plot of LOM + D. Li, LBL * A. Nassiri, APS/ANL

ICFA – FSBSR05 16 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 Design B with Waveguide Dampers: Dipole Modes Coaxial input coupler considered to permit variable coupling. Deflecting dipole mode couples to waveguide as TE 20 mode and is rejected by > 30 dB in current configuration due to waveguide cutoff frequency. “Degenerate” deflecting mode couples to TE 10 waveguide mode and is strongly damped. Asymmetric cavity may no longer be necessary depending on HOM spectrum.

ICFA – FSBSR05 17 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 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 Input Coupler Waveguide Damper Coaxial Damper

ICFA – FSBSR05 18 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 R&D Plan 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) Feasibility study completed SC rf technology chosen

ICFA – FSBSR05 19 A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005 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