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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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Presentation Transcript
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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

acknowledgements
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

feasibility study group
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

outline
Outline
  • Introduction
  • SC vs. RT option
  • Crab cavity modeling
  • Summary

A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

parameters constraints what hv is required
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

slide6

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

rt vs sc rf
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

slide8

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

sc rf cavity study for aps
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

damping parasitic modes f fc
Damping Parasitic Modes f < fc

A. Nassiri, G. Waldschmidt APSINFN – LNF 8 November2005

lom damping

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

single cell deflecting cavity rejection filter

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

design a configuration
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

issues with kek type layout @ 2 8 ghz
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

design b with waveguide dampers monopole modes
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

design b with waveguide dampers dipole modes
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

design b configuration

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

r d plan
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

summary
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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