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Ali Nassiri and Geoff Waldschmidt Accelerator System Division Advanced Photon Source

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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

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

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

- Introduction
- SC vs. RT option
- Crab cavity modeling
- Summary

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

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

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

- 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

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

- 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

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

Coaxial transmission lines

Rejection filter not shown

- 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

Deflecting mode filter

Waveguide to damper load

- 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

- 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

- 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

- 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

- 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

Input Coupler

Waveguide Damper

Coaxial Damper

- 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

- 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

- 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