Ali Nassiri and Geoff Waldschmidt Accelerator System Division Advanced Photon Source

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

2A. Nassiri, G. Waldschmidt APS INFN – LNF 8 November 2005ICFA – FSBSR05

Acknowledgements

Special thanks to Kenji Hosoyama (KEK), Derun Li and J. Shi ( LBNL), and Tim Koeth (Fermilab) for many productive and useful discussions.


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


Outline

Introduction SC vs. RT option Crab cavity modeling Summary


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)


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 (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

frequency

TM010

Accelerating mode

TM110v

APS crabbing mode

TM110h/TE111h

TM011

TE111v


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


9 Cells SW Deflecting Structure

V. Dolgashev, SLAC, APS seminar, June 2005

Limited available power ≤ 5 MW

EMAX < 100 MV/m (surface)

Pulsed heating < 100 deg. C

BMAX < 200 kA/m for 5 μs pulse

(surface)


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)

Frequency 2.81 GHz

Deflecting Voltage 6 MV

Qo 1 x 109

BMAX < 100 mT

RF loss at 2 K < 100 W

HOM: Rt @ 100 mA < 2.5 MV/m

HOM: Rs*fp @ 100 mA < 0.8 M- GHz

Available length 2.5 m

Superconducting Deflecting Cavity Design Parameters


Damping Parasitic Modes f < fc

HOM Q Values

1.E+00

1.E+04

1.E+08

1.E+12

2.0 2.5 3.0 3.5 4.0 4.5 5.0

Frequency (GHz)

Q

Unloaded Q

Max Stable Q


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.

Rejection filter not shown

Coaxial transmission

lines

Excitation antenna

Rejection filterCoaxial

transmission line


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


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


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.


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

+ D. Li, LBL

* A. Nassiri, APS/ANL


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.


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


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


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

Documents

Ali Nassiri and Geoff Waldschmidt Accelerator System Division Advanced Photon Source