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Rf Deflecting Cavity Design for Generating Ultra-short pulses at APS. Geoff Waldschmidt Radio Frequency Group Accelerator Systems Division Advanced Photon Source LHC IR Workshop, October 3, 2005. Feasibility study group*. RF K. Harkay D. Horan R. Kustom A. Nassiri G. Pile

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rf deflecting cavity design for generating ultra short pulses at aps

Rf Deflecting Cavity Design for Generating Ultra-short pulses at APS

Geoff Waldschmidt

Radio Frequency Group

Accelerator Systems Division

Advanced Photon Source

LHC IR Workshop, October 3, 2005

feasibility study group
Feasibility study group*

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

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

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

outline
Outline
  • Science case for storage ring ps pulses
  • Summary of beam and optics simulations and beam studies
  • Deflecting cavity designs
    • Room temperature vs. superconducting
    • Multi-cell and single-cell superconducting cavity designs
  • R&D plan
  • Summary

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

science drivers for ps x rays
Science Drivers for ps X-rays

APS Strategic Planning Workshop (Aug 2004): Time Domain Science Using X-Ray Techniques

“…by far, the most excitingelement of the workshop was exploring the possibility of shorter timescales at the APS, i.e., the generation of 1 ps x-ray pulses whilst retaining high-flux. This important time domain from 1 ps to 100 ps will provide a unique bridge for hard x-ray science between capabilities at current storage rings and future x-ray FELs.”

Atomic and molecular dynamics, coherent/collective processes:

  • Atomic and molecular physics
  • Condensed matter physics
  • Biophysics/macromolecular crystallography
  • Chemistry

APS User’s Meeting: Workshop on Generation and Use of Short X-ray Pulses at APS (May 2005)

Slide Courtesy K. Harkay

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

slide5

Atomic

Ex. States

Proton

Transfer

Period of

Moon

Bound Water

Relaxation

Ex. State

Fe57

Cell

Division

Rotations

Bond

Breakage

DNA

Unfolding

Protein

Folding

Phonon

Frequency

Lasers

APS ps source

X-ray Techniques

Workshop on Time Domain Science Using X-ray Techniques

D. Reiss, U. Michigan

106

104

102

100

10-2

10-4

10-6

10-8

10-10

10-12

10-14

10-16

Ultrasonic

EPR

NMR

Storage Ring Sources

X-ray FELs

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

storage ring ps sources offer unique capability

XFELs can provide

  • fs pulses
  • Ultrahigh peak power
  • Ultrahigh brightness
  • Lower avg. repetition rate
  • Storage rings can provide
  • ~1 ps pulses
  • Energy tunability
  • Spectral stability
  • Flux comparable to 100 ps
  • High repetition rate
Storage Ring ps Sources Offer Unique Capability

Note: Femtoslicing not practical at APS (calc per A. Zholents)

Energy modulation DE=7se requires:

~10 mJ laser pulse at lL=400 nm and t=50 fs

giving ~ 100 fs x-ray pulse with ~ 105 photons per pulse

BUT: looks difficult at a high rep. rate

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

generation of ps pulses

Δt

Slitting alone

or add compression using asymmetric cut crystal; throughput ~ 2-15x better

Compressed pulse length (linear rf):

Generation of ps Pulses

Fig courtesy A. Nassiri

For APS: h=8, 6 MV deflect. voltage, σy’,e = 2.2 μrad, and σy’,rad = 5 μrad; the calc’d compressed x-ray pulse length is ~0.36 ps rms.

A. Nassiri

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

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

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

x ray compression optics simulation r dejus
X-ray Compression Optics Simulation(R. Dejus)

Pulse histogram using optics compression and a slit of 8.0 mm (half-width). The rms width is 1.19 ps (FWHM ~ 2.8 ps), and the transmission is 62%.

2D scatter plot, undulator A irradiance at 30 m, 10 keV. Red = 10% level, green = 37% (1/e).; ~ 20% are thrown away. The tilt angle β is ~ 55°.

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

ultrashort x ray pulse generation test 1
Ultrashort X-ray Pulse Generation Test 1

Transient, short pulses are generated using an alternate scheme based on synchrobetatron coupling (suitable for study, but not operation). At left, an image of the beam 0.3 ms after a vertical kick, using APS sector 35 optical streak camera. At right, a short pulse is observed using a 40 μm vertical slit (fit gives 5.8 ps rms, nominal bunch length 30 ps).

W. Guo, K. Harkay, B. Yang, M. Borland, V. Sajaev, Proc. 2005 PAC

Carry out a demonstration short-pulse x-ray experiment on APS beamline

Structural molecular reorganization following photoinduced isomerization/ dissociation can be studied on a finer timescale. Transient pulse requires acquisition of entire x-ray absorption spectrum in a single shot. (L. Young and colleagues)

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

room temperature rt vs superconducting sc rf k harkay a nassiri
Room Temperature (RT) vs. Superconducting (SC) RF(K. Harkay, A. Nassiri)
  • No installed “crab cavity” in existing synchrotron facilities
  • Need to study and understand transients during pulsing of RT structure and its effect on SR beam. How does it affect “non-crab” beamline users?
  • RT pulsed system allows user to “turn off” ps pulse via timing (~1 s pulse, 0.1 – 1 kHz rep rate) (M. Borland, P. Anfinrud)
  • KEKB plans to install two SC single cell cavity in March 2006
  • SC system runs CW, rep rate up to bunch spacing; some experiments desire high rep rate (no pump probes) (D. Reiss)
  • Either option requires dedicated R&D effort

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

slide12

9 Cells SW Deflecting Structure

V. Dolgashev, SLAC, APS seminar, June 2005

  • Pulsed heating < 100 deg. C
  • BMAX < 200 kA/m for 5 μs pulse (surface)
  • Limited available power ≤ 5 MW
  • EMAX < 100 MV/m (surface)

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

sc rf cavity study for aps g waldschmidt g pile d horan r kustom a nassiri k harkay
SC RF Cavity Study for APS (G. Waldschmidt, G. Pile, D. Horan, R. Kustom, A. Nassiri, K. Harkay)
  • 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

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

squashed rf crab cavity designs based on kek design

21.3 cm

Squashed RF Crab Cavity Designs (based on KEK design)

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

slide15

Dipole mode electric field

Coaxial beam pipe damper

Waveguide input coupler

Single-Cell Deflecting Cavity: Coaxial Beam Pipe Damper

  • Multiple cells produce multiplicity of parasitic modes. Single-cell cavity chosen due to stringent HOM requirements.
  • Coaxial beam pipe damper (CBD) is located on one side of the cavity and extracts LOM’s and HOM’s from cavity
  • LOM / HOM’s can couple to the coaxial beam pipe as a TEM mode. HOM’s can also couple as higher order coaxial modes
  • HOMs above beam pipe cutoff, propagate along CBD and / or through other beam pipe

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

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.

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

lom damping

Rejectionfilter

Coaxialtransmissionline

Coaxial transmission lines

Excitationantenna

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.

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

instability thresholds from parasitic mode excitation per y c chae
Instability Thresholds from Parasitic Mode Excitation(per Y-C. Chae)

APS parameters assumed: I= 100 mA, E = 7 GeV,a=2.8e-4, ws/2p=2 kHz,ns=0.0073,bx= 20 m

[1] A. Mosnier, Proc 1999 PAC.

[2] L. Palumbo, V.G. Vaccaro, M. Zobov, LNF-94/041 (P) (1994; also CERN 95-06, 331 (1995).

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

damping parasitic modes f fc
Damping Parasitic Modes f < fc

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

space requirements
Space Requirements
  • Total available space at the APS for deflecting cavity assembly is 2.5 m.
  • Assuming ten single-cell cavities, and 0.4 m on each end for an ion pump/valves/bellow assembly, the total space required by the following physical arrangement is ~ 2.6 m.
  • Additional dampers may also be required.
  • Coupling between cavities must be addressed
    • Impedance bump in coaxial beam pipe damper or shorting plane may be used
  • Beam impedance considerations may require different cavity configuration

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

summary of rf design considerations
Summary of RF Design Considerations
  • SC CW option is more attractive
    • May offer a greater degree of compatibility with normal SR operation
    • Compatible with future development of higher rep rate pump probe lasers
    • Opportunity for APS to gain SC rf expertise
  • Goal 1 ps in crab insertion
  • No impact of crab cavities on performance outside insertion
  • Availability of 100-kW class rf amplifiers limits study to h = 8 (2.8 GHz)
  • Available insertion length for cavities nominally 2.5 m (could be extended)
  • Effects of errors: emittance growth or orbit kicks (M. Borland)
    • Intercavity phase error < 0.04 for <y’>/σy’ < 10%
    • Intercavity voltage difference < 0.5%
  • Significant parasitic mode damping requirement (Y-C Chae)

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

sc rf issues to be resolved
SC RF Issues to be Resolved
  • Physically realizable coaxial beampipe damper and optimized filter.
  • Extraction of power from parasitic modes
    • Removal of heat load outside of cryo-module.
    • Quantify power handling requirements
    • Additional dampers and/or enlarged beam pipe on input coupler side
  • Examination of inter-cavity coupling
  • Compare coaxial and waveguide input couplers for ease of implementation and HOM damping.
  • Cavity optimization
  • Tuner design
  • Beam impedance calculation
  • Evaluation of microphonics
  • 2.5 m space limitation !!

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

r d draft plan
R&D Draft Plan
  • Feasibility study completed
  • SC rf technology chosen
  • Model impedance effects (parasitic modes, head-tail)
  • Finalize RF system design, refine simulations
  • System review of crab cavities at KEKB during assembly and testing
  • Refine x-ray compression optics design, end-to-end ray tracing
  • 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)
    • Install warm model of SC rf cavity (passive), parasitic mode damping (K. Harkay)

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005

summary
Summary
  • We believe x-ray pulse lengths ≤ 1 ps achievable at APS
  • SC RF chosen as baseline after study of technology options
  • RF priorities include parasitic mode damping and cavity assembly optimization with physical length constraints
  • Beam impedance calculation may have appreciable effect on final design
  • Optics design performed in parallel with RF. Optics compression increases throughput 2-15x better than slits alone.
  • Proof of principle R&D is underway: beam/photon dynamics
  • Operational system possibly ≤ 3 yrs from project start

G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005