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

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

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

4 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 Science Drivers for ps X-rays APS Strategic Planning Workshop (Aug 2004): Time Domain Science Using X-Ray Techniques “…by far, the most exciting element 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.” APS User’s Meeting: Workshop on Generation and Use of Short X-ray Pulses at APS (May 2005) Atomic and molecular dynamics, coherent/collective processes:  Atomic and molecular physics  Condensed matter physics  Biophysics/macromolecular crystallography  Chemistry Slide Courtesy K. Harkay

5 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/ Ultrasonic NMR Workshop on Time Domain Science Using X-ray Techniques Bound Water Relaxation Rotations Bond Breakage Phonon Frequency Ex. State Fe 57 DNA Unfolding Period of Moon Cell Division Protein Folding Proton Transfer Atomic Ex. States Lasers Storage Ring Sources X-ray Techniques EPR X-ray FELs D. Reiss, U. Michigan APS ps source

6 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 Storage Ring ps Sources Offer Unique Capability Storage rings can provide ~1 ps pulses Energy tunability Spectral stability Flux comparable to 100 ps High repetition rate XFELs can provide fs pulses Ultrahigh peak power Ultrahigh brightness Lower avg. repetition rate Note: Femtoslicing not practical at APS (calc per A. Zholents) Energy modulation  E=7  e requires: ~10 mJ laser pulse at L =400 nm and  =50 fs giving ~ 100 fs x-ray pulse with ~ 10 5 photons per pulse BUT: looks difficult at a high rep. rate

7 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 Generation of ps Pulses For APS: h=8, 6 MV deflect. voltage, σ y’,e = 2.2 μrad, and σ y’,rad = 5 μrad; the calc’d compressed x-ray pulse length is ~0.36 ps rms. Compressed pulse length (linear rf): or add compression using asymmetric cut crystal; throughput ~ 2-15x better Slitting alone Δt Fig courtesy A. Nassiri A. Nassiri

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

9 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 X-ray Compression Optics Simulation (R. Dejus) 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°. 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%.

10 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 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 PAC C arry 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)

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

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

13 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) 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 MW/m HOM: R s *f 100 mA< 0.8 MW - GHz Available length2.5 m Superconducting Deflecting Cavity Design Parameters

14 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 Squashed RF Crab Cavity Designs (based on KEK design) 1-cell4-cell Frequency (GHz)2.81 No. of cavities104 Cavity radius (cm)7.8 Iris radius (cm) Beam pipe rad (cm) / 1.8 Defl voltage (MV)66 Defl gradient (MV/m) QoQo 1 x 10 9 R T /Q (  m  50.6 * * 4 Active length (cm)5.3 * * 4 B MAX (mT)10090 P beam 1mm (kW) ~ 100 RF loss at 2K (W)7.1 * * cm

15 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 Coaxial beam pipe damper 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 Dipole mode electric field Waveguide input coupler

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

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

18 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 Instability Thresholds from Parasitic Mode Excitation (per Y-C. Chae) APS parameters assumed: I = 100 mA, E = 7 GeV,  =2.8e-4,  s / 2  =2 kHz, s =0.0073,  x = 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).

19 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 Damping Parasitic Modes f < fc

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

21 G. Waldschmidt, APS RF Deflecting Cavity Design at APS LHC, 10/3/2005 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 /σ y’ < 10% –Intercavity voltage difference < 0.5% Significant parasitic mode damping requirement (Y-C Chae)

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

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

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