# Presentation: 363807

## 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*

- 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

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## Workshop on Time Domain Science Using X-ray Techniques

**10****-16**

**10****-14**

**10****-12**

**10****-10**

**10****-8**

**10****-6**

**10****-4**

**10****-2**

**10****0**

**10****2**

**10****4**

**10****6**

**Ultrasonic**

**NMR**

***Workshop on Time Domain Science Using X-ray Techniques***

***Storage Ring Sources***

**EPR**

***X-ray FELs***

- D. Reiss, U. Michigan

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## Storage Ring ps Sources Offer Unique Capability

**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 ~ 105 photons per pulse
- BUT: looks difficult at a high rep. rate

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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.**** **

- Fig courtesy A. Nassiri

- A. Nassiri

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## Parameters / Constraints: What  hV  is Required? 

- 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)

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## X-ray Compression Optics Simulation(R. Dejus)

***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%.

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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. 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)

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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)

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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)

| 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 M/m |
| HOM: Rs*fp @ 100 mA | < 0.8 M- GHz |
| Available length | 2.5 m |

- Superconducting Deflecting Cavity Design Parameters

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## Squashed RF Crab Cavity Designs (based on KEK design)

|  | 1-cell | 4-cell |
| --- | --- | --- |
| Frequency (GHz) | 2.81 | 2.81 |
| No. of cavities | 10 | 4 |
| Cavity radius (cm) | 7.8 | 7.8  |
| Iris radius (cm) | --- | 1.8 |
| Beam pipe rad (cm) | 1.9  | 2.1 / 1.8 |
| Defl voltage (MV) | 6 | 6 |
| Defl gradient (MV/m) | 11.3 | 7.0 |
| Qo | 1 x 109 | 1 x 109 |
| RT/Q (m | 50.6 * 10 | 220 * 4 |
| Active length (cm) | 5.3 * 10 | 21.3 * 4 |
| BMAX (mT) | 100 | 90 |
| Pbeam (300mA)               @ 1mm (kW) | ~ 100 | ~ 100 |
| RF loss at 2K (W) | 7.1 * 10 | 10.2 * 4 |

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## Single-Cell Deflecting Cavity: 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

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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.

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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 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.

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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, =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).

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## Damping Parasitic Modes f < fc

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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 < 0.04 for <y’>/σy’ < 10%
- Intercavity voltage difference < 0.5%
- Significant parasitic mode damping requirement (Y-C Chae)

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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 !!

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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)

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005

## 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

- K. Harkay          RF and X-ray optics for generating ultra short pulses at APS         9/16/2005