1. A Multi Purpose X-band structure for FELs
M. Dehler, A. Citterio, A. Falone, J.-Y. Raguin
Paul Scherrer Institut
A. Grudiev, G. Riddone, R. Zennaro, D. Gudkov, A. Samoshkin, W. Wuensch
CERN
G. D‘Auria
Sincrotrone Trieste

2. Why - 250 MeV Injector at PSI
Introduction of X band technology at PSI - the shopping list:
Klystron at European X Band (development by SLAC for CERN/PSI/ELETTRA)
Accelerating structure described in the following
X Band component – loads, splitters etc (developments by I. Syratchev)
joined in (similar needs for their project).

3. A CERN/PSI/ST collaboration
Motivation for CLIC:
Another data point in high gradient test program
Validation of design and fabrication procedures
A true long term test in another accelerator facility
Motivation for the FEL projects:
An X band structure to compensate long. phase space nonlinearities
High gradient/power requirements of CLIC = a design for safe operation at the more relaxed parameters of the PSI X-FEL
RF design (mostly) by PSI, fabrication & LL RF test (mostly) at CERN, mechanical support & other parts by FERMI

4. Special considerations for FEL
Operating structure at relatively low beam energies (PSI injector: 250 MeV)
High sensitivity to transverse wakefields!
Strategy:
Passive: Try to have open structure while maintaining good efficiency and breakdown resilience
Active: Wake field monitors
See offsets before they show up as emittance dilution
Possibly measure higher order/internal misalignments (tilts, bends ….)

5. A priori specifications
Beam voltage 30 MeV at a max. power of 45 MW
Mechanical length <1017 mm
Iris diameter > 9 mm
Wake field monitors
Operating temperature 40 deg. C
Constant gradient design, no HOM damping
Fill time < 1 usec
Cooling assuming 1 usec/100 Hz RF pulse

6. Do a castrated NLC type H75 without damping manifolds!
The strategy
Use 5π/6 phase advance:
Longer cells: smaller transverse wake
Intrinsically lower group velocity: Good gradient even for open design with large iris
Needs better mechanical precision
Long constant gradient design (efficiency!)
No HOM damping
Wake field monitors to insure optimum structure alignment
Do a castrated NLC type H75 without damping manifolds!

7. NLC type H75
Well optimized design (iris aperture, thickness and ellipticity varying along structure)
Original design gives 65 MV/m for 80 MW input power
Sucessfully tested up to 100 MV/m with SLAC mode launcher (below)
|E| r z
|B| r z

8. Constant gradient design
72 cells, active length 750 mm
Relatively open structure: mean aperture 9.1 mm
Average gradient 40 MV/m (30 MeV voltage) with 29 MW input power
Group velocity variation: %
Fill time: 100 nsec
Average Q: 7150

9. Lower dipole band versus cell number
Measuring the alignment via dipole wakes
Lower dipole band versus cell number
May be damped by output coupler, strong coupling to beam
Trapped modes
Strong coupling to beam
Modes may couple to input coupler
Weak kick (nowhere coupling to fsync )
From distribution, we see distinct frequency bands

10. Cell 36 as upstream monitor
See contributions from the first half of the structure in the band
GHz

11. Cell 63 as downstream monitor
Restricted by bandwidth of dipole band:
Contributions from cells 40-63
Signal bandwidth
GHz

12. HOM coupler a la NLC DDS
Electric short on one side
TE type coupling minimizes spurious signals from fundamental mode and longitudinal wakes
Need only small coupling (Qext<1000) for sufficient signal
Minor loss in fundamental per- formance: 10% in Q, <2% in R/Q
Output wave guides with coaxial transition connecting to measurement electronics
Axial signal output wave guides

13. Modelling transverse wakes via Bane-Gluckstern equivalent circuit model
Quasi physical model of hybrid two mutually coupled chain of resonators describing TE and TM components.
Parameters derived from numerically calculated dispersion curves and kick factors
Advantages:
Modelling lowest two dipole bands
Very good fit of dispersion relations
Physically reasonable variation of kick factor with phase.
K.L.F. Bane, R.L. Gluckstern, The transverse wakefield of detuned X-band accelerator structure, SLAC-PUB-5783, 1992
Extension to include damping manifold:
R.M. Jones et al., PRST-AB 9:102001

14. Transverse wake spectrum

15. Output signal spectra

16. Alternative time domain measurement?
An approximate idea about time domain behavior (looking only at cell 45)
As the beam passes cell 45, it will excite a backward wave at the synchronous frequency
The wave will travel back to where this frequency has a phase advance of pi, where it is reflected
From there it advances forward until it reaches the point with phase advance zero (cell 63..)
Wake signals from different cells (with their respective synchronous frequency) will arrive with a different delay at the coupler …
Alternative time domain measurement?

17. Signal envelopes of wake monitors
Signal at time t is correlated with frequency – is correlated with cell number…..
Can we learn something about internal misalignments?

18. Beam axis
Structure tilt
Tilted
Ref. - offset

19. What limits the resolution?
Dilution by
Noise in RF front – only an issue for low bunch charges
Spurious signal from fundamental, longitudinal wakes - negligible due to TE coupling, waveguide length plus (if still necessary) additional filtering
Random misalignment of individual cells:

20. Comparing random misalignment systematic offset
Comparing both signals gives a estimate for the resolution being double the random cell to cell misalignment
Syst. offset

21. Validation results with full structure simulation using the ACE3P codes from SLAC
Computing full structure using ACE3P codes on NERSC super computer Franklin (Cray XT4)
Goals:
Validate electrical/mechanical design
Get refined information about longitudinal/transverse wakes
See influence of mechanical tolerances on performance

22. The accelerating mode |E| of 5 π/6 mode Below: monitor at cell 36
66 cell substructure:
Omit power couplers, matching cells
500’000 elements, 10’000’000 unknowns (3rd order approach required)
Computed resonance frequency:
F = GHz (w/o losses)
~ F= GHz (including losses)
Design: F= GHz
|E| of 5 π/6 mode
Below: monitor at cell 36

23. Ez on axis Some reflections due to monitors Monitor locations
|E| [a.u.]
Some reflections due to monitors
z [mm]

24. Phase error of accelerating gradient
Monitor locations
Δφ [deg]
z [mm]
Only 25 deg phase error!
Excellent!!

25. Driven problem in s3p Include matching cells and power couplers
Include conduction losses
Using 2nd order elements (somewhat higher numerical error)

26. Ez on axis

27. Phase error

28. Acceleration at nominal power (29 MW)
V [MeV]
F [GHz]

29. Reflection at input port
S11 [dB]
F [GHz]

30. Transverse wakes from eigenmodes
72 cell model (omitting power couplers)
2nd order elements
Comparison to equivalent circuit distorted by
Matching cells
Waveguide stubs

31. Transverse spectrum
Im(Zt) [kOhm/mm/m]
F [GHz]

32. Comparing both to TD wake calculation
WT [V/pC/mm/m]
TD wake uses full structure (including power couplers) and include damping effects of all waveguide ports!!
s [m]

33. Mechanical model
Each two structures for structures for PSI (SwissFEL) and ST (Sincrotone Trieste) with wakefield monitors under fabrication
Wakefield monitor details 33

34. Prototype disks from VDL ETG – flatness profile
Done by Didier GLAUDE 34

35. Short test stack done with diffusion bonding
Bonding at 1040°C for 90 minutes under H2
Metallurgical polishing + etching 75 s in Ammonium peroxodisulfate (NH4)2S2O8 35

36. Site of Interest 1: Outer side of disc stack
Done by Markus AICHLER
21/07/2010
Site of Interest 1: Outer side of disc stack
Grains grew down across the joining plane
Joining plane 36

37. Summary
Design for joint CERN/PSI/FERMI X band structure
Special challenge transverse wake fields
5π/6 design: simultaneously open and efficient
Wake field monitors
Separate decoupled signals for up and downstream half
Additional information from time domain envelopes of output signals
Resolution of 10 μm determined by internal structure alignment
Validating of circuit simulations/full structure simulations using SLAC codes S3P/Tau3P
Successful metrology and bonding tests with prototype disks at CERN
Delivery of disks for first two structures expected until the end of the year.
☺ Big thanks to ACE3P group at SLAC for access to codes, lots of support, specially A. Candel, R. Lee, Z. Li ☺