1. Franck Peauger, Riccardo Zennaro
Status of Wakefield Monitors developments for CLIC accelerating structures
25 Sept. 2009
Franck Peauger, Riccardo Zennaro
Alexandre Samoshkin

2. Outline
Context and requirements
Time domain simulations of wakefields in AS
Design of WFM RF transition
Integration in the Two-Beam Test Stand
180° Hybrid coupler
Test set-up in CTF3

3. Context
Wakefield Monitors are Beam Position Monitors integrated to the drive beam Accelerating Structures (AS)
It allows beam-based alignment of AS to remove wakefield effects and emittance growth
Emittance growth is very well improved by aligning the AS to an RMS accuracy of 5 µm to the beam

4. Wakefield kicks from misaligned AS can be cancelled by another AS
Context
Girder
Movers
AS with WFM
Electron bunch
D. Schulte
Wakefield kicks from misaligned AS can be cancelled by another AS
One WFM per structure and mean offset of the 8 AS computed

5. WFM dev. plan & requirements
Step 1 ( ): build one WFM prototype and integrate it into a CERN structure and test on TBTS with CALIFES probe beam
Step 2 ( ): build 2 or 3 structures fully instrumented and test on TBTS + CALIFES

6. WFM additionnal requirements
Since there is no place available in the linac, the damping waveguides of the middle cell are used to measure the beam position inside the structure
we cannot define the geometry, frequency mode, Q factor … as we do for BPM
the WFM works necessarily with strong damped modes (Q ~ 10) !!
The WFM design consists in studying the HOM modes and their sensitivity to a beam offset, choosing an operating mode, designing an RF transition to couple this mode and processing the signal
We must attenuate the 12 GHz high power accelerating signal to -150 dB !!
The RF transition must not reflect any signal to the cell (typically -20 dB from 10 to 35 GHz)
The middle cell equipped with the WFM must keep its strong damping functionality (SiC loads)
The WFM must be cheap, easy to integrate mechanically in the Two Beam Module and avoid if possible any major additional machining of the cell

7. Wakefield simulation with beam offset
GDFIDL Simulations:
Five cells meshed (no symmetry) with a mesh step of 0.1 mm
volume limited to +/- 15 mm in the X and Y directions
PML set at the waveguide extremities (Xmin, Xmax, Ymin, Ymax, Zmin, Zmax)
Beam: 1 bunch of 0.6 nC, σz=3 mm with offset
Simulation stopped at 6.66 ns.
Rectangular ports at the end of the damped waveguides of the middle cell. The two first modes are selected in GdfidL :
ydamphaut
xdamphaut
xdampbas
ydampbas
1st port mode: Fc = 13.3 GHz
2st port mode: Fc = 21.4 GHz
Total of 90e6 meshs
Time computation of 9 to 13 hours with 36 hosts on LXCLIC cluster
E field
E field
TM like mode for the beam
TE like mode for the beam

8. 18 GHz = First dipole-band mode
TM like modes with beam offset of 1 mm Y+
Port signal amplitude (voltage)
X- & X+
Beam dx=1mm offset X- X+
Y+ & Y-
Time (s) Y-
Port signal amplitude (voltage)
Port signal amplitude (voltage)
11.94 GHz
18 GHz = First dipole-band mode
Time (s)
F (Hz)

9. monopole / dipole mode configuration
Monopole mode
Dipole mode
E Field
E Field
Opposite ports signals are in phase
Opposite ports signal have opposite phase
When we substract the opposite port signals, the monopole mode is cancelled and the dipole mode amplitude is increased

10. TM modes after 180° perfect recombination
Recombined port signal amplitude (voltage) Y+
Beam dx=1mm offset
DX=X+-X- X- X+
DY=Y+-Y- Y-
Time (s)
Recombined port signal amplitude
Recombined port signal amplitude (voltage)
18.19 GHz
14.81 GHz
11.95 GHz
F (GHz)
Time (s)

11. Variation of beam offset amplitude
Recombined port signal amplitude DX (voltage)
dx = 1 mm
dx = 0.8 mm
dx = 0.6 mm
dx = -1 mm
Time (s)

12. The 18.2 GHz mode works quite well as a cavity BPM mode !
Linearity
Max. amplitude of the delta signal (voltage)
Offset dx (mm)
The 18.2 GHz mode works quite well as a cavity BPM mode !
But we need to design an RF transition that couple this mode, attenuate 12 GHz mode, avoid reflection, allows strond damping, etc…

13. RF transition design
Long waveguide with cut-off above 12 GHz + 90° E-bend
Coax coupler designed to have -10 dB transmission
55 mm
Load location
15 mm
Cell radius of CLIC accelerating structure (D = 140 mm)
70 mm =

14. Geometry of WFM RF transition
Coaxial waveguide (K type)
Damped waveguide F1
Antenna F2 p
Load R2 R1 d b L2 a
Cell L1
Cell axis

15. RF transition transfer function T(f)
-11 dB 1 2 3
S11
T (f) (S parameters)
S13
-145 dB
S12
12 GHz
18 GHz

16. ~ 1 – 3 GHz band pass filter around 18.1 GHz
TM modes after RF Transition and 180° perfect recombination
with dx=1mm beam offset
Recombined port signal amplitude (voltage) Y+ U0 U1 X-
DX=X+-X- X+
DY=Y+-Y- Y-
Time (s)
Recombined port signal amplitude
18.14 GHz
13.88 GHz
Given by HFSS
Given by Gdfidl
~ 1 – 3 GHz band pass filter around 18.1 GHz
F (GHz)

17. Resolution Dipole mode 18 GHz: Accelerating mode 12 GHz:
The power and voltage are linked by:
Dipole mode 18 GHz:
Simulation results (at the coax pick-up, after the RF transition)
dx=1mm, q=0.6nC → U1 ≈ 1 V
Extrapolation for the commissioning case (voltage varies linearly with offset and charge)
dx=5µm, q=0.06nC, (s=70µm) → U1 = 500 µV
Extrapolation for the nominal case
dx=5µm, q=0.6nC, (s=60µm) → U1 = 5 mV
Accelerating mode 12 GHz:
For 60 MW input power, there is 3 nW at the coax pick-up, after the RF transition
→ equivalent to UHP = 56 µV
+ Thermal noise and noise from signal processing to be evaluated

18. Integration in the Two Beam Test Stand
Design a support and ensure good electrical contact between the WFM and the structure
Must make a hole in U support for the “Y- waveguide”

19. Integration in the Two Beam Test StandOr
We would like to reserve one available flange (150 mm diameter) for a special CF flange with four feedthroughs

20. 180° Hybrid coupler Tapered Coupled Line Hybrid Magic Tees
Hybrid couplers are the special case of a four-port directional coupler that is designed for a 3dB (equal) power split and a 180 degree phase shift between two output ports
Ring Hybrid Junction
Tapered Coupled Line Hybrid
Magic Tees
When using as a combiner, input signals are applied at port 2 and 3, the sum of the inputs will be formed at port 1 while the difference at port 4.
In theory: S12 = S13 = -3dB and S11 = S14 = -40 dB

21. 180° Hybrid coupler Two tapered-line directional couplers cascaded
Excellent phase and amplitude matching
Realized with a three-layer stripline configuration
Etched on opposite sides of a thin coupler circuit board, sandwiched between a pair of equal thickness Duroid boards

22. Layout of WFM prototype test with Hybrid
: WFM – transition, qty 4, output connector = K female, UHV compatible
: semi rigide cable, Type?, qty 4, length ≈ 400 mm , input connector = K male, output connector = K female, UHV compatible
: CF flange feedthrough, qty 1, 4 connectors, input connector = K male (vacuum side), output connector = K female, UHV compatible
: Flexible cable, qty 4, length ≈ 30 m , input connector = K male, output connector = K female,
: 180° Hybrid coupler, input connector = K male, output connector = K female
TBTS rack
Scope
Diode detector
Bandpass filter
≈ 30 m
Klystron gallery
Front
Back
Accelerating structure
TBTS Accelerating structure tank
CLEX – beam tunnel

23. Conclusion (1/2)
The WFM Context, specifications and development program well defined
with the objective to demonstrate the WFM concept before end of 2010
We have developed a methodology to run Gdfidl from CEA Saclay on the CERN Cluster, and couple the results to HFSS simulations
The TM like mode at 18.2 GHz has been identified and well studied in time domain. It will be used as classical cavity BPM dipole mode with 180° recombination
We proposed a simple design of the WFM RF transition which should meet the long list of requirements
We would like to investigate TE like modes around 23 GHz. Additional simulations are required
But we also would like to freeze the design of the WFM prototype soon in order to start the mechanical study and procurement of components
We should not forget to work on the electronic for signal processing

24. Thank you for your attention
Conclusion (2/2)
Important topic for CLIC
Very interesting R&D program
Good continuation after CALIFES for CEA Saclay
Thank you for your attention

25. Extra - slides

26. Wakefield simulation with one symetry
GDFIDL Simulations:
Five cells meshed with one symetry (half of the structure is meshed)
Perfect magnetic boundary condition on XZ plane
mesh step of 0.05 mm
PML set at the waveguide extremities (Xmin, Xmax, Ymax, Zmin, Zmax)
Beam: 1 bunch of 0.6 nC, σz=1 mm, offset Δx = 1 mm
Simulation stopped at 6.66 ns.
Rectangular ports at the end of the damped waveguides of the middle cell. The first modes is selected in GdfidL so that longitudinal (TM) modes can be recorded
Total of 156.8e6 meshs
14 hours with 36 hosts machines lxclic

27. TM modes responses
Beam 1mm offset Y+ X- X+
11.83 GHz
15.15 GHz

28. TM modes after 180° perfect recombinationY+
DX=X+-X- X- X+
11.98 GHz
15.19 GHz
18.47 GHz

29. RF cable under vacuum

30. CF Flange feedthroughs
CF Flange: Dext152mm x2
Dimensions (inches)
A=0.63
B= 0.92
C=2.75 (69.85mm)
D= 1.38
E=0.87
Flange Size=2.75"CF
Number of feedthrough = 4
Grounded 50-Ohm
Ref. = IFDCG042013

31. Two Beam Module integration
Vac. Manifold: 30 x 30 mmm²