1. Update on the impedance studies of the SPS wirescanners
Mauro Migliorati, Benoit Salvant and Carlo Zannini for the impedance team
Many thanks to Hannes Bartosik, Nicolas Critin, Heiko Damerau, Bernd Dehning, Jonathan Emery, Federico Roncarolo, Ray Veness, and Christophe Vuitton

2. Still work in progress, and a lot more studies are needed
Many thanks for the very useful comments and suggestions at and following the previous MSWG and LIU-SPS meetings.

3. Agenda Objectives New design Current design Previous measurements
Summary

4. Objectives
Need to find solutions for mechanical design of the new prototype
Significant impedance issues found with the proposed design
However current design is not transparent either…
Which is best?
Wire usually breaks during measurement
Not so critical in parking position?
simulation also of the wire when it gets closer to the beam
Does it make sense to try to protect the wire in parking position?
Several options were on the table after the last meeting
Ferrites as usual
All mode damper (Fritz)
Change the inner pipe to two plates (Heiko and Hannes after MSWG)
Remove completely the inner pipe (Ray and Bernd)

5. Agenda Objectives New design Current design Previous measurements
Summary

6. New model for SPS
CATIA model from Nicolas Critin
Model heavily simplified to be able to mesh the geometry
Wire and gaps very difficult to mesh
Same as LHC but bigger aperture
SPS geometry + Use of more realistic 316 LN conductivity (1.35 MS/m) instead of Steel clarification of Linac/Circuit convention for both wakefield and eigenmode solver
First resonant mode now at Rs=40 kLinacOhm (20 kOhm) in parking position with around f=160 MHz (Q=650) without ferrites.
Still significant power loss and impedance contributions in parking position. What about when the wire is scanning the beam?

7. 120 degrees (parked OUT)
60 degrees (crossing beam)
0 degrees (parked OUT)
When the wire is moving, the
shunt impedances are in the
15 to 20 kOhm range.
 >1kW of dissipated power
Once the wire is in the pipe,
strong fields are close to the
wire. Is the wire really protected
Shunt impedance in LinacOhm
160 MHz
~400 MHz
900 MHz

8. New wire scanner
Dangerous mode below 200 MHz due to the cut in the pipe.
The mode strongly depends on the geometries around the pipe (high sensitivity of the impedance versus small geometrical details)
The mode is weakly dependent from wire and fork materials
Possible solutions
damping of the mode using ferrites  need input from BI to know what is feasible (first proposals rejected)
Metal Rods ending on feedthrough (proposed by Fritz: to be simulated, but difficult also with the constraints of wire scanner)
Removing the inner pipe?

9. Removing the inner pipe
120 degrees (parked OUT)
60 degrees (crossing beam)
0 degrees (parked OUT)
Factor 2 increase in shunt impedance
Strong correlation with angle
All fields located close to the wire
300 MHz
650 MHz

10. Agenda Objectives New design Current design Previous measurements
Summary

11. Current wire scanner More realistic current design:
H fork has smaller width
Should implement taper
Ferrites (random location)

12. Present wire scanner: longitudinal impedance
The longitudinal impedance significantly depends on the wire material. Simulation model account for a carbon wire (with steel wire, the shunt impedance is much larger). There are no ferrites.
Measurements
F. Roncarolo et al. 2003
Measurement and simulations show a good agreement in the spectral behaviour: strong modes around 300 MHz and 700 MHz.

13. Beam induced heating on the wire worst case scenario
angle=0 (close to parking)
Mode number
f [MHz] Q
R [Ω]
Total losses [W]
Losses on the wire [W]
Mode 1
284
670
3600
224 44
Mode 7
658
2000
8500
196 52
Mode 9
681
1750
22072
433
160
25 ns beam
p = 450 GeV
288 bunches
N=1.4e11 ppb
not excited at injection
angle=40 (in beam)
Mode number
f [MHz] Q
R [Ω]
Total losses [W]
Losses on the wire [W]
Mode 1
284
1000
6000
373 73
Mode 2
290
7200
444 81
Mode 7
657
2580
25280
583
155
Mode 10
693
2510
23187
454
172
25 ns beam
p = 450 GeV
288 bunches
N=1.4e11 ppb
not excited at injection
 Potentially large losses on the wire
A wire breaking occurs with these beam during measurements
Strongest heating from mode at ~700 MHz (not excited at injection)

14. Adding “random” ferrites (still Steel wire and no taper)

15. With injection beam (Gaussian)
With top energy beam (Gaussian)
Modes at 700 MHz heavily damped, but not so much the first mode at 300 MHz

16. Agenda Objectives New design Current design Previous measurements
Summary

17. Observations from 2003 F. Roncarolo et al. DIPAC 2003
Wire heating stronger with shorter bunch length (no ferrite)
Indicates that the 2nd mode at 700 MHz is probably very important for the heating

18. Wire diagnostics (Jonathan Emery)
Maximum of the current is later: transient of the dissipation or real? In simulations, larger current inside the wire when the beam is passing through the fork

19. Current inside the wire
We do not use this data usually, not sure we can trust it, requires simple benchmarks
Seems to be strongly affected by mesh issues (PEC mesh cells)
Due to oscillating Wake fields
Beam outside of the fork loop
Current in the wire (for bunch of 1nc)
Beam inside the fork loop
Time in ns
Could this explain that there is more heating after the beam is passed?
Needs to check if there is voltage data with an IN/OUT scan with beam at injection

20. Agenda Objectives New design Current design Previous measurements
Summary

21. Where are we? Models and assumptions more refined
Need to understand clearly the constraints of the new design
Try to see what solution can be implemented:
Current ideas: dielectric fork (as BSRT), all mode coupler (better than the wire…), ferrite at appropriate location.
Dissipated power predicted in the kW range if no solution is applied
Heating of the wire for the current scanner increases with
Smaller bunch length
Approaching wire
Beam inside the fork?
Need to implement the taper
Bench measurements on the current and prototype scanners with moving fork would help as the meshes are very unstable with thin wire and small gaps.

23. Current SPS model

24. Remaining issues
Can we trust the simulated current curves?

25. New Model (SPS) CATIA model from Nicolas Critin
Model heavily simplified to be able to mesh the geometry
Only one longitudinal gap (2.3 mm)
Wire and gaps very difficult to mesh

26. Solutions with ferrite?
With ferrite in green
With less ferrite in red
Re(Zlong) in Ohm
Re(Zlong) in Ohm
Frequency (GHz)
Frequency (GHz)
To be checked if these solutions can be implemented
To be compared with the current wire scanner design

27. Proposal of Heiko and Hannes
Frequency in GHz
Longitudinal impedance in Ohm
 Better than the initial solution, still worse than the current design
 However, solution not feasible for BI (“the fork should rotate completely around the axis”)

28. Agenda New design Current design Power loss Mode geometry
Previous measurements
Comparison with wall current monitor
Summary

29. Current design (also simplified)
Wakefield simulations
Very high Q resonances, solver
Resonant frequencies are much lower for the new design

30. Subtleties of the current design
Taper in 416 H
vertical fork much wider than horizontal fork
 resonance frequency change
416H Wire out
416 Wire in

31. Agenda New design Current design Power loss Mode geometry
Previous measurements
Comparison with wall current monitor
Summary

32. Comparison for the resonant modes (no ferrites)
Frequency
Shunt impedance (kOhm) Q
Fields close to the wire?
Power loss at 450 GeV
(from LHC spectrum)
140 MHz
250
2500 No
60 kW
470 MHz 15
3500
Yes (partly H)
0.3 kW
New design
Frequency
Shunt impedance (kOhm) Q
Fields close to the wire?
Power loss at 450 GeV
(from LHC spectrum)
290 MHz
18.5
1660
Yes
1.3 kW
658 MHz 55
7000
0.8 kW
682 MHz
146
5500
2.4 kW
1020 MHz
210
12000
0.5 kW
Current design
 Hopefully there are ferrites to partially damp these modes!
 Need to do that also for the SPS spectrum at injection energy?

33. Spectrum at injection From C. Zannini72 8 72 8 72 8 72
25ns buckets
First lobe of the beam spectrum after around 700 MHz

34. Comparison for the resonant modes (no ferrites) modes also significant at injection?
Frequency
Shunt impedance (kOhm) Q
Fields close to the wire?
Power loss at 450 GeV
(from LHC spectrum)
140 MHz
250
2500 No
60 kW
470 MHz 15
3500
Yes (partly H)
0.3 kW
New design
Frequency
Shunt impedance (kOhm) Q
Fields close to the wire?
Power loss at 450 GeV
(from LHC spectrum)
290 MHz
18.5
1660
Yes
1.3 kW
658 MHz 55
7000
0.8 kW
682 MHz
146
5500
2.4 kW
1020 MHz
210
12000
0.5 kW
Current design
 Hopefully there are ferrites to partially damp these modes!

35. Simulated power loss with 50 ns SPS beam with ferrite as a function of bunch length
Carlo Zannini
Still between 50 and 100 W in the ferrite
Is that ok with the duty factor of LHC beams in the SPS?
Would cooling be needed and is it possible to cool there?

36. Power loss (with ferrites)
HL-LHC
New design
modified design (Heiko)
Current design
Predicted for post LS1

37. Agenda New design Current design Power loss Mode geometry
Previous measurements
Comparison with wall current monitor
Summary

38. New design
472MHz
138 MHz
H field
H field
E field
E field

39. Old design 293 MHz 658 MHz 681 MHz 1018 MHz
All fields for all modes are located around the wire forks
Ferrite location not ideal.

40. Agenda New design Current design Power loss Mode geometry
Previous measurements
Comparison with wall current monitor
Summary

41. Previous measurements by Federico et al in 2003
Cavity Mode Related Wire Breaking of the SPS Wire Scanners and Loss Measurements of Wire Materials" F. Caspers, B. Dehning, E. Jensen, J. Koopman, J.F. Malo, F. Roncarolo - Proc DIPAC 2003
Simulation results (from eigenmode, 2013)
Frequency
Of main modes
Shunt impedance (kOhm) Q
290 MHz
18.5
1660
658 MHz 55
7000
682 MHz
146
5500
1020 MHz
210
12000
 Modes around 700 MHz
To be checked with the actual location of shielding and ferrite
To be checked with simulations of transmission measurements

42. Agenda New design Current design Power loss Mode geometry
Previous measurements
Comparison with wall current monitor
Summary

43. Example of Wall current monitor (J
Example of Wall current monitor (J. Belleman - BI) impedance reduction of the pipe interruption with coaxial line
 This reduction of the impedance of the gap does not seem possible with the wire scanner

44. Agenda New design Current design Power loss Mode geometry
Previous measurements
Comparison with wall current monitor
Summary

45. Outlook
New design of wire scanner seems very unfavourable from the impedance point of view due to the abrupt unmatched beam pipe interruption
Interesting solution proposed by Heiko and Hannes after MSWG helps reducing impedance, but not feasible for BI.
Putting ferrites reduces efficiently the heat load, but:
Is it possible to put ferrites there?
If needed, is it possible to cool the ferrites?
Work still on-going to:
Find acceptable solutions
Compare with previous measurements of Federico
See the impact of the position of the wire in the tank