0. Cavity BPM Pickups for SwissFEL
Paul Scherrer Institut
Cavity BPM Pickups for SwissFEL
Boris Keil for the PSI/GFA Beam Diagnostics Team Paul Scherrer Institut
DEELS Workshop 2014

1. Introduction SwissFEL Linac-based FEL, photocathode RF gun
Injector and linac: 2 bunches, 28ns spacing, 100Hz
Fast beam distribution kicker
2 Undulators, 1 bunch each
Hard X-ray ("Aramis"), nm
Soft X-ray ("Athos"), 0.7-7nm
1st construction phase
2nd construction phase
Linac 3
Linac 1
Injector
Linac 2
Athos 0.7-7nm
Aramis nm
0.35 GeV
2.0 GeV
3.0 GeV
GeV
user
stations
GeV
BC1
BC2

2. SwissFEL Site
PSI East
PSI West
SwissFEL
SLS

3. SwissFEL Site

4. Experiment End Stations
SwissFEL Site 6/2013
PSI East
PSI West
Experiment End Stations
Injector

5. SwissFEL Site 4/2014 Gun area
Injector & Linac below ground level, technical gallery (incl. klystrons) on top

6. SwissFEL Site 4/2014
Experimental area

7. BPM Usage Alignment of beam trajectory. Orbit feedback.
Measurement of beam Energy:
- In bunch compressors:
Standard BPMs in bunch compressor “arms”
(no special large-aperture BPM needed), plus two
non-dispersive REFerence BPMs for x/y jitter removal.
- In dog-legs / beam dumps
Standard BPMs used to measure energy in log-leg /
beam dump “arms”. No special ultra-large-aperture
beam dump BPM needed.
Relative beam charge measurement (absolute
calibration via dedicated Bergoz charge monitor).
Correction of position-/charge-dependent measurement
errors of other systems (BAM, wire scanner, ...).
REF E
beam
REF E E
REF

8. BPM Requirements / Specifications
Type 1 "BPM38"
Type 2 "BPM16"
Type 3 "BPM8"
Inner Beam Pipe Aperture
38 mm
16 mm
8 mm
Pickup Length
250 mm
100 mm
Usage
Injector & Linac
Undulators
Position Range*
±10 mm
±5 mm
±1 mm
RMS Position Noise
<10 μm
<5 μm
<1 μm
Position Drift (per week)
Relative RMS Charge Noise
<0.1%
Nominal Charge pC
# Bunches per Train
1-3 1
Max. Bunch Train Rep Rate
100Hz
Min. Bunch Spacing
28 ns -
* Desired: Support of larger/smaller range (via remote gain control), but with lower/higher resolution.

9. BPM Type Choice Evaluation of BPM Types For SwissFEL Pickup Button
Fulfill requirement for injector, linac, TL
Fullfill requirements for all BPMs
Pickup
Button
Matched Stripline
Resonant Stripline
Cavity
Frequency Spectrum
(M=Monopole, D=Dipole Mode)
Monopole Mode Suppression
Modal (hybrid) / electronics
Modal (coupler),
frequency,
phase (sync. det.)
Typical RMS Noise, 10pC, 20mm pipe
~200μm
<80μm
<4μm
~1μm
Typical Electronics Frequency
300…800MHz
MHz
3-6GHz
“Typical” noise: Examples & estimates (scaling, …) based on existing systems, not theoretical limit …

10. Pickup Parameters Pickup Name: BPM38 BPM16 BPM8.1 BPM8.2 Usage
Injector, Linac, TL, BC
Undulators
Undulators (Alternative)
Quantity 6
111
27+23 ←
Pickup Type
Cavity (2 Resonators, Mode-Suppressing Couplers)
Frequency
3.2844GHz
4.8552GHz
Loaded Q
~40
~200
~1000
Material
Stainless Steel
Copper/Steel Hybrid
Gap Width
14mm
7mm
12mm
Reson. Distance
180mm
60mm
50mm
Signal [V/mm/nC]
5.7
7.1
5.2
4.3
RFFE
IQ Downconversion*
IF Frequency
~0Hz
~50MHz
ADC
16-Bit 160MSPS (Linac/Injector: 12-Bit 500MSPS Option)**
* Undulators (Alternative Option): Single-channel downconversion feasible, being evaluated.
** Sample rates of available ADCs for European XFEL (E-XFEL) BPM electronics built by PSI
*** E-XFEL Undulator: 2.9 V/mm/nC (Q=70) -> ~3x improved low charge resolution for SwissFEL.

11. SwissFEL BPM16 Pickup Based on E-XFEL/SACLA design
Optimized for low charge & low
production costs.
Position resonator (used signal ~ position*charge).
Resonator gap width
“Waveguide depth”
Reference resonator (used signal ~ charge)
Waveguides connected to beam pipe

12. SwissFEL BPM16 Pickup
16 mm
100 mm

13. SwissFEL BPM38 Pickup 38 mm 255 mm
Reference resonator (2 RF feed-throughs): Signal ~ charge
38 mm
255 mm
TM010-suppressing waveguide
Dipole resonator (4 RF feed-throughs) Signal ~charge*pos.

14. SwissFEL BPM8 Pickup
8 mm inner beam pipe aperture. Pickup length 100 mm.
Motorized X-Y mover (BPM+quad. magnet

15. BPM16 Pickup Production Steps
Production Steps (Complete Pickup):
Machining of three pickup body parts from metal block [Company]
Mechanical measurement [Company]
RF test (Q, frequency) [PSI]
Brazing of three body parts (foil) [PSI]
Leak test, RF test (Q, frequency) [PSI]
Welding of RF feed-throughs to body [PSI]
Final vacuum & RF test. [PSI]
Production Steps (Feedthroughs):
Machining of pickup metal parts [Company]
Production of boro-silicate "pill" (sintered granulate) [Company]
Loose assembly, then oven to melt glass [Company]
Tests: Vacuum, dimensions [Company]
Test: RF (reflection) [PSI]

16. BPM16 Pickup Costs Body Parts (316LN Stainless Steel)
Design already well optimized by SACLA/DESY
SwissFEL: Only low-charge performance optimized
Costs of different companies differ a lot (1400CHF
to 3800EUR per pickup, material + machining + meas. ...).
Feedthroughs (FTs)
Few years ago: SACLA designed FT for their cavity
BPMs. Single supplier, PSI paid ~500EUR per FT in 2010
= ~half of overall pickup costs!
Several companies offered compatible type for E-XFEL,
typ. few 10% cheaper
PSI developed FT in collaboration with Swiss company
specialized in high-volume low-cost glass FTs (airbags:
few million glass ceramic FTs per year!, medical, ...).
Price reduced ~5x compared to initial design.

17. SwissFEL Cavity BPM Feedthrough
Feedthrough production AG): Some iterations were necessary until our
requirements were met (size of glass pearl, modification of graphite stamp, ...)

18. Bad vacuum design: Risk of inner leaks.
Neutron Scattering Images of FTs
Also feedthtoughs from other companies evaluated ...
Vacuum side
Borosilicate glass seal (good neutron absorber, not well visible with X-rays ...)
Idea: M. Rohrer (had neutron scattering image of gun bullet on his desk ...). Did not show difference between good & bad VSWR. But: ...
air side
Bad vacuum design: Risk of inner leaks.

19. Feedthrough RF Testing Tool
50 Ω broadband load
Feedthrough
to be tested
APC7-N adapter
Series production: Tool for fast RF test of all feedthroughs.

20. S11 for Feedthrough Pre-Series

21. S11 for Feedthrough Pre-Series

22. S11 for Feedthrough Pre-Series

23. BPM16 Pickup & Support Most pickups: Cheap rigid support,
adjustment via shimming (~10um
X/Y steps/reproducibility)
Few pickups: Support adjustable
via screws with differential threads
(~1um X/Y steps/reproducibility)

24. Mechanical Dimension Def.
Position Cavity
Reference Cavity

25. BPM16 Mechanical & RF Tolerances

26. BPM16 Mechanical & RF Tolerances

27. Pre-Brazing Pickup RF Test
Tool fixes body
parts and RF
feedthroughs in
correct position
pressure/weight
used to get
contact.
Measure Q and
frequency of
all pickups
before and after
brazing/welding.

28. RF Meas. Before Brazing

29. RF Meas. After Brazing

30. BPM16 Pickup: Beam Signals
Decay to 0.07%
Decay to 1.6%
Raw signals of SwissFEL BPM16 (QL=40) & E-XFEL undulator cavity pickup (QL=70)

31. BPM16 RFFE Output Signals
SwissFEL BPM prototype: RFFE output signals (IQ outputs, just Q shown)
Low bunch-bunch crosstalk
28ns bunch spacing

32. BPM16 Position Resolution
SwissFEL BPM16 position resolution measurement: Difference of SwissFEL & E-XFEL (extrapolated) BPM position reading. <0.8μm RMS noise at 135pC & 0.35 mm offset (range > ±1mm)

33. BPM16 Charge Resolution
SwissFEL BPM16 Charge resolution measurement: Correlation with E-XFEL undulator BPM. <0.1pC RMS charge noise at 135pC bunch charge.

34. Summary & Conclusions SwissFEL uses only cavity BPM pickups.
In-house feedthrough design & collaboration with
Swiss large-scale (automotive/medical) non-RF feed-
through manufacturer allowed significant price reduction.
BPM16 prototypes meet requirements. Currently doing
minor redesign, removing systematic Q and frequency
shift of final version (with BC-Tech feedthroughs)
BPM38 and BPM8 prototype beam tests 7-9/2014
Neutron scattering allowed non-desctructive analysis
of feedthroughs from alternative manufacturer.

35. Team & Acknowledgements
F. Marcellini, M. Rohrer (Cavity pickup & feedthrough design & test)
M. Stadler (Cavity RFFE, algorithms, overall system tests)
M. Roggli, R. Ditter, R. Kramert (ADC Mezzanine, BPM crate)
R. Baldinger (FPGA carrier board)
G. Marinkovic, W. Koprek (Software & FPGA firmware)
and
PSI Mechanical Department (Pickup construction & prototyping)
Colleagues from DESY and SACLA (Pickup infos & tips)
C. Bargähr (RF feedthroughs,

36. Thank you for your attention!
Paul Scherrer Institut
Thank you for your attention!

37. Supplementary Slides …

38. BPM16 Pre-Series: Dimensions
pickup 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Position cavity: Deviation from nominal mechanical dimension R -5
-10
-4,5
-6,5
-1,5 -8
-8,5
-2,5
-5,5 -3 L -1 -6 -4
-15 G T -2 27 a
-30
-50
-40
-60
-80
-70 b
-20
wgH 18 17 -9
wgL 20 -7
wgW
-12,5
1,5
-25
Reference cavity: Deviation from nominal mechanical dimension
-12
-3,5
-16
-13
-11
-17 22 D 44 21 31

39. BPM16 Pre-Series: Frequency
Position Cavity
Reference Cavity