1. SPS – RFD Experience and Evolution to LHC
International Review of the Crab Cavity Performance for HiLumi
SPS – RFD Experience and Evolution to LHC

2. Outline PoP and SPS RFD Cavities SPS-RFD Experience
Cavity processing plan
Frequency recipe  Expected and measured
SPS-RFD Lessons Learned
Changes proposed in cavity processing plan
Evolution to LHC
Design modifications of RFD cavity for LHC

3. Proof of Principle (PoP) – RFD Cavity
53 cm
34 cm
Operates in TE11 like mode
Cylindrical shaped cavity
Fabricated at Niowave Inc.
Parameters
Value
Units
Frequency
400
MHz
Nearest HOM
590
Deflecting Voltage (Vt*)
0.375 MV
Peak Electric Field (Ep*)
4.02
MV/m
Peak Magnetic Field (Bp*)
7.06 mT
Bp*/Ep*
1.76
mT/
(MV/m)
Stored Energy (U*)
0.195 J
[R/Q]t
287.0 Ω
Geometrical Factor (G)
140.9
RtRs
4.0×104 Ω2
At Et* = 1 MV/m
E Field
B Field
Teflon Bead
Al Bead

4. PoP RF Test Results
Surface treatment and rf testing done at Jefferson Lab
Test I – April, 2012
Test II – June, 2014
Test at CERN – October, 2014
JLab results were confirmed
Cavity reached a VT of 7.0 MV
Cavity was retested with Nb coated flanges provided by CERN
Q0 increased by a factor of 3 from 4×109 to 1.2×1010
Multipacting was processed easily and did not reoccur
3.4 MV
JLab
3.4
CERN

5. SPS – RFD Cavity Properties
Frequency
400.79
MHz
Aperture 84 mm
Nearest HOM
633.5
Ep*
3.6
MV/m
Bp*
6.2 mT
[R/Q]t
429.7 Ω
Geometrical Factor (G)
106.7
RtRs
4.6×105 Ω2
At Et* = 1.0 MV/m Vt
3.4 MV 33 56
E Field
H Field
281 mm
194 mm
Operates in TE11 like mode
Next HOM is 1.5 times fundamental mode
Multipacting is improved compared to PoP cavity
Surface fields are lower compared to PoP cavity
Shunt impedance higher compared to PoP cavity
Reduced multipoles components with curved poles

6. Full SPS – RFD Cavity

7. HOMs for SPS-RFD Cavity Design
Pick Up probe
FPC
H-HOM
(Hi-pass filter)
V-HOM
(selective coupling)
Only 2 HOM couplers
FPC/HOM ports oriented to keep clearance for the second beam pipe
Selective coupling (V-HOM) to reduce the number of filters
Couplers at locations of low field region on cavity body
7 mm offset incorporated into the probe tip to enhance coupling to the dipole modes at around 2 GHz
Minimizes RF heating on the coupler components
Small RF power leakage through the coupler, ~1.5W, due to asymmetry
Location preserves field symmetry
FPC coupler: QL = 5.0×105
H-HOM coupler with 30 degree hook orientation with no change to filter elements
Hook shaped coupler reduces RF heating  69 W

8. HOMs for SPS-RFD Cavity Design
Qext calculated using Omega3P (SLAC ACE3P suite) for modes up to 2 GHz
Solid lines are the impedance budget for dipole HOMs (blue) and accelerating HOMs (red) respectively
Damping scheme meets the LHC High-Luminosity Upgrade impedance requirements

9. SPS – RFD Cavity Processing Plan
Surface Polishing
Bulk BCP
Trimming
Final Weld
Heat Treatment
Light BCP
High Pressure Rinse
Assembly
Low Temp Bake
RF Test
Mechanical grinding un-even weld / surface pits
Total removal of 140 microns
Re-processing of RFD-001
Trimming of center body for frequency
3 mm weld trimming
Bulk BCP
Heat Treatment
Light BCP
High Pressure Rinse
Assembly
Low Temp Bake
RF Test
Welding of end plates to center body
600 oC for 10 hours in furnace
Total removal of 20 microns
Rinsing in 2 iterations
Proper rotation to drain cavity completely
Cavity assembly of cavity in clean room
120 oC bake for 36 hours
Test with and without HOM couplers

10. Frequency Recipe Step (Recipe for 20 C, 50 % and 1013.25 mbar)
Δf [kHz]
fn [MHz]
Cavity after trimming and thinning
Shift due to bulk BCP (140 microns)
Cavity after bulk BCP
Weld shrinkage
Weld bead
5.000
Cavity after final weld
Shift due to light BCP (20 microns)
-5.762
Cavity after light BCP
Shift due to mounted couplers
4.906
Fully assembled cavity with HOM couplers
Pressure effect (760 Torr differential)
Dielectric effect air to vacuum
Evacuated cavity at 20 C
Thermal shrinkage
Cooled down cavity at 4.2 K
Shift due to change in skin depth
28.000
Pressure from 760 Torr to 23 Torr in He tank
58.960
Cooled down cavity at 2.0 K
Shift due to tuner activation to its mid range
Cavity with tuner activated
Lorentz detuning
Operational cavity with RF on

11. Trimming Sensitivity Trimming sensitivity for RFD cavities
df/dz = kHz/mm
Measured sensitivity:
CAV001: kHz/mm
CAV002: kHz/mm
dz/2

12. Lessons Learned – 1 Perform bulk BCP after final welding of the cavity
Bulk BCP of the two SPS-RFD cavities were done before final weld to control the frequency shift due to processing
Due to quality of final welds additional bulk BCP was done on both RFD cavities
Frequency shift due to bulk BCP  40 kHz (140 microns removal)
Frequency shift is comparatively small  Not a critical parameter in frequency control

13. Horizontal BCP/EP tool – (ANL)
Lessons Learned – 2
Perform chemical etching and high pressure rinsing in horizontal orientation
Currently bulk BCP, light BCP and high pressure rinsing are done in vertical orientation
Horizontal orientation preferred
To achieve uniform removal
Better acid circulation and drainage
Horizontal BCP/EP tool – (ANL)

14. Lessons Learned – 2
Warm tuning of RFD cavity can be done by push/pull of center body
Experience from fixing the center body of RFD-CAV-001

15. RFD-CAV-001 RF Test Preparation
Beam Port – VHOM Side
Beam Port – HHOM Side
FPC Port
HHOM Port
Pick Up Port
VHOM Port
Calculations at 1.0 J with Stainless Steel flanges
Conductivity of stainless steel 6.99×106 S/m
Port
Power Loss
[W] Q
Beam Port (HHOM)
2.64×10-5
9.53×1013
Beam Port (VHOM)
2.71×10-4
9.30×1012
FPC Port
5.76×10-5
4.37×1013
HHOM Port
6.52×10-4
3.86×1012
VHOM Port
3.02×10-9
8.34×1017
Pick Up Port
1.69×10-18
1.49×1027
Total
1.01×10-3
2.50×1012

16. RFD-CAV-001 RF Test Preparation
VHOM and HHOM ports are blanked
HHOM blanked with a Nb coated flange
RFD test results
Two rf tests on RFD-001
RFD-002 ready to be testsed
RF results
Multipacting was processed easily
Surface resistance  9 nΩ
Lorentz detuning 
Pressure sensitivity  55 kHz/torr
FPC Port
(Iput test probe)
Qext ~ 5×109
Pick Up Port
(PickUp test probe)
Qext ~ 2×1011
Safety valve

17. RFD-CAV-001 RF Test Results

18. RFD Cavity Design Changes for LHC
Modified pickup port (To be confirmed with CERN)
Pickup port radius increased to 40 mm from 20 mm for easy cavity processing
Pickup probe coupling: 2.7×1010 to extract 1 W at operating field
No change on cavity properties or on HOM spectrum and damping
Increase height of FPC, HHOM and VHOM ports for convenience in assembly

19. Summary No major modifications to RFD cavity design for LHC
Two SPS-RFD cavities have been fabricated and processed
RFD-CAV-001  Completed
RFD-CAV-002  To be tested
Cavity reached Vt=4.4 MV
Epk=45 MV/m and Bpk=77 mT  Much room for improvements
Achieved 3.4 MV with a Q0 of 8.5×109
No major alteration to the cavity processing plan; Recommendations for future cavities
Preference to do bulk BCP after final welding
Horizontal orientation while cavity processing (BCP and HPR)
No major modifications to RFD cavity design for LHC
Modified pick up port
Increased port height

20. Frequency Measurements – RFD-CAV-001

21. Frequency Measurements – RFD-CAV-002