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

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

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

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

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

Full SPS – RFD Cavity

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

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

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

Frequency Recipe Step (Recipe for 20 C, 50 % and 1013.25 mbar) Δf [kHz] fn [MHz] Cavity after trimming and thinning 399.840296 Shift due to bulk BCP (140 microns) -39.441 Cavity after bulk BCP 399.800855 Weld shrinkage 115.645 Weld bead 5.000 Cavity after final weld 399.921500 Shift due to light BCP (20 microns) -5.762 Cavity after light BCP 399.915738 Shift due to mounted couplers 4.906 Fully assembled cavity with HOM couplers 399.920644 Pressure effect (760 Torr differential) -60.800 Dielectric effect air to vacuum 130.341 Evacuated cavity at 20 C 399.990185 Thermal shrinkage 572.877 Cooled down cavity at 4.2 K 400.563062 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 400.650022 Shift due to tuner activation to its mid range 150.000 Cavity with tuner activated 400.800022 Lorentz detuning -10.022 Operational cavity with RF on 400.790000

Trimming Sensitivity Trimming sensitivity for RFD cavities df/dz = -119.85 kHz/mm Measured sensitivity: CAV001: -129.3 kHz/mm CAV002: -116.1 kHz/mm dz/2

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

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)

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

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

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

RFD-CAV-001 RF Test Results

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

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

Frequency Measurements – RFD-CAV-001

Frequency Measurements – RFD-CAV-002