Performance of SPL cavities at CEA & IN2P3

1. Performance of SPL cavities at CEA & IN2P3 Juliette PLOUIN, in behalf of Franck PEAUGER, Guillaume OLRY and SRF Cavity teams at CEA and IPNO Slhipp-4, 15-16 May 2014, cern

2. Spl cavity beta = 1 Cavity design and tests performed at CEA-Saclay Cavity manufactured by E. Zanon

3. Dumbbells and end group trimming Jan-Feb 2013 @ Zanon Dumbbell frequency measurement step 1 - length step 1 - length step 2 - length step 2 - length step 3 - length step 1 to 3 step 1 to 3 • 1 DB and the 2 EG have reached the target frequency • Frequency of the 3 other DB is lower than expected (702.2 MHz) • This is due to the cell deformation during the turning operation which cannot be controlled perfectly. step 3 - length

4. Choice of the dumbbell sequence before welding Endgroup tuner Endgroup coupler C D A B Final welding of the cavity completed in May 2013. RF simulations with measured frequencies to find the best value of Field Flatness according to dumbbell sequence Endgroup tuner Endgroup coupler D A C B Endgroup tuner Endgroup coupler A C B D Measured field flatness at cavity reception : 80% Simulated field flatness for the best configuration : 89%

5. Field flatness tuning Target freq. at 2 K and assuming 200 µm chemical removals : 704.4 MHz • Calculated target freq. at 293 K after fabrication : 703.837 MHz Measured frequency after fabrication is 703.707 MHz which is 130 kHz below the target frequency. After few iterations, the field flatness ratio has been improved from 80% to 92%. The frequency is now 703.755 MHz (82 kHz below target) Cavity on the tuning bench Field flatness after tuning

6. First electropolishing July-Sep 2013 @ Saclay • Cathode with a constant diameter of 70 mm along the cavity • Electrolyte : H2SO4(95%) – HF (40%) mix (9:1 proportion) • Applied voltage : 7.5 V • Current varies from 170 to 190 A depending on the acid temperature in the cavity (14° to 16°C). HPR 100 bars Dried under laminar flow Assembly in clean room Leak test OK Average total Nb removal = 115 mm

7. First test in vertical cryostat Oct 2013 @ Saclay • Q0 very low ! Why ? • Insufficient chemical etching • 100K effect • locally high surface roughness • external magnetic field from cavity support Q0 =1.5e9 150 W dissipated Test stopped Difficult to evaluate Magnetic measurements have shown no field enhancement T=1.7 K It has been decided to pursue the chemical removal process of Nb by electropolishing in order to reach an average removal of 200µm.

8. Second electropolishing Average total Nb removal = 200 mm More details on EP : see “CEA expertise with SRF cavity treatment”, F. Eozenou, this afternoon HPR 100 bars Dried under laminar flow Assembly in clean room Leak test OK

9. Second test in vertical cryostat Feb 2014 @ Saclay Previous test (Oct 2013) 1.5 K (Feb 2014) 1.8 K (Feb 2014) Q0 decreases down to 5e8 at 7 MV/m… Q0 slope very pronounced : 100 K effect highly suspected !

10. Thermal treatment March 2014 @ CERN Ion pump currents during the full thermal treatment cycle at CERN 24 hours

11. Thermal treatment result Mass spectrum The quantity of different elements decreased by two decades after thermal treatment Hydrogen Before thermal treatment Field flatness Unaffected by thermal treatment After thermal treatment

12. First test in vertical cryostat April 2014 @ Saclay The cavity has been cleaned in ultrasound bath and slightly chemically treated using a standard BCP mixture. Two tests at 1.6 K Previous tests • Some processing has been necessary from 10 to 12MV/m • Tests have been stopped 17.6 MV/m because high dose rate was measured out of the bunker(> acceptable rate in the Supratech area)

13. Results analysis Surface resistance measured vs temperature Residual resistance around 8-9 nOhms • @ T = 1.6 K • Rs ~ 10 nOhms • Q0 = G/Rs=2.7e10 • Consistent with measured Q0 at low field (between 2e10 and 3e10) • Thermal treatment has been efficient ! • We were limited at 17.6 MV/m • We still have a slight Q-slope

14. Future plans • The test stand at Supratech will be modified (stop the sky effect) to allow a higher dose rate of the cavity  new test to reach higher gradient • The helium tank (stainless steel) will be we welded to the cavity  new tests in CV, test in Cryholab with coupler • This cavity could be used for R&D activities at CEA • All that can be learned will also be used for following projects, in particular ESS Thank you to Sergio Calatroniand Antonio Mongelluzzo for the thermal treatment of the b = 1 cavity

15. 5-CELL ELLIPTICAL CAVITY, BETA 0.65 FOR SPL Slides from Guillaume Olry (IPN Orsay)

16. Fabrication and preparation FABRICATION Kick-off meeting: nov 2011 Delivery #1 for field flatness tuning done @ IPN Orsay: march 2013 Cavity sent back to RI Then final delivery after He tank welding: june 2013 (fabrication time=19 months!) CAVITY PREPARATION 1st etching (BCP): 200 µm Field flatness tuning 2nd etching (BCP): 20 µm 2-hour HPR through beam tubes only (6 passes) No 600°C annealing nor 120°C baking Cavity w/o He tank before field flatness tuning 2nd etching pass: cavity with He tank

17. Field flatness tuning [1] Coupler • Refurbishment of the tooling used by CEA for the HIPPI cavity • Manual tuning, 1 cell after another • Tuning done after BCP (200µm) • Cells 1, 2 and 4 deformed of few mm (between 1.4 to 2.5 mm max) Cell 5 Cell 1 Cavity w/o He tank during field flatness tuning Before: field flatness ~48% After: field flatness ~6%

18. Field flatness tuning [2] • 2 degradations • Before sending back the cavity, after leak tightness test: field flatness from 6% (blue curve) to 16% (red curve) • After He tank welding: field flatness from 16% (red curve) to 38% (green curve)

19. test • Cavity delivered to CEA this week. • Tested @ 2K by CEA in Vertical cryostat (Thanks Franck Peauger and his team!) • Test planned in June