Vertical EP at Marui & KEK November 2016 TTC High-Gradient WG Meeting T Saeki KEK V Chouhan, Y Ida, K Ishimi, K Nii & T Yamaguchi (Marui Galvanizing Co., Ltd., Japan) H Hayano, S Kato, H Monjushiro & M Sawabe (KEK, Japan) V Chouhan, MGI (Nov 2016) 1
Outline • VEP setups • Coupon cavity & Ninja cathode • Optimization of cathode design and VEP parameters • Vertical test results • Fabrication of 9-cell coupon cavity • Summary V Chouhan, MGI (23 Sep 2016) 2
VEP Setups at Marui For 9-cell/1-cell Cavity For 1-cell Cavity Air Cooling Ducts Both setups were prepared with PVC materials in order to reduce cost of surface treatment of cavities in mass production. V Chouhan, MGI (Nov 2016) 3
Single Cell Coupon Cavity Coupon Nb Coupons View Ports Coupon View Port Ø 8 mm • Cavity contains 6 coupons at beam pipes, irises and equator. • An EP current can be measured for individual coupon. • Coupon surfaces are analyzed with several surface analytical tools. • The cavity is having 4 view ports on the top iris, bottom iris and equator for light introduction and in-situ observation. V Chouhan, MGI (Nov 2016) 4
Ninja Cathode Side View Four re-tractable wings (Al or EP 時 EP 時 挿入時 挿入時 insulating) Top View 空洞へセット 空洞へセット Ninja in した状態 した状態 Cavity Cell Ninja cathode was developed in order to agitate EP acid. The wings are either insulating or metallic/partially metallic which act as a cathode. V Chouhan, MGI (Nov 2016) 5
VEP with Rod and Ninja Cathode (Al Wings) Ninja Cathode- Parameter Value Rod Cathode Al Wings H 2 SO 4 :HF 9:1 Electrolyte Flow Direction Bottom to Top Flow Rate 5 L/min Cathode Rotation Speed 1 rpm Applied Voltage ~9 V 25-30 mA/cm 2 EP Current Density Target Removal Thickness 50 µm Cavity Surface Temperature ~25 °C Upper Iris Coupon Ninja Cathode Rod Cathode • Ninja Cathode (Al wings) results in: o Smooth surface o Asymmetric removal similar as rod cathode 6 V Chouhan, MGI (Nov 2016)
Ninja Improvement • Al wings were replaced with insulating and partial metallic wings. Partial Metal Wings Insulating Wings Side View P 時 P 時 P 時 P 時 Cathode Top View V Chouhan, MGI (Nov 2016) 7
Effect of Stirring in VEP At 50 rpm At 1 rpm P 時 P 時 Coupon Ø 8 mm • The top iris coupon current was the highest at 1 rpm because of accumulation of H 2 bubbles on the top iris. • Ninja cathode rotating at 50 rpm displaced the bubbles and might make uniform acid flow in the cavity cell. 8 V Chouhan, MGI (Nov 2016)
I-V Characteristic Typical I-V Curve S. Jin et al. Phys. Rev. ST Accel Beams 13, 061001 (2010) • An I-V curve gives an information about phenomenon occurs on Nb surface based on applied voltage. • A particular range of voltage (in polishing region) needs to be applied for EP. V Chouhan, MGI (Nov 2016) 9
Effect of Partial Cathode Wings Insulating Wings Partial Cathode Wings Cathode • H 2 bubbles can screen the cathode and reduce ion migration to the cathode. • A distance between anode and cathode might be important for a small area of cathode. • Electric field on equator was obviously lower due to a large distance from cathode. • Wing cathode enhanced ion transportation and electric field on equator surface. 10 V Chouhan, MGI (Nov 2016)
Removal Thickness W/O and W/ Wings Removal thickness was measured with ultrasonic thickness gauge. Partial Metal Wings Rod Cathode Partial Metal Wings Cathode Rotation: 50 rpm P 時 P 時 Ninja wings were found to be effective as a stirrer in order to minimize longitudinal asymmetry of Nb removal. 11 V Chouhan, MGI (Nov 2016)
Surfaces of Coupons Partial Metal Insulating Wings Rod Cathode at 50 rpm Wings at 50 rpm Top Iris Top Iris P 時 P 時 P 時 P 時 Equator Equator Top Bottom Top Bottom Equator BP Iris Iris BP Typical roughness of coupons before VEPs: Rz=~4 µm Bottom Iris Bottom Iris 500 µm 500 µm 500 µm • Rough equator surface of equator with rod cathode and insulating wings might be due to Nb removal in etching region. (Since equator remains in etching region at the applied voltages.) • Ninja with partial metal wings resulted in smooth surface of the equator. 12 V Chouhan, MGI (Nov 2016)
What We Learned • Ninja cathode with Insulating Wings: – Cannot be used since it resulted in rough equator surface due to cathode screening by H 2 bubbles. • Ninja cathode with Partial Metal Wings: – May not be appropriate for VEP of a 9-cell cavity because bubbles generating on the wings will finally accumulate in the upper cells of a cavity. The problem of cathode screening by H 2 bubbles should be solved. V Chouhan, MGI (Nov 2016) 13
Ninja with Enhanced Cathode Area P 時 P 時 • Cathode surface area was enhanced to reduce the screening effect. • The cathode was covered well with a meshed shield to trap H 2 bubbles and guide them along the cathode. V Chouhan, MGI (Nov 2016) 14
Effect of Enhanced Cathode Area Insulating Wings with Insulating Wings Enhanced Cathode area • EP plateau for equator was obtained with Ninja cathode having larger cathode area. 15 V Chouhan, MGI (Nov 2016)
Surfaces of Coupons Enhanced Cathode Partial Metal Insulating Wings at area with Insulating Wings at 50 rpm 50 rpm Wings at 50 rpm Top Iris Top Iris Top Iris P 時 P 時 P 時 P 時 P 時 P 時 Equator Equator Equator Bottom Iris Bottom Iris Bottom Iris 200 µm 200 µm 200 µm The enhanced cathode area resulted in smooth surface of the equator surface. V Chouhan, MGI (Nov 2016) 16
VEP at Lower Rotation Speed • Since the Ninja cathode was prepared with bubble guide, the rotation speed of the cathode was reduced. • VEP was carried out at 20 rpm. Partial Metal Wings Enhanced Cathode area At 50 rpm At 50 rpm At 20 rpm Top Iris Top Iris Top Iris P 時 P 時 P 時 P 時 P 時 P 時 Equator Equator Equator • Surface roughness of equator was reduced in case of Bottom Iris Bottom Iris Bottom Iris 20 rpm. 200 µm 17 V Chouhan, MGI (Nov 2016)
Removal Trend at Lower Rotation Speed Partial Metal Wings Enhanced Area Rod Cathode at 50 rpm at 20 rpm Almost symmetric removal was obtained with the Ninja cathode due to bubble guide and uniform acid flow in cavity cell. Based on the concept of the Ninja cathode (enhanced cathode area), a Ninja cathode for VEP of 9-cell cavities has been fabricated. 18 V Chouhan, MGI (Nov 2016)
Vertical Test Results • Two single-cell cavities (NR1-2 and C1-19) were VEPed with the Ninja cathodes and tested in vertical cryostats. NR1-2 Cavity (Cornell Cavity) • Pre-treatment : Tumbling, BCP, degassing at 800 °C Vacuum Leak occurred • Cathode : Ninja cathodes (partial metal wings and enhance area) with the VEP setup of Cornell University Quench • VEP : VEP with each cathode (20+20 µm removal) 35 MV/m at Q 0 =9×10 9 • Ninja rotation speed : 50 rpm 21 MV/m at Q 0 =1.4×10 10 • VT : Performed at 2K at Cornell after 120 °C baking C1-19 Cavity (Saclay Cavity) • Pre-treatment : BCP • Cathode : Ninja cathode (enhanced area) with Marui VEP setup • VEP : Two VEP for 31 and 55 µm removal, degassing at 750°C and final VEP for 11 µm removal • Ninja rotation speed : 30 rpm • VT : Performed at 1.6K at Saclay 19 The both cavities showed good performance in the vertical tests.
9-Cell VEP Experiment P 時 P 時 Power Supply Cathode Motor Parameter Value H 2 SO 4 :HF 9:1 Water Waste Tank Water Electrolyte Flow Direction Bottom to Top Pump-2 Flow Rate 5 L/min Chiller EP Tank Cathode Rotation Speed 50 rpm System Controller Applied Voltage ~11 V Pump-1 30-35 mA/cm 2 EP Current Density Cavity Surface Temperature ~ 20 °C Cooler 20 V Chouhan, MGI (Nov 2016)
VEP of 9-Cell Cavity with Ninja Cathode of Al Wings (Old Version) θ =0º (a) (b) Top Top View View Removal thickness at iris is around 2-3 θ =180º times larger than that at the equator. V Chouhan, MGI (Nov 2016) 21
9-Cell Coupon Cavity • The world’s first 9-cell coupon cavity was fabricated to optimize VEP parameters. Coupon with View Port near Iris Equator Coupon Nb Coupon with view port 1 Nb Coupon 2 3 4 3 coupons at 5 Equator positions 6 6 coupons near iris positions 7 8 9 22 V Chouhan, MGI (Nov 2016)
Summary VEP setup and unique Ninja cathodes were developed for surface treatment of Nb cavities. VEP parameters and design of the Ninja cathode were optimized for single cell cavity. Smooth surface and symmetric removal of Nb along cavity length were obtained with the Ninja cathode. The Ninja cathode and optimized VEP parameters were applied to two single-cell cavities which showed good performance in a vertical tests performed at Cornell University and CEA-Saclay. The world’s first 9-cell coupon cavity has been fabricated. Based on the VEP results of 1-cell cavity, 9-cell Ninja has been fabricated. V Chouhan, MGI (Nov 2016) 23
Conference Presentations • V. Chouhan et al., THPP098 , LINAC2014. • K. Nii et al., MOPP108 , LINAC2014. • K. Nii et al., TUPP101 , LINAC2014. • V. Chouhan et al., MOPB105 , SRF2015. • V. Chouhan et al., THBA02 , SRF2015. • K. Nii et al., MOPB098 , SRF2015. • V. Chouhan et al., MOPLR037 , LINAC2016. • S. Kato et al., MOPLR038 , LINAC2016. • K. Nii et al., MOPLR039 , LINAC2016. V Chouhan, MGI (Nov 2016) 24
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