Series Tests of High Gradient Single- cell Superconducting Cavity for the Establishment of KEK Recipe T. Saeki (1), F. Furuta (1), K. Saito (1), M. Ge (2), Y. Higashi (1), T. Higo (1), S. Kazakov (1), H. S. Kim (3), H. Matsumoto (1), Y. Morozumi (1), R. S. Orr (4), N. Toge (1), K. Ueno (1), and H. Yamaoka (1) Affiliation: (1)KEK, (2)IHEP, (3)Kyungpook National Univ., (4)Univ. of Toronto/JSPS EPAC, June 2006 (Presentation ID: MOPLS087) We have performed a series of vertical tests of single cell Niobium superconducting cavities at 2 degrees Kelvin. These tests aimed at establishing the feasibility of reaching an accelerating gradient of 45 MV/m on a routine basis. The cavity profiles were all of the KEK low loss design and were fabricated from deep drawn Niobium half shells using electron beam welding. The cavity surface preparation followed an established KEK procedure of centrifugal barrel polishing, light chemical polishing, high temperature annealing, electro-polishing, and finally a high pressure water rinse. Of the six cavities tested, three exceeded 45 MV/m on the first test. This clearly establishes the feasibility of this gradient. In this paper we describe these tests and our future program for optimizing the surface preparation. Abstract 1

TreatmentProcess Removal Thickness (um) Purpose Centrifugal Barrel Polishing (CBP) Mechanical grinding by stones and water 135 – 235 Remove defects of Nb material and smooth EBW seam. Light Chemical Polishing (CP) Chemical reaction 10 Remove dusts after CBP and prepare smooth surface before EP Annealing / degassing 750 O C for 3 hours by furnace - Release mechanical stress and degass hydrogen Electro-Polishing (EP) Chemical + electronic reaction 80Prepare very smooth surface High-Pressure Rinsing (HPR) Rinse with pressured Ultra-Pure Water (UPW) - Remove particles and make clean surface Baking 120 O C for 48 hours with pumping vacuum -Defuse oxidized layer KEK recipe Parameters of each process are optimized for good cavity performance with low cost. 2

Step 0 : Proof of principle for 50 MV/m, done => Poster MOPLS084 by Furuta for details. Step I : Proof of principle for 45MV/m on the first trial, done Step II : Yield rate goal = 85% for 45MV/m on the first trial, 50% Level III : Why yield rate is 50%? Investigation is ongoing. No H Q-disease in series tests. The establishment of KEK recipe with single-cell cavity 3

Six ICHIRO-shape Single-cell (IS) cavities were fabricated. Electron Beam Welding (EBW) Both KEK in-house machine and industrial machine were used Six IS cavities were fabricated (IS#2, #3, #4, #5, #6, #7). Series of tests was done to establish the KEK recipe. Deep-drawing Nb cups 4

Centrifugal Barrel Polishing (CBP) / Mechanical grinding Stones and water Rough stones Fine stones KEK recipe Rough stones + water : 4 hours x 3 times Removal thickness = 25 um x 3 = 75 um Fine stones + water : 4 hours x 3 times Removal thickness = 20 um x 3 = 60 um Total removal thickness = 135 um. 5

CBP removal thickness depends on surface roughness at EBW seam Before CBP Equator EBW seam (inner surface) 2nd CBP Good EBW 1 st CBP(30um) Bad EBW 3 rd CBP (90um) 12th CBP Removal thickness > 200 um. Removal thickness = 60 um. Bad EBW with a pit 2 nd CBP (60um) 13th CBP Removal thickness > 200 um. 6

Light Chemical Polishing (CP) and annealing / degassing Light Chemical Polishing (CP) HF(46%) : HNO 3 (60%) : H 3 PO 4 (85%) = 1 :1 : 1 in volume CP for 1 minute at 25 O C. Removal thickness = 10 um. Prepare smooth surface before EP. Annealing/Degassing by furnace 750 O C for 3 hours Degassing of hydrogen is important. Temperature and time is optimized for cavity softness and cost. 7

Cavity Beam Pipe Cathode bag Acid Level EP acid circulates Circulating EP acid H 2 SO 4 (95%) : HF(46%) = 10 : 1 in volume Electric Current Voltage ~ 20 V, Current ~ 40A Horizontally Rotated Electro-Polishing (EP) Back to EP-acid reservoir tank (100L) Well established method 8

High Pressure Rinse (HPR) Ultra Pure Water Specific resistance =18 M Ohm cm TOC = 12 – 22 ppb bacteria = 0 – 3 count / mL HPR for 1 hour HPR nozzle Water pressure = 7 MPa Flow rate = 10 L/min. HPR is a very powerful tool to remove particle contamination on the inner surface of Nb cavity. 9

Proof of principle for 50 MV/m with ICHIRO single-cell cavity Press release ’05 28th Sept. NHK news, “Good morning Japan” ’05 12th Oct. Nikkan Kogyo News ’05 21st Oct. Energy News Weekly ’05 1st Nov. Daily Yomiuri ’06 24th Jan. Nihon Keizai News Eacc = 53.5 MV/m was achieved. This is the world record. 10

~45MV/m achievement with six single-cell cavities on the first trial (Goal is 85%) yield rate for ~45 MV/m = 50% on the first trial Statistics Accidental 16.7% FE initiated by MP16.7% Quench 16.7% No problem 50% Oxidation, EBW, field enhancement ? EP acid contamination ? Assembly or HPR problem Quench FE Accidental 3 cavities in 6 achieved the gradient > 44 MV/m. 3 cavities failed. 11

Why three cavities (50 %) failed What 50% failed Easy mistake in HPR or careless assembly in clean room Sulfur or oxidization Contamination in EP process Defect of material or roughness at EBW seam Particle on the surface should be removed Particle on the surface should be removed Much thicker removal Re-HPR and careful assembly Light EP with fresh EP acid or light CP EP with thick removal or CBP How to improve? Additional treatments Possible sources of failure No removalLight removal Heavy removal 12

50 % failure from very thin surface? Pilot study for new recipe HF rinse (0.2 um) + HPR(UPW) Field emission disappeared after re-HPR. But no significant improvement in the gradient….. 13

Failed 50% were recovered by light removal IS#7 IS#3 IS#2 IS#2, EP(20 um) + EP(fresh acid, 3um) Eacc, Q0 = 47.1 MV/m, 1.06e10 IS#3, EP(20-30 um) + EP(fresh acid, 3 um) + HF rinse Eacc, Q0 = 44.7 MV/m, 0.98e10 IS#7, EP(20-30 um) + EP(fresh acid, 3 um) + HF rinse Eacc, Q0 = 43.9 MV/m, 1.17e10 Conclusion: No problem in the material quality, deep-drawing, CBP (mechanical grinding), EBW and all mechanical issues. The source of failure (50%) is coming from thin surface < ~20 um. Oxidation layer is most suspicious? The KEK recipe needs some modifications for higher yield rate > 85%. But change might be minor. ~20 um removal All 6 cavities Eacc > ~44 MV/m IS#6 IS#4 IS#5 Fresh EP acid was used for last 3 um to avoid contamination. 14

IS#2IS#3IS#4IS#5IS#6IS#7 KEK Recipe Eacc Qo1.53e108.66e99.07e95.38e99.56e91.94e9 +re-HPR e107.27e95.66e97.78e91.1e10 +HF rinse +HPR e101.43e109.97e93.9e93.33e9 +CP(10um) +HPR+Baking e95.57e9 +EP (fresh acid, 3 um) +HPR+Baking e101.28e101.17e10 +EP(20-30um)+EP(3um) +HPR+Baking e107.81e9 +EP(20-30um)+EP(3um) +HF rinse+HPR+Baking e107.83e91.17e10 Results of series tests for six ICHIRO Single-cell (IS) cavities 15

IS#2IS#3IS#4IS#7 KEK recipe (EP 80um) +HF rinse +HPR Keep 100K for 12h Keep 100K for 21h Keep 100K for 12h +EP (fresh acid, 3 um) +HPR+Baking Keep 100K for 12h Keep 100K for 38h Keep 100K for 40h +EP(20-30um)+EP(3um)+ HF rinse +HPR+Baking Keep 100K for 24h CP(10um)+EP(20-30um)+EP(3um) + HF rinse +HPR+Baking Keep 100K for 24h No H Q-disease H Q-disease was checked by keeping cavities at 100K for >12 hours in 8 tests with 4 cavities. No H Q-disease was found after treatments and warming-up to 100K in the above table. 16

Histograms for series tests ICHIRO shape + KEK recipe FE and Q-slope removed Mean = 44 MV/m. Sigma = 6 MV/m. Eacc [MV/m] Repeated tests removed Mean = 44 MV/m. Sigma = 7 MV/m. All tests Mean = 38 MV/m. Sigma = 9 MV/m. Eacc [MV/m] 17

Summary Six ICHIRO-shape single-cell cavities were fabricated for series tests to establish the KEK recipe. The KEK recipe : CBP (mechanical grinding), light CP (10um), annealing / degassing (750 O C for 3 hours), EP (80 um), HPR (Ultra-Pure Water) for 1 hour, Baking (120 O C for 48 hours) Proof of principle for Eacc=50 MV/m was done. Eacc = 53 MV/m (the world record). Yield rate of the KEK recipe for Eacc>44 MV/m = 50% (3 in 6) on the first trial. Pilot study of new recipe. HF rinsing + HPR, EP(3 um) + HPR, EP(20+3 um) + HPR etc.. 3 failed cavities were recovered by additional treatments: EP (20-30 um +3 um) + HPR, EP(20-30 um +3 um) + HF rinse + HPR => All 6 cavities reached Eacc > 44 MV/m => The source of failure exists within the depth of <~30 um. No H Q-disease was found in 8 tests with 4 cavities. Eacc histograms were made for the series tests. Mean(Eacc) = 44 MV/m and Sigma(Eacc) = 6 MV/m, if removing FE and Q-slope. KEK recipe needs some modifications for higher yield rate (>85%) but modification might be minor. 18

OPTIONAL POSTERS

TESLA design cavities Operation at 31.5 MV/m Four 9-cell ICHIRO LL cavities Already fabricated. STF Phase 1 Operation at 36 MV/m KEK OP1

ICHIRO 9-cell cavities MP barrier was severe. (1)Contamination in EP acid? (2)Ununiform gas-absorption during cooling-down cavity? Feed-back the results from single-cell cavities to the 9- cell cavities. Even if Eacc<45MV/m, we will install the four 9-cell cavities into STF. 8th Vertical Test (V.T.) ICHIRO 9-cell #0 Cavity Eacc = 15.9 MV/m Q0 = 3.4e9 MP Eacc = 29.3 MV/m Q0 = 1.02e10 HOM study Tuner study MP at HOM coupler? 1 st V.T. #1 #2 #3 #0 OP2

9-cell ICHIRO high-gradient Low-Loss (LL) cavity Our goal is 51 MV/m ! (36 MV/m in operation) Record breaker, 262 hits in single season in Most famous Japanese baseball-player. Hp/Eacc = 36 Oe/(MV/m) (Designed at KEK in collaboration with DESY.) OP3

Snowmass ILC 2nd Workshop （ 20 th Aug ） Cavity shape Baseline Configuration (BC): TESLA shape Alternative Configuration (AC): Re-entrant (RE) / Low-Loss (LL) shapes Cavity gradient (acceptance by pulsed-power sampling test) BC: 35MV/m (TESLA shape) AC: 41MV/m (RE/LL shapes) Operation gradient (in the cryostat of main linac) BC: 31.5MV/m (TESLA shape) with RF distribution for 35MV/m AC: 36MV/m (RE/LL shapes) Our target at the beginning of 2005 was “to include the possibility of the operation gradient of 40 MV/m in the appendix of CDR”. The result was “the operation gradient of 36 MV/m (LL/RE) was adopted as AC”. OP4

Water quality control R&D PW TOC (ppb) Bacteria (count / mL) HPR with Updated Pure Water (PW) at KEK is OK up to the cavity performance of ~46 MV/m PW PW is OK up to 36 MV/m PW KEK is OK up to 46 MV/m KEK PW facility updated Correlation between HPR water-quality and cavity performance was checked. OP5