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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/JSPSEPAC, 26-30 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
Treatment ProcessRemoval
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 10Remove dusts after CBP and
prepare smooth surface before EP
Annealing / degassing
750 OC for 3 hours by furnace -
Release mechanical stress and degass hydrogen
Electro-Polishing (EP)
Chemical + electronic reaction 80 Prepare very smooth surface
High-Pressure Rinsing (HPR)
Rinse with pressured Ultra-Pure Water
(UPW)-
Remove particles and make clean surface
Baking120 OC 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 toestablish the KEK recipe.
Deep-drawing Nb cups
4
Centrifugal Barrel Polishing (CBP) / Mechanical grinding
Stones and water
Rough stones Fine stones
KEK recipeRough stones + water : 4 hours x 3 timesRemoval thickness = 25 um x 3 = 75 um
Fine stones + water : 4 hours x 3 timesRemoval thickness = 20 um x 3 = 60 um
Total removal thickness = 135 um.
5
CBP removal thickness depends on surface roughness at EBW seam
Before CBPEquator EBW seam (inner surface)
2nd CBPGood EBW
1st CBP(30um)
Bad EBW
3rd CBP (90um) 12th CBPRemoval thickness> 200 um.
Removal thickness= 60 um.
Bad EBWwith a pit
2nd CBP (60um) 13th CBPRemoval thickness> 200 um.
6
Light Chemical Polishing (CP)and annealing / degassing
Light Chemical Polishing (CP)HF(46%) : HNO3(60%) : H3PO4(85%) = 1 :1 : 1 in volumeCP for 1 minute at 25 OC.Removal thickness = 10 um. Prepare smooth surface before EP.
Annealing/Degassingby furnace750 OC for 3 hoursDegassing of hydrogenis important. Temperature and time is optimized for cavity softness and cost.
7
Cavity
Beam Pipe
Cathode
Cathodebag
AcidLevel
EP acid circulates
Circulating EP acidH2SO4(95%) : HF(46%)
= 10 : 1 in volume
ElectricCurrent
Voltage ~ 20 V, Current ~ 40A
Horizontally Rotated Electro-Polishing (EP)
Back to EP-acidreservoir
tank (100L)
Well established method
8
High Pressure Rinse (HPR)
Ultra Pure Water Specific resistance =18 M Ohm cmTOC = 12 – 22 ppbbacteria = 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 contaminationon the inner surface of Nb cavity.
9
10
20
30
40
50
60
70
Eac
c,m
ax [
MV
/m]
Date (Year)'91 '00'95 '05'93 '97 '03
High pressuer water rinsing
(HPR)
Electropolshing(EP)
+ HPR + 120 OC Bake
New Shape
Chemical Polishing
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
108
109
1010
1011
0 10 20 30 40 50
ICHIRO Single cell 1st round
IS#2-2nd
IS#3-1st
IS#4-1st
IS#5-1st
IS#6-1st
IS#7-1st
Qo
Eacc[MV/m]Statistics
Accidental16.7%
FE initiated by MP16.7%
Quench16.7%
No problem50%
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 mistakein HPR or
careless assembly in clean room
Sulfur or oxidizationContaminationin EP process
Defect of materialor roughnessat EBW seam
Particle on the surface should
be removed
Particle on the surface should
be removed
Much thicker removal
Re-HPR andcareful assembly
Light EP with fresh EP acid
or light CP
EP with thick removal
or CBP
How toimprove?
Additionaltreatments
Possiblesources offailure
No removal Light removal Heavy removal
12
108
109
1010
1011
0 10 20 30 40 50
IS#2-3rd
IS#3-3rd
IS#4-1st
IS#5-1st
IS#6-1st
IS#7-2nd
Qo
Eacc[MV/m]
After HP-rins (20min) for IS#2 and 3
50 % failure from very thin surface?
Pilot study for new recipeHF 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%) iscoming 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 cavitiesEacc > ~44 MV/m
IS#6IS#4
IS#5
Fresh EP acid was used forlast 3 um to avoid contamination.
14
IS#2 IS#3 IS#4 IS#5 IS#6 IS#7
KEK RecipeEacc 36.9 31.4 45.1 44.2 48.8 28.3
Qo 1.53e10 8.66e9 9.07e9 5.38e9 9.56e9 1.94e9
+re-HPR37.6 32.7 42.7 51.4 29.9
1.42e10 7.27e9 5.66e9 7.78e9 1.1e10
+HF rinse
+HPR
37.1 36.7 50.4 50.2 30.0
1.64e10 1.43e10 9.97e9 3.9e9 3.33e9
+CP(10um)
+HPR+Baking
41.0 40.5
6.65e9 5.57e9
+EP (fresh acid, 3 um)
+HPR+Baking
41.6 40.3 41.1
1.00e10 1.28e10 1.17e10
+EP(20-30um)+EP(3um)
+HPR+Baking
47.1 47.8
1.06e10 7.81e9
+EP(20-30um)+EP(3um)
+HF rinse+HPR+Baking
44.7 53.5 43.9
0.98e10 7.83e9 1.17e10
Results of series tests for six ICHIRO Single-cell (IS) cavities
15
IS#2 IS#3 IS#4 IS#7KEK recipe (EP 80um)
+HF rinse +HPRKeep 100K
for 12hKeep 100K
for 21hKeep 100K
for 12h
+EP (fresh acid, 3 um)
+HPR+BakingKeep 100K
for 12hKeep 100K
for 38hKeep 100K
for 40h
+EP(20-30um)+EP(3um)+ HF rinse
+HPR+BakingKeep 100K
for 24h
CP(10um)+EP(20-30um)+EP(3um)
+ HF rinse +HPR+BakingKeep 100K
for 24h
No H Q-disease
H Q-disease was checked by keeping cavities at 100K for >12 hoursin 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 recipeFE and Q-slope removed Mean = 44 MV/m.Sigma = 6 MV/m.
Eacc [MV/m]
Repeated tests removedMean = 44 MV/m.Sigma = 7 MV/m.
All testsMean = 38 MV/m.Sigma = 9 MV/m.
Eacc [MV/m]
Eacc [MV/m]
17
Summary• Six ICHIRO-shape single-cell cavities were fabricated for series tests t
o establish the KEK recipe. • The KEK recipe : CBP (mechanical grinding), light CP (10um), annealing / degassing (750 OC for 3 hours), EP (80 um), HPR (Ultra-Pure Water) for 1 hour, Baking (120 OC 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 cavitiesOperation at 31.5 MV/m
Four 9-cell ICHIRO LL cavities
Already fabricated.
STF Phase 1
Operation at 36 MV/m
STF @ 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/mQ0 = 3.4e9
MPEacc = 29.3 MV/mQ0 = 1.02e10
HOM study
Tuner study
MP at HOM coupler?
1st 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 2004.
Most famous Japanese baseball-player.
Hp/Eacc = 36 Oe/(MV/m)(Designed at KEK in collaboration with DESY.)
OP3
Snowmass ILC 2nd Workshop( 20th Aug. 2005)
• 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
UPW@Nomura
PW (old)@KEK
TOC (ppb)
Bacteria(count / mL)
HPR with Updated Pure Water (PW) at KEK is OK up to the cavity performance of ~46 MV/m.
10 - 20 50 - 150
PW (new)@KEK
50 - 150
0 - 5 10 - 200
PW (old) @KEK is OK up to 36 MV/m
5 - 15
PW (new) @ KEK is OKup to 46 MV/m
KEK PW facilityupdated
Correlation between HPR water-quality and cavity performance was checked.
OP5