High Gradient Study in Superconducting RF Cavities

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1 High Gradient Study in Superconducting RF Cavities Kenji Saito KEK Accelerator Lab Outline 1. Fabrication and Surface Defects 2. Particle Contamination Control 3. Importance of Smooth Surface 4. Fundamental Field Limitation and A Way to TESLA 1000

2 Deep Drawing of half cell Talk by K.Saito in the High Gradient Workshop RF on 7 Oct in ANL Cavity Fabrication

3 Electron Beam Welding

4 Surface Defects on Niobium SC Cavities No foreign materials found Surface defects, holes can also cause TB Inclusion Cracks Sputter balls on EBW seam

5 Quench at Defects - Experience in the TRISTAN - Quench in the center cell T-mapping Field profile in pass bands Eacc [MV/m] T-mapping to detect the heat location (TRISTAN)

6 Quench defects - Experience in the TRISTAN - Electron beam weld seam Defect Tempeature mapping on the defects

7 Material Inspection Eddy current scanning Nb discs, dia. 265 mm at DESY Global view, rolling marks and defect areas can be seen Real and imaginary part of conductivity at defect, typical Fe signal

8 Thermal instability(quench) - Cures k ( Tc - Tb) ( Tc - Tb) ( K) Better quality High RRR niobium material: Hq= 4 µ RRR 4 r R r R 300 D n D n 3 Hp/Eacc ~ 43 Hp = 43 x 40 ~ 1720 Oe r D ~ 2mm For high gradient Eacc ~ 40 MV/m, RRR should be higher than several hundreds and defect size (radius :r D ) less than 1mm.

9 Simple Mechanical Grinding Barrel Polishing Soupy water Simple but slow material removal Polishing stones Centrifugal barrel polishing High material removal speed Succesufully developed CBP One week 4 hours

10 Field Emission Field Emission Electron loading due to field emission 1 2 Ê 1 ˆ 1 2 = A + B Ep[ MV / m], DÁ = -( A + B E MV m Q Q 2 p[ / ] ) Ë Q ( E ) o o o p Ê Flower-Nord Hiem Plot: = D 1 ˆ E15 E MV m15 Q 546 =S. f Á p exp( - ) = S p[ / ]. exp( -. E4 b E b E [ MV/ m] ) Ë O P P

11 Electron Trajectory in Field Emission Non-resonant electron loading due to field emitted electrons by tunneling effect mk

12 High Pressure Water Rinsing (HPR)

13 Effect of High Pressure Water Rinsing (HPR) TRINSTAN rinsing method without HPR TRISTAN rinsing method + HPR The first confirmation of HPR by P.Kneisel HPR is a very powerful tool to eliminate particle contamination on cavity surface!

14 Importance of Smooth Surface Chemical Polishing Electropolishing Rtm [mm] #15 #16 #19 Roughness (Rtm) [mm] #6 Roughness (Rtm) #13 Roughness (Rtm) #22 Roughness (Rtm) Removal Thickness [mm] Removal Thickness [mm]

15 High Gradient Performance with EP Electropolished Cavities in KEK (1300MHz single cells) After EP ~ 1.8 K Qo CP 10 9 K-14 : half cell annealed at 1400 o C, EP, HPR, Bake K-8 : BP, 760 o C anneald, EP, HPR, Bake K-9 : BP, 760 o C annealed, EP, HPR, Bake JL-1 : fabricated at CEBAF, CP, EP, HPR, Bake K-11 : CP, 760 o C annealed, EP, HPR, Bake K-22 : CP, EP, HPR, Bake Eacc [MV/m]

16 High Gradient TTF Cavities by EP Vertical Test Qo TESLA500 Specification TESLA800 Specification DESY CP+HPR (lower Q) DESY CP+HPR (higher Q) AC72 : EP(KEK)+HPR(DESY)+Bake(DESY) AC73 : EP(KEK)+HPR(DESY)+Bake(DESY) AC78 : EP(KEK)+HPR(DESY)+Bake(DESY) Eacc [MV/m] 35 MV/m has been achieved in 4 cavities of 8 electropolished TTF ones in the vertical test.

17 High Power Test Result of an Electropolished TTF Cavity Successful stable operation at 35 MV/m for more than 1000 hours. No degradation was seen in such a high gradient performance.

18 What happens in CP cavities at the high gradient? Field enhancement by J.Knobloch Chemicaly polished 130mm Started local sc breaking Qo Q-slope Eacc [MV/m] Eacc,max = 40 MV/m Field enhancement ~ 2 40/2 = 20 MV/m Local sc braking starts around 20MV/m heating Q-slope CClean Assembly

19 Clean Assembly

20 Cleanness and Field Emission Class 1000 Cleanness must be better class 100 against field emission.

21 Procedure for High Gradient SC Cavity Fabrication Use of high quality material Careful material inspection Remove large surface defects Mechanical grinding Make smooth & clean surface Electropolishing, HPR with ultra-pure - Clean Assembly Clean-room

22 Fundamental Field Limitation Vortex line nucleation model (VLNM) ( lh ) = ( xh ) x Hc l fi Hsh = Hc =, k (GL - parameter) l k x H sh a ( T) 2 2 c ( T ) ( T ) = H c, for AC application Hsh( T) = 2H c k( T ) k( T ) Flux line nucleation 2 Hc 2 Hc f = fcore + fmag = - px + pl 8p 8p Normal core : condensation energy magnetic energy Fluxoid core r~ x fcore 2 2 Hc 2 Hc = - px fmag = pl 8p 8p 2

23 Talk in the Workshop on High Gradient RF by K.Saito on 7 9 0ct in ANL A way to TESLA 1000 Eacc,max [MV/m] Saclay(HT,CP) CEBAF(HT,CP) CEBAF(CP,HPR) Cornell(HT,CP,HPP) KEK(EP,HPR) DESY/CERN/Saclay/KEK(EP,HPR) DESY/JLAB/KEK(Nb seamless cavity, EP,HPR) DESY/JLAB (Nb/Cu clad cavity, CP,HPR) INFN-LNL/KEK(Nb spun cavity, EP,HPR) 0 '90 '92 '94 '96 '98 '00 '02 Date (Year) The RF critical field of niobium might be limited around 1800 Oe by vortex line nucleation. [Oe] H c rf Hc1 Hc MJC VLNM VPNM Hcr Cornell: Puls Hcr KEK: CW T: Temperature [K] Resent cavity shape: Hp/Eacc = 42 ~ 44 Eacc =1800/(42 ~ 44) = 43 ~ 41 MV/m For beyond 40 MV/m, one should go to the cavity design with smaller Hp/Eacc ratio. 50 MV/m Hp/Eacc=36 ; TESLA 800 (35MV/m) TESLA 1000 (45MV/m)

24 Talk in the Workshop on High Gradient RF by K.Saito on 7 9 0ct in ANL Summary 1) Today, the high gradient of niobium superconducting RF cavity has been achieved 40 MV/m with single cell cavities. 2) This achievement also has been done in TESLA cavities. 3) Current my understanding for high gradient cavity production is : Use high pure Nb material (RRR=200~300) with careful material inspection, Remove surface defects by mechanical grinding prior to chemical process, Make smooth surface using electropolishing, Finish clean surface by high pressure water rinsing. Assembly cavities in a clean-room better than class ) For the TEALA1000, the gradient must be upgraded to 45MV/m. Cavity shape with a smaller Hp/Eacc rato ~36 has to be redesigned for such a high gradient.