State-of-the-Art and Future Prospects in RF Superconductivity

Transcription

1 State-of-the-Art and Future Prospects in RF Superconductivity Kenji Saito* High Energy Accelerator Research Organization (KEK) / Michigan State University (MSU), FRIB Specifics and Constrains in SRF Cavities SRF Performance on Niobium Cavities Over The Past 50 Years Future Prospects * Moving from KEK to MS, FRIB K.Saito IPAC2012 Talk 1

2 SRF Specifics and Constrains : London penetration depth ~500Å Surface resistance is very small and very small intrinsic surface heating. Unexpected phenomenon dominates the heating and limits the performance. Surface current flows in very shallow skin surface (). Cavity performance strongly depends on the surface. Thermal superconducting state is very small. K.Saito IPAC2012 Talk 2 High thermal conductivity of the cavity low temperature is very important.

3 Limited SRF Cavity Performance Thermal instability Ideal performance : Fundamental limit due to RF magnetic field Qo Multipacting Q-slope Field emission Quench Hydrogen Q-disease Eacc Field emission and Multipacting are related to surface contamination, which are mitigated by making clean SRF surface. Thermal instability occurs at surface defects, which is suppressed by using high thermal conductivity Nb: High pure niobium. Hydrogen Q-disease happens by niobium-hydride K.Saito IPAC2012 doped Talk hydrogen during surface 3 processing. This is cured by hydrogen degassing.

4 Understanding &Technologies Over Past 50 Years 1965 ~1975 Stanford University IHEP 1978~1985 Cornell University Muffinten High current 1996 ~ A KEKB CESER Eacc ~ 5MV/m Storage Ring 90~2000 TRISTAN(KEK) HERA(DESY) LEP-II(CERN) Recirculating machine CEBAF(Jlab), DALLINAC(Darmstadt) Many light source applications 1975 ~ 1980 Heavy Ion application Germany, Kalushuhe Nuclear Institute(KFK) Argonne National labs (ANL) Mechanical Tuner California Institute of Technology (CLTEC), State University of New York FEL 1995 ~ Jlab-FEL, JAERI FEL Eacc ~ 30MV/m HPR, CBP High Gradient application 1990~2008 TESLA(DESY) High gradient cavity shape EURO XFEL 2015~ ILC Heavy ion 1978 ~ ATLAS(ANAL) SNS 2006 ~ EP+Bake against Q-slope Large Grain Nb 2014 ~ FRIB,... K.Saito IPAC2012 JAERI Tandem Talk buster RIA 4 INFN LEGNRO

5 Multipacting Multipacting /Field MP Emission limited the gradient seriously in the early SC cavity R&D in Stanford Uni. Courtesy of H.Padamsee 1 point MP K.Saito IPAC2012 Talk 5

6 SRF Clean Surface by Adopting Semiconductor Technologies Courtesy of Cornell Courtesy of Cornell Particles are seeds of FE! Use ultrapure water, clernroom assembly K.Saito IPAC2012 Talk 6

7 Vendor High Purity Nb Material Production T [K] Vapor Pressure vs. T RRR R R R Change of RRR over melting times A/R Max Average Min R R R Courtesy of Tokyo Denkai Melting Cycles Evaporation Crucial conditions RRR 1) higher vacuum 2) larger molten surface 3) multi-melting Maximum RRR Titanification K.Saito IPAC2012 Talk

8 Chemical Processing for Production Cavities Buffered Chemical Polishing, CEBAF Horizontal Electropolishing, TRISTAN K.Saito IPAC2012 Talk 8

9 World Wide SC Accelerators in 1990' Nb coated on Copper cavities K.Saito IPAC2012 Talk 9 ~ 5MV/m CW operation

10 Understanding &Technologies over 50 years 1965 ~1975 Stanford University IHEP 1978~1985 Cornell University Muffinten High current 1996 ~ A KEKB CESER Eacc ~ 5MV/m Storage Ring 90~2000 TRISTAN(KEK) HERA(DESY) LEP-II(CERN) Recirculating machine CEBAF(Jlab), DALLINAC(Darmstadt) Many light source applications 1975 ~ 1980 Heavy Ion application Germany, Kalushuhe Nuclear Institute(KFK) Argonne National labs (ANL) Mechanical Tuner California Institute of Technology (CLTEC), State University of New York FEL 1995 ~ Jlab-FEL, JAERI FEL Eacc ~ 30MV/m HPR, CBP High Gradient application 1990~2008 TESLA(DESY) High gradient cavity shape EURO XFEL 2015~ ILC Heavy ion 1978 ~ ATLAS(ANAL) SNS 2006 ~ EP+Bake against Q-slope Large Grain Nb 2014 ~ FRIB,... K.Saito IPAC2012 JAERI Tandem Talk buster RIA 10 INFN LEGNRO

11 High Pressure Water Rinsing (HPR) Courtesy of P.Kneisel Q-slope TRISTAN rinsing method HPR rinsing K.Saito IPAC2012 Talk 11

12 Superiority of EP on HG and Baking Effect of High Field Q-Slope EP S-3 Cavity : CP after EP & EP after CP No Heat treated, RRR=230 Qo BCP limits HG to ~25MV/m but EP pushes it to 40MV/m 10 9 KEK3: EP50 m, HPR, Bake KEK4: EP+70 m, HPR, Bake KEK5: CP60 m, HPR, Bake KEK6: CP+70 m, HPR, Bake KEK7: EP50 m, HPR, Bake KEK8: EP+50 m, HPR, Bake KEK9: EP+50 m, HPR, Bake BCP degrades HG but EP recovers the HG Eacc [MV/m] Courtesy of Saclay Qo S-3 cavity : EP, HPR, No bake S-3 cavity : EP, HPR, Bake(120 o C) After Bake BCP+ Bake works partially on Q-slope Before Bake EP+ Bake eliminates the Q-slope K.Saito IPAC2012 0Talk Eacc [MV/m]

13 High Gradient Cavity Shape for 50MV/m Cavity shape designs with low Hp/Eacc TTF: TESLA shape Reentrant (RE): Cornell Univ. Low Loss(LL): JLAB/DESY Ichiro ー Single(IS): KEK Courtesy of J.Sekutowicz TESLA LL RE IS Diameter [mm] Ep/Eacc Hp/Eacc [Oe/MV/m] R/Q [W] G[W] K.Saito IPAC2012 Talk 13 東大物理大学院集中講義ノート 2011 Eacc max

14 Eacc,max [MV/m] History of upgraded gradient in SRF High pressuer 1 st Breakthrough! water rinsing (HPR) Electropolshing(EP) + HPR O C Bake 2 nd Breakthrough! RE, LL, IS shape New Shape 20 Chemical Polishing 10 '91 '93 '95 '97 '99 '00 '03 '05 '07 Date [Year] K.Saito IPAC2012 Talk 14

15 12GeV CEBAF Upgrade by State-of-Art Courtesy of C.Reece K.Saito IPAC2012 Talk 15

16 ILC Baseline Gradient and Recent Achievement Courtesy of R.Gen Hpk mt K.Saito IPAC2012 Talk 16

17 ILC Baseline Gradient and Recent Achievement Courtesy of R.Gen Hpk mt Hpk mt K.Saito IPAC2012 Talk 17

18 ILC Baseline Gradient and Recent Achievement Courtesy of R.Gen 85% accept Presumed distribution accept performance Theoretical limit TESLA shape Theoretical limit TESLA shape Hpk mt 36.9+/-1.85MV/m (-10% from the limit) Hpk mt Gradient [MV/m] K.Saito IPAC2012 Talk 18

19 Large Grain Nb Cavities Courtesy of DESY Directly sliced Nb sheet from Ingot 270mm BCP LG+BCP+HPR EP LG+BCP+ final EP+HPR+Bake K.Saito IPAC2012 Talk 19

20 Improvement of High Gradient in Various SRF Fields DESY AC155, AC158 Hpk Oe New 9-cell record Courtesy of R.Gen Best elliptical ILC 1 TeV Best QWR, spoke ESS Elliptical, spoke Project X XFEL ILC 500 GeV Best HWR FRIB QWR, HWR CEBAF 12 GeV 650 Oe > 95% confidence CEBAF 4 GeV K.Saito IPAC2012 Talk 20

21 Fundamental Field Limit with Nb Cavity Courtesy of Cornell Superheating Model H sh ( T ) c H c 1 T T c 2 C()= ( Oe) Eacc Hsh /( H p / Eacc) 56 MV / m 36( Oe/[ MV / m]) K.Saito IPAC2012 Talk 21

22 Future Prospects in SRF Cavity High gradient shape Improved space factor Martial ~15% upgrade + + ~10% upgrade improve ~10% upgrade Nb bulk Cavity 50~60MV/m fundamental Limit: Vortex Hc1 Exploring New Materials for 100~200MV/m & High Q Stopped Nb 3 Sn(~1995) by weak link grain boundary effect or Failed in HTS (1990') due to fundamental reason (d-wave) Thin Film Multi -Layer Cavity Alex Gurevich, APL 88, (2006) 20~30 years beyond Understand Substrate boundary Niobium Coated Film Cavity Develope best coating technology K.Saito IPAC2012 Talk 22

23 Superconducting Materials 23 K.Saito IPAC2012 Talk 23

24 Superconducting Materials No way to SRF application : High-Tc d-wave cuprates are SRF unsuitable : Rs T 2 Classic HTSs are s-wave superconductors: Rs ~ exp(-/k B T). If weak link mechanism is solved, excellent performance is expected. K.Saito IPAC2012 Talk 24 24

25 Superconducting Materials 1E+12 1E+11 Nb 3 Sn Nb No way to SRF application : High-Tc d-wave cuprates are SRF unsuitable : Rs T 2 Qo 1E+10 1E+09 Nb 3 Sn QUENCH 1E Epk (MV/m) 1.5 GHz Nb 3 Sn cavity (Wuppertal, 1985) 1.3 GHz Nb cavity (Saclay, 1999) Classic Nb 3 Sn is HTSs a classic are HTS, s-wave however, superconductors: K.Saito not yet IPAC2012 achieved Rs Talk ~ exp(-/k the expected B T). performance 25 due If to weak link mechanism is at solved, grain boundary. excellent performance is expected. 25

26 Overcoming niobium limits (A.Gurevich, 2006) : Keep niobium but shield its surface from RF field to prevent vortex penetration Use nanometric films (w. d < ) of higher Tc SC : => H C1 enhancement Example : NbN, = 5 nm, = 200 nm H C1 = 0,02 T 20 nm film => H C1 = 4,2 T Nb Alex's Idea of TFML(complicated) Applied x 200 Courtesy of A. Gurevich I-S-I-Sv v V1 V2...Vn Outside wall H H Nb Nb H appl e Nd (similar improvement expected with MgB 2 or Nb 3 Sn) Cavity's internal surface High H C1 => no transition, no vortex in the layer Applied field is damped by each layer Insulating layer prevents Josephson coupling between layers Applied field, i.e. accelerating field can be increased without vortex nucleation K.Saito IPAC2012 Talk 26 Thin film w. high Tc => low R at low field => higher Q 26

27 Material T c (K) H c [T] Potential TFML Materials H c1 [mt] H c2 [T] 0) [nm] [mev] Courtesy of A.Gurevich Nb B 0.6 K 0.4 BiO Nb 3 Sn NbN MgB ; 7.1 Ba 0.6 K 0.4 Fe 2 As > >5.2 High-T c d-wave cuprates are SRF unsuitable (R s T 2 instead of R s exp(-/t) Large s-wave gap (good for SRF) is usually accompanied by low H c1 (bad for SRF) d-wave s-wave MgB 2 K.Saito IPAC2012 Talk 27 M. Iavarone et al., PRL 89, (2002)

28 Material T c (K) H c [T] Potential TFML Materials H c1 [mt] H c2 [T] 0) [nm] [mev] Courtesy of A.Gurevich Nb B 0.6 K 0.4 BiO Nb 3 Sn x 200 NbN MgB ; 7.1 Ba 0.6 K 0.4 Fe 2 As > >5.2 High-T c d-wave cuprates are SRF unsuitable (R s T 2 instead of R s exp(-/t) Large s-wave gap (good for SRF) is usually accompanied by low H c1 (bad for SRF) d-wave s-wave MgB 2 K.Saito IPAC2012 Talk 28 M. Iavarone et al., PRL 89, (2002)

29 Thin Films: Niobium State of The Art 1.5 GHz Nb/Cu cavities, sputtered w/ Kat 1.7 K (Q 0 =295/R s ) Nb Nb/Cu CERN 2000 Bulk Nb like performance Find best film coating technology Study the effect K.Saito of substrate; IPAC2012 TalkGrain, Orientation, Preparation 29

30 Courtesy of SLAC New Material Hunting p Klystron High Peak short pulsing method T=7K T=10K T=4K 55dB 1 Cryostat Qo Cavity dB d B 5 45dB 6 Mode converter Bend X-band 100nm MgB 2 /20nm Al 2 O 3 /Nb Load Forward and reflected power trace S15-1, H~23mT Hpeak (mt) Power (W) Reflected Forward K.Saito IPAC2012 Talk 30 Time(s)

31 Summary The SRF performance of Nb bulk cavity has been very much improved in last two decades and is reaching the fundamental limit ~60MV/m. This state-of-the-art technology is based on the well understanding of performance limits and thanks to its suitable feedback to technology R&D; High Purity Nb material vendor production, Electropolishing, High pressure water rinsing, Ultrapure water, Cleanroom assembly, Baking, Hydrogen degas annealing and so on. Thin film multi-layer (TFML) coated cavity is proposed as the post niobium cavity for the gradient MV/m. MgB 2, NbN, Nb 3 Sn are most candidate materials for TFML. Thin film coating technology has its specific issues learning from Nb film coated cavity, therefore TFML might not be straightforth to achieve the expected performance, it will take a time. High peak puls measurement method is very suitable for new material hunting. This method could make faster finding the new material. K.Saito IPAC2012 Talk 31

32 Backup Slides K.Saito IPAC2012 Talk 32

33 Bulk Nb ultimate limits : not far from here! Cavité 1DE3 : Saclay T- DESY Film : courtoisie A. Gössel + D. Reschke (DESY, Début 2008) The hot spot is not localized : the material is ~ equivalent at each location => cavity not limited /local defect, but by material properties? K.Saito IPAC2012 Talk 33 33

34 EBW defects limitig the gradient around 20MV/m Fine grain EP twin defects causing quench 17 MV/m. Cavity by a new vendor K.Saito IPAC2012 Talk 34

35 Defect Elimination bytumbling/cbp Water stone FNAL ILC 9-cell cavity equator Before CBP (equator EBW seam) After CBP December 2010, FNALsucceeded the imprving gradient by CBP+Annealing+EP(40m) from 18MV/m limit to 35MV/m K.Saito IPAC2012 Talk 35