Review of Frontier Workshop and Q-slope results

Transcription

1 Review of Frontier Workshop and Q-slope results Gianluigi Ciovati Jefferson Lab 12 th SRF Workshop, July 10 th -15 th 2005, Cornell

2 Workshop on Pushing the Limits of RF Superconductivity September 22-24, 2004 Hold at Argonne National Laboratory 64 participants invited to discuss how to push the limits of RF superconductivity for particle accelerators 30 talks divided in three sessions: Ultimate field limits, new materials, new geometries High Q, field emission, Q-slopes Future research paths to ultimate performance 2

3 Workshop s website: Workshop s presentations available on-line ( Workshop s proceedings published as Argonne Report ANL-05/10, March

4 Outline Overview of knowledge on high field Q- slope (or Q-drop ) Improved oxygen diffusion model and possible explanation of Q-drop Summary 4

5 Q-drop: obstacle before the ultimate limit Q 0 Recovered by low temperature in-situ baking Sharp drop of the quality factor at B p 100 mt without field emission Peak surface field 5

6 BCP treated cavities (1) Fine grain ( 50µm), rough surface (5-10µm) 1E+11 T=1.55K T=1.55K after bake Q 0 1E+10 1E GHz Single Cell (JLab) E acc [MV/m] P. Kneisel et al.-pac 05-TPPT076 R.L. Geng et al.-srf 99-TUP021 Before bake After bake 1.3 GHz Single Cell (Cornell) H. Padamsee et al.-frontier Workshop-p. 291 Q-drop STILL PRESENT after baking 6

7 BCP treated cavities (2) Larger grains (1-5mm) by post-purification, rough surface (5-10µm) 1.00E+11 T=1.37K T=1.37K, 120C 48hr bake Q E E GHz CEBAF Single Cell (JLab) E acc [MV/m] G. Ciovati-J. Appl. Phys. 96, p (2004) 1.3 GHz Single Cell (Saclay) B. Visentin et al.-epac 02-THPDO013 Q-drop RECOVERS after baking 7

8 BCP treated cavities (3) 1.0E+11 T=2K, after 120C, 48h air baking T=2K Air baking Fine grain Larger grain (post-purif.) Q 0 1.0E E E acc [MV/m] 1.5 GHz CEBAF Single Cell (JLab) G. Ciovati, unpublished 1.3 GHz Single Cell (Saclay) B. Visentin et al.-srf 03-MOP19 Reduced Q-drop improvement Higher residual resistance 8

9 EP treated cavities (1) Fine grain ( 50µm), smooth surface (2-5µm) 1.E+11 Before bake After bake Q0 1.E GHz Single Cell (Saclay) B. Visentin et al.-srf 03-TUO01 1.E E acc [MV/m] 1.3 GHz 9-Cell Cavity AC76 (DESY) R.L. Geng et al.-srf 99-TUP021 L. Lilje -EPAC 04- WEAOCH03 Q-drop RECOVERS after baking 9

10 EP treated cavities (2) Larger grains ( 1mm) by post-purification, smooth surface (2-5µm) 1E+11 T=2K T=2K after 120C 48h bake 1.E+11 Before bake After bake Q 0 1E+10 Q0 1.E+10 1E GHz CEBAF Single Cell (JLab) E acc [MV/m] G. Ciovati, J. Mammosser, unpublished 1.E GHz 9-Cell E Cavity acc [MV/m] AC78 (DESY) L. Lilje -EPAC 04- WEAOCH03 Q-drop RECOVERS after baking 10

11 Single grain BCP cavities One grain, very smooth surface (<1µm) with BCP 1E+11 Baseline After 120 C, 24 h bake T = 2 K 1E+11 After 120C 48h bake Baseline T = 2 K Q 0 1E+10 Q 0 1E+10 1E E acc [MV/m] A. Wu - unpublished 1E Q-drop RECOVERS after baking E acc [MV/m] 2.3 GHz CEBAF LL Single Cell (JLab) 2.2 GHz CEBAF HG Single Cell (JLab) P. Kneisel et al.-pac 05-TPPT076 11

12 Statistic on Q-drop onset field No. of tests JLab data on 1.5 GHz single cells Before post-purification Large grain After post-purification Large grain Electropolished Q-drop onset field range (mt) 12

13 Conclusions (1) Q-drop is common to BCP, EP and Single crystal cavities The onset field is the the density of grain boundaries is The baking effect is different on polycrystalline fine grain Nb treated by BCP and EP, but is similar for density of grain boundaries Air baking less effective than UHV bake 13

14 Origin of Q-drop: E- or B-field? Interface Tunnel Exchange Model 1 : resonant absorption of energy due to tunneling of normal e - between metal and oxide in the presence of strong E- field R SE = b [exp(-c/e rf ) - exp(-c/e 0 )] Excellent fit to experimental data All other models 2,3,4,5 : Q-drop is a magnetic field effect 1 J. Halbritter, IEEE Trans. on Appl. Superc., 11 No. 1 (2001) K. Saito, Proc. 11th SRF Workshop, Travemuende, Germany (2003), ThP17 3 A. Gurevich, Argonne Report ANL-05/10, (2005) p H. Safa, Proc. 10th SRF Workshop, Tsukuba, Japan (2001) p J. Knobloch et al., Proc. 9th SRF Workshop, Los Alamos, NM (1999) p

15 Measurements on TE 011 mode 1E+11 TE011 TM010 TE011 after 120C 30h bake TM010 after 120C 30h bake T = 2 K 1E+11 TM010 TE011 TM010, 115 C 22h bake TE011, 115 C 22h bake T = 2 K Q 0 1E+10 1E+10 1E GHz CEBAF Single Cell 1E B peak [mt] B p [mt] G. Ciovati and P. Kneisel Frontier Workshop p. 74 After post-purification, Q-drop appears in TE 011 mode (zero E-field on surface) with the same field dependence as for TM 010 mode and recovers after baking 15

16 HF treatment HF used to remove oxide layer after baking. The surface is being re-oxidized after rinsing and air exposure 1E+11 Q 0 1E+10 1E+09 1E+08 Test E5 - March 2000 Test I1 - January 2004 C1-03 ( KEK - Saclay) E5 : after baking 120 C/60h I1 = E5 + air exposure 46 months + 20' HF + HPR quench E acc ( MV/m ) 1.3 GHz Single Cell (Saclay) B. Visentin Frontier Workshop p. 94 Q-drop is NOT restored after a new oxide layer is formed 16

17 Temperature maps before baking Q 0 = B p = 110 mt DESY JLAB T (K) L. Lilje et al.-srf 99-TUA001 CORNELL Azimuth S1 S3 S5 S7 S9 S11 S13 S15 Top Iris G. Ciovati-unpublished Equator 0 Bottom Ii G. Eremeev et al.-srf 03-MoP18 Patchy losses develop at the cavity equator (high magnetic field region) 17

18 Conclusions (2) The Q-drop is due to high magnetic field The benefit of baking is maintained after exposure to air for 3 years high pressure water rinsing build-up of new oxide layer 18

19 Baking effect on low field R s Cornell Saclay JLab, Desy Decrease of R BCS due to strong of l and slight of energy gap The physics of the niobium surface changes from CLEAN (l > 200 nm) to DIRTY LIMIT (l 25 nm ξ 0 ) (300 nm depth) G. Ciovati-J. Appl. Phys. 96, p (2004) 19

20 Surface studies on samples Natural oxide (Nb 2 O 5 ) decomposes into sub-oxides (NbO, NbO 2 ), thinner oxide layer Segregation of interstitial oxygen near metal-oxide interface, measured conc. up to 10 at.% Hydrogen? Very difficult to measure accurately, no clear data yet Oxygen (oxide) Niobium Carbon Hydrogen Thickness [Monolayers] 20,0 18,0 16,0 14,0 12,0 10,0 8,0 6,0 4,0 2,0 0,0 7,4 3,1 0,7 2,2 0,2 1,5 2,1 0,1 0,4 2,3 0,1 4,2 7,6 4,9 7,7 2,5 11,5 3,8 3,7 5,8 2,2 1,7 0,5 0,1 1,9 1,4 2, Carbon H2O Nb2O5 NbO2 NbO NbO0.2 Baking Time [hours] ~ oxide-metal interface C. Antoine Frontier Workshop p. 65 A. Dacca` et al., Appl. Surf. Sci., 126 (1998) p

21 How deep is the baking benefit? H. Padamsee et al. Frontier Workshop p. 293 Cavity test result: original Q-drop is obtained after 20 nm thick Nb was converted to oxide Sample analysis: oxygen concentration in 20 nm depth reduced by baking 21

22 Susceptibility meas. on samples r GL (Ginzburg-Landau) =1.695 In these measurements: r 32 >1.8 r 32 =B c3 /B c2 : depends on bake temperature and duration Two possible explanations: B c2 surf > B c2 bulk B c3 =r GL B c2 surf ( naïve model) impurities in a layer d ξ r 32 =1.67[1+(1-χ G ) 1.7 d/ x ] T bake, time: d, l S. Casalbuoni et al. SRF 03 WEO13 22

23 Magneto-optical meas. on samples Study flux penetration in Nb samples Optical image, large grain sample (by post-purif.) BCP treated MO image at 57 mt, 7 K Evidence of flux penetration along grain boundaries! P. J. Lee et al. Frontier Workshop p

24 Conclusions (3) Impurity diffusion occurs during baking Decrease of mean free path (cavity meas.) Susceptibility measurements (samples) XPS analysis (samples) High O concentration near the surface ( 20nm deep) reduced by baking Flux penetration may occur at grain boundaries Is the enemy? 24

25 Oxygen diffusion models Solve diffusion equation Infinite source of oxygen at the surface Fixed amount of oxygen S. Calatroni et al. SRF 01 PR025 H. Safa SRF 01 MA008 25

26 Improved model Contribution from dissociation of oxide layer Nb2O5 k NbO ( ) a k T Ae E RT = A /s E a 135 kj/mol ( ) u x t t ( ) u x t = + q x t T x 2,, D ( T) e Solution: 112kJ mol RT 2 cm s 0 (,, ) ( ) uk T e ( ) ktt δ ( x) Diffusion equation with source at x=0 First order reduction kinetic (, ) u x t = u 0 ( ) π DT t 0 ( ) k T e ( ) kts t s e x 4D T 2 ( )( t s) Concentration of oxygen produced by the oxide reduction 1000 at.% nm, obtained from comparison with data B. R. King et al., Thin Solid Films 192, p. 351 (1990) Q. Ma and R. Rosenberg SRF 01 PR007 26

27 Diffusion of interstitial oxygen existing before baking ( ) v x t t Solution: ( ) v x t = D ( T) v 2 ( 0,0) = v x 2,, 0 (, ) v x t = General solution: v 0 ( ) 4π DT t e 2 x 4D T t ( ) 10 at.% nm I. Arfaoui et al., J. Appl. Phys. 91, p (2002) (, ) = (, ) + (, ) c x t u x t v x t 27

28 Comparison with data 0.35 Oxygen concentration (at. %) Data c(x, 45 h) u(x, 45 h) v(x, 45 h) Comparison with data obtained by successive anodizations on cavity baked at 145 C, 45h x (nm) Data are from: P. Kneisel SRF 99 - p

29 Oxygen concentration at the surface as function of baking temperature 1.2 Oxygen concentration (at. %) Initial interstitial oxygen 0.2 c(0, 48 h) u(0, 48 h) v(0, 48 h) From oxide decomposition T ( C) Optimum baking temperature: lowers O conc. near the surface 29

30 Schematic of the Nb surface Before baking After baking λ λ Interstitial oxygen Nb 2 O 5 Suboxides (NbO 2, NbO) Oxide cluster 30

31 How is O related to the Q-drop? J. R. Waldram, Superconductivity of metals and cuprates, IoP, 1996 Q-drop onset κ 2.4 Interstitial oxygen concentration of 0.56 at.% # # C.C. Koch et al.,phys. Rev. B 9 (1974) p

32 Flux penetration at reduced B c1 There exist a surface barrier which prevents 1 vortices from penetrating even above B c1 BUT the surface barrier can be reduced to zero in non-uniform (rough) surfaces 2 Surface morphology Changes in surface barrier Onset of Q-drop ( smoother surfaces) 1 C. Bean and J. D. Livingston, Phys. Rev. Lett. 12 p. 14 (1964) 2 R. D. Blois and W. de Sorbo, Phys. Rev. Lett. 12 p. 499 (1964) 32

33 Losses due to flux penetration Rabinowitz 1 calculated the power loss due to a normal conducting fluxoid in rf field R s (B p ) exp(d/b p ) D 1 kf NRa B ln ( f d ) a k D 2.2 T (with ρ=ρ n ) D 5 8 mt from experiments 1 M. Rabinowitz, J. Appl. Phys. 42 p. 88 (1971) 33

34 Q-drop onset freq. dependence Q-drop onset B p (mt) Polycrystalline Nb cavities Saclay data No Q-drop found on low-beta, low frequency cavities up to B p 100 mt 2 Frequency (GHz) TE mode data Rabinowitz 3 calculated that for the case of negligible viscous damping and negligible pinning B p_flux_penetration ω R BCS, global heating 1 B. Visentin Frontier Workshop p M. Kelly et al. Frontier Workshop p M. Rabinowitz, J. Appl. Phys. 42 p. 88 (1971) 34

35 Summary The onset field of the Q-drop and the effect of baking seem to depend on the density of grain boundaries The Q-drop appears to be driven by magnetic-field There is a high oxygen concentration at the oxide/metal interface which seems to be diluted by baking An improved oxygen diffusion model can be linked to a change of the onset field for flux penetration to explain the Q-drop and the baking effect 35

36 Open issues Is there enough experimental evidence to exclude H from playing a role in the Q- drop? How can we test the hypothesis of flux penetration during Q-drop? Interpretation of experimental data against O hypothesis: Saclay data: Q-drop is not restored after HF rinsing of baked cavity (O conc. near surface restored as before baking) 36

37 Acknowledgements P. Kneisel, G. Myneni, J. Halbritter, J. Delayen, A. Gurevich, H. Padamsee, C. Antoine 37