Magnetic Screening of Nb. N Multilayers Samples C. Z. ANTOINE, J. LECLERC, Q. FAMERY CEA, Irfu, Centre d'Etudes de Saclay, 91191 Gif-sur-Yvette Cedex, France J-C. VILLEGIER, CEA, INAC, 17 Rue des Martyrs, 38054 Grenoble-Cedex-9, France G. LAMURA CNR-SPIN-GE, corso Perrone 24, 16124 Genova, Italy A. ANDREONE CNR-SPIN-NA and Dipartimento di Scienze Fisiche, Università di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 1
Limits in a RF cavity Classical theory BCS (GL) + RF : Magnetic RF field limits Eacc : Eacc HRF Phase transition when magnetic HRF ~> HSH (superheating field) For Nb HSH ~1. 2. HC (thermodynamic), for higher TC SC HSH ~ 0. 75. HC Higher Tc => higher Hc => higher Eacc But… Bulk Nb 3 Sn cavity : relative failure Nb 3 Sn Nb High Q 0 @ low field => low surface resistance => good quality material Early Q slope !!! Note : GL valid only near TC , SH model = for 1 rst order transition (Type I SC) we work ~ 2 -4 K, type II SC). SH model needs to be completed 1. 5 GHz Nb 3 Sn cavity (Wuppertal, 1985) 1. 3 GHz Nb cavity (Saclay, 1999) CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 2
High field dissipations : due to vortices ? Theoretical Work from Gurevich : temperature correction Non linear BCS resistance at high field : quadratic variation of RBCS Vortices : normal area ~ some nm can cause “hot spots” ~ 1 cm (comparable to what is observed on cavities) At high field vortices => thermal dissipation => Quench Nb is the best for SRF because it has the highest HC 1, (prevents vortex penetration) α HC 1 Nb 3 Sn (0. 05 T) α HC 1 Nb (0. 17 T) Nb 3 Sn Nb is close to its ultimate limits Nb 1. 5 GHz Nb 3 Sn cavity (Wuppertal, 1985) 1. 3 GHz Nb cavity (Saclay, 1999) CEA/DSM/Irfu/SACM/Lesar (normal state transition) avoiding vortex penetration => keep below HC 1 increasing the field => increase HC 1 “invent” new superconductors with HC 1> HC 1 Nb A. Gurevich, "Multiscale mechanisms of SRF breakdown". Physica C, 2006. 441(1 -2): p. 38 -43 A. Gurevich, "Enhancement of RF breakdown field of SC by multilayer coating". Appl. Phys. Lett. , 2006. 88: p. 12511. P. Bauer, et al. , "Evidence for non-linear BCS resistance in SRF cavities ". Physica C, 2006. 441: p. 51– 56 Claire Antoine –SRF 2011 - Chicago July, 25 -29 3
Breaking Niobium monopoly Overcoming niobium limits (A. Gurevich, 2006) : Keep niobium but shield its surface from RF field to prevent vortex penetration Use nanometric films (w. d < l) of higher Tc SC : => HC 1 enhancement Example : Nb. N , x = 5 nm, l = 200 nm HC 1 = 0, 02 T 20 nm film Nb I-S- => Happlied H’C 1 = 4, 2 T x 200 (similar improvement expected with Mg. B 2 or Nb 3 Sn) high HC 1 => no transition, no vortex in the layer applied field is damped by each layer HNb Outside wall Cavity's internal surface → insulating layer prevents Josephson coupling between layers applied field, i. e. accelerating field can be increased without vortex nucleation thin film w. high Tc => low RBCS at low field => higher Q 0 CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 4
High Tc nanometric SC films : low RS, high HC 1 In summary : take a Nb cavity… deposit composite nanometric SC (multilayers) inside Nb / insulator/ superconductor / insulator /superconductor… (SC with higher Tc than Nb) Magnetic field B (m. T) I-S-… Quality coefficient Q 0 Nb Good candidates : Nb 3 Sn Mg. B 2 Nb. N… CEA/DSM/Irfu/SACM/Lesar Accelerating Field Eacc (MV/m) Increasing of Eacc AND Q 0 !!! Claire Antoine –SRF 2011 - Chicago July, 25 -29 5
Samples and HC 1 issues Choice of model samples: It is easier to change parameters on samples than on cavities : Easier to get good quality layers on small surfaces Change of substrate nature : sapphire, monocrystalline Nb, polycrystalline Nb, surface preparation. . . Optimization of SC thickness, number of layer, etc. But ! HC 1 measurement is more difficult with classical means (DC/AC). Note that HC 1 DC ≤ HC 1 RF ≤ HC ~ HSH => any DC or low frequency measurement is conservative compare to what is expected if RF. HC 1 give an estimation of the maximum field achievable without dissipation : if I keep below HC 1, I don't really have to care about what exactly HSH is, what the (complex!) behavior of vortices is, etc. Still need RF test to estimate RS/Q 0 CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 6
First exp. results on high quality model samples Choice of Nb. N: ML structure = close to Josephson junction preparation (SC/insulator compatibility) Use of asserted techniques for superconducting electronics circuits preparation: Magnetron sputtering Flat monocrystalline substrates ~ 25 nm Nb. N ~ 15 nm insulator (Mg. O) 250 nm Nb “bulk” Reference sample R, Tc = 8. 9 K Test sample SL Tc = 16. 37 K Monocrystalline sapphire ~ 12 nm Nb. N x 4 14 nm insulator (Mg. O) 250 nm Nb “bulk” Monocrystalline sapphire Test sample ML Tc = 15. 48 K Collaboration with J. C. Villégier, CEA-Inac / Grenoble CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 7
Magnetic characterization : SQUID (1) Principle of measurement (5 x 5 mm 2 samples) : Parallel and perpendicular field tested Thin films in parallel configuration (B//): 1. Strong sensitivity of M to applied field orientation (alignment should be better than 0. 005°). 2. Strong transverse signal vs longitudinal => superposition of 2 signals => development of a dedicated fitting procedure was necessary B //, transverse moment B //, longitudinal moment Quartz holder Detection coils B Oriented quartz holder Sample B Sample Detection coils CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29
SQUID (2) : references @ 4. 5 K Nb(250 nm) : Nb. N(30 nm) "Elemental" layers : isotropic. HP ~ 18 m. T = compatible with magnetron sputtered films No field enhancement on 30 nm Nb. N layer (due to full penetration of field ? ) CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29
SQUID (3) : SL Sample @ 4. 5 K High quality Nb. N film (Tc =16. 37 K) Strong anisotropic behavior Longitudinal moment : Fishtail shape characteristic of layered SC HP ~ 96 m. T (+ 78 m. T /Nb alone) !!! Similar behavior with ML ~ “ 20 MV/m” !? CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29
SQUID (4) : ML Sample @ 4. 5 K ML sample : 250 nm Nb + 4 x (14 nm Mg. O + 12 nm Nb. N ) Similar behavior as SL Instabilities in 1 rst and 3 rd quadrant (vortices jumps ? ) Nb. N lowest quality (Tc= 15. 48 K) CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29
SQUID : ISSUES Strong screening effect observed although sample is in uniform field (!? ) Edge, shape, alignment issues => is HP ~ HC 1 ? Perpendicular remnant moment => what is the exact local field ? DC instead of RF : not a problem; HC 1 is expected to be even higher in RF => need to get rid of edge/orientation effects => need to get local measurement ! CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29
Local magnetometry (1) 3 rd harmonic measurement, coll. INFM Napoli M. Aurino, et al. , Journal of Applied Physics, 2005. 98: p. 123901. Perpendicular field : field distribution can be determined analytically. If rsample> 4 rcoil : Sample ≡ infinite plate approximation Applied field : perpendicular, induction (B ) // surface (below BC 1) Differential Locking Amplifier Excitation/Detection coil (small/sample) CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 14
Local magnetometry (2) 3 rd harmonic measurement, coll. INFM Napoli b 0 cos (wt) applied in the coil temperature ramp third harmonic signal appears @ Tb 0 , when b 0 reaches BC 1 (Tb 0) series of b 0 => series of transition temperature => BC 1 (T)) increasing I = increasing B TCi at Bi = T/Tc Sample SL : third harmonic signal for various b 0 CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 15
Local magnetometry (3) 18 16 ref 14 SL B (m. T) 12 10 8 6 4 2 0 0 5 10 T (K) 15 20 SL sample : 250 nm Nb + 14 nm Mg. O + 25 nm Nb. N 8. 90 K < Tp° < 16 K : behavior ~ Nb. N alone Tp°< 8. 90 K, i. e. when Nb substrate is SC , => BC 1 SL >> BC 1 Nb CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 16
Local magnetometry (4) Sample SL : small Nb signal @ ~Tc. Nb : Nb is sensed through the Nb. N layer ! Since the Nb layer feels a field attenuated by the Nb. N layer, the apparent transition field is higher. This curve provides a direct measurement of the attenuation of the field due to the Nb. N layer 2 Nb Ref B (m. T) 1. 5 Nb/SL 1 0. 5 0. 04 0 0. 03 8 0. 02 0. 01 0 9 BC 1 curves for Niobium in the reference (direct measurement) and in SL (under the Nb. N layer). CEA/DSM/Irfu/SACM/Lesar 8. 5 T (K) Claire Antoine –SRF 2011 - Chicago July, 25 -29 9 17
Local magnetometry (5) 18 16 ref 14 SL B (m. T) 12 10 8 6 4 2 0 0 5 10 T (K) 15 20 SL sample : 250 nm Nb + 14 nm Mg. O + 25 nm Nb. N Tp°< 8. 90 K, i. e. when Nb substrate is SC , => BC 1 SL >> BC 1 Nb Need to extend measure @ higher field and lower temperature CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 18
Local magnetometry (5) SL sample : 250 nm Nb + 14 nm Mg. O + 25 nm Nb. N Tp°< 8. 90 K, i. e. when Nb substrate is SC , => BC 1 SL >> BC 1 Nb Need to extend measure @ higher field and lower temperature CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 19
Local magnetometry (6) copper rod (thermalization of electrical wires) spring coil support (high conductivity copper) glass bead coil samplethermal regulation : 1. 6 K <Tp°< 40 K, automated thermal braid 100 to 200 m. T available High conductivity copper plate CEA/DSM/Irfu/SACM/Lesar heating wire sample support temperature steel rods (high conductivity sensor copper) Claire Antoine –SRF 2011 - Chicago July, 25 -29 20
Local magnetometry (7) 100 90 Référence 80 70 SL B (m. T) 60 50 (B SL_S) B = I/9, 4 40 30 20 10 0 3 8 13 18 T (K) SL sample : 250 nm Nb + 14 nm Mg. O + 25 nm Nb. N Tp°< 8. 90 K, i. e. when Nb substrate is SC , => BC 1 SL >> BC 1 Nb Need to extend measure @ higher field and lower temperature CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 21
Future : Depositing and testing RF cavities depositing and testing RF Cavities: TE 011, ~3 GHz IPNO IPN (Orsay) : 3 GHz, LKB (Paris) : 50 GHz Cavités 1. 3 GHz @ Saclay (what deposition technique? !) 1. 3 GHz Irfu 50 GHz LKB CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 22
Conclusions and perspectives If Gurevich approach is correct, ML structures are the only way to go beyond Nb Magnetic screening of nanometric layers seems effective even in perpendicular field An increase of first penetration field ~ 80 m. T has been observed with only one 25 nm Nb. N layer !!! next challenges : confirm the squid data with local magnetometry @ 2 -4 K deposit sample RF cavity (conventional techniques) develop deposition techniques for “real” cavities CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 23
Compléments CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 24
Adjustment of coil distance glue glass bead glue + copper powder wedge : 60 µm CEA/DSM/Irfu/SACM/Lesar coil Claire Antoine –SRF 2011 - Chicago July, 25 -29 25
Multilayers optimization SC structure optimization Deposition techniques optimization Magnetron sputtering Inac (Grenoble), Atomic Layer Deposition INP (Grenoble) Nb Nb. N Al 2 O 3 Mg. O Cu Metallic substrates more realistic): From samples to cavities : ALD involves the use of a pair of reagents Application of this AB Scheme Reforms a new surface Adds precisely 1 monolayer Viscous flow (~1 Torr) allows rapid growth No line of site requirements => uniform layers, larges surfaces, well adapted to complex shapes : cavities! up grade of existing cavities ? CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 27
Bulk Nb ultimate limits : not far from here ! Cavité 1 DE 3 : EP @ Saclay T- map @ 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 ? CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29 28
Rappels sur les principaux supras rn (µWcm) Pb 7, 1 Nb 9, 22 Mo 3 Re 15 Nb. N 17 V 3 Si 17 Nb. Ti. N 17, 5 35 Nb 3 Sn 18, 3 20 Mg 2 B 2 40 YBCO 93 CEA/DSM/Irfu/SACM/Lesar 70 l. L (nm)* Type n. a. 48 I 0, 17 0, 4 40 II 0, 03 3, 5 140 II 0, 23 2 HC 2 (Tesla)* 0, 2 HC 1 (Tesla)* 0, 43 TC (K) HC (Tesla)* 0, 08 Matériau 0, 02 15 200 II 0, 03 II 151 II 0, 54 0, 05 30 85 II 0, 43 0, 03 3, 5 140 II 2 gaps 1, 4 0, 01 100 150 d-wave Claire Antoine –SRF 2011 - Chicago July, 25 -29 29
Radial field repartition 200µm away from the coil Coil radius CEA/DSM/Irfu/SACM/Lesar Claire Antoine –SRF 2011 - Chicago July, 25 -29
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