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RF breakdown in multilayer coatings: a possibility to break the Nb monopoly

RF breakdown in multilayer coatings: a possibility to break the Nb monopoly. Alex Gurevich National High Magnetic Field Laboratory, Florida State University. " Thin films applied to Superconducting RF:

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RF breakdown in multilayer coatings: a possibility to break the Nb monopoly

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  1. RF breakdown in multilayer coatings: a possibility to break the Nb monopoly Alex Gurevich National High Magnetic Field Laboratory, Florida State University "Thin films applied to Superconducting RF: Pushing the limits of RF Superconductivity" Legnaro National Laboratories of the ISTITUTO NAZIONALE DI FISICA NUCLEARE, in Legnaro (Padova) ITALY, October 9-12, 2006.

  2. Motivation • Why Nb? BCS surface resistance: Rs 4exp (-/kBT) Minimum Rs implies maximum , that is, maximum Tc 1.87/kB and minimum London penetration depth  Exceptions: • Does not work for the d-wave high-Tc superconductors for which Rs T2due to nodal lines in (k) = 0. • Two gap MgB2 with   2.3 meV   7.2 meV: Tc is proportional to , while Rs exp (- /kBT) is limited by  Nb3Snhas maximum Tc and minimum  to provide the optimum Rs Vanishing (k) = 0 along [110] directions in HTS

  3. Background KEK&Cornell Best KEK - Cornell and J-LabNb cavities are close to the depairing limit (H  Hc = 200 mT) How far further can rf performance of Nb cavity be increased? Theoretical SRF limits are poorly understood …

  4. Superconducting Materials Very weak dissipation Strong vortex dissipation Hc Hc2 H 0 Hc1 - M Very weak dissipation at H < Hc1(Q = 1010-1011) Q drop due to vortex dissipation at H > Hc1 Nb has the highest lower critical field Hc1 Thermodynamic critical field Hc (surface barrier for vortices disappears) Higher-Hc SC Nb

  5. 2 2 Single vortex line • Core region r < • where (r) is suppressed • Region of circulating • supercurrents, r < . • Each vortex carries the • flux quantum 0 B  r   Important lengths and fields • Coherence length  and magnetic (London) penetration depth λ For clean Nb, Hc1  170 mT, Hc  180 mT

  6. Two forces acting on the vortex at the surface: • Meissner currents push the vortex in the bulk • Attraction of the vortex to its antivortex image pushes • the vortex outside J H0 image to ensure J = 0 Thermodynamic potential G(x0) as a function of the position x0: b G  Image Meissner H < Hc1 H = Hc1 Vortices have to overcome the surface barrier even at H > Hc1 (Bean & Livingston, 1964) Surface barrier disappears only at the overheating field H = Hc> Hc1 at which the surface J becomes of the order of the depairing current density x0 H > Hc1 H = Hc Surface barrier: How do vortices get in a superconductor at H > Hc1?

  7. Vortex in a thin film with d <  London screening is weak so 22B = - 0(r) Vortex field in a film decays over the length d/ instead of  (interaction with many images) G. Stejic, A. Gurevich, E. Kadyrov, D. Christen, R. Joynt, and D.C. Larbalestier, Phys. Rev. B49, 1274 (1994) Vortex free energy as a function of the position x0 Magnetic energy Lorentz force  x0 Self-energy

  8. Enhanced lower critical field and surface barrier in films Use thin films with d <  to enhance the lower critical field Field at which the surface barrier disappears Example: NbN ( = 5nm) film with d = 20 nm has Hc1 = 4.2T, and Hs = 6.37T, Much better than Hc = 0.18T for Nb

  9. Higher-TcSC: NbN, Nb3Sn, etc Nb, Pb Insulating layers How one can get around small Hc1 in SC cavities with Tc > 9.2K?AG, Appl. Phys. Lett. 88, 012511 (2006) Multilayer coating of SC cavities: alternating SC and insulating layers with d <  Higher Tc thin layers provide magnetic screening of the bulk SC cavity (Nb, Pb) without vortex penetration For NbN films with d = 20 nm, the rf field can be as high as 4.2 T ! No open ends for the cavity geometry to prevent flux leaks in the insulating layers

  10. H0 = 2T Hi = 50mT d How many layers are needed for a complete screening? Example: N Nb3Sn layers with d = 30nm 0 = 65 nm and Hc1 = 2.4T Peak rf field H0 = 2T < Hc1 Internal rf field Hi = 50 mT (high-Q regime) Nb N = (65/30)ln(40) = 8 Strong reduction of the BCS resistance by Nb3Sn layers due to larger  and shorter :

  11. H0 = 324mT Hi = 150mT d A minimalistic solution A Nb cavity coated by a single Nb3Sn layer of thickness d = 50nm and an insulator layer in between If the Nb cavity can withstand Hi = 150mT, then the external field can be as high as Lower critical field for the Nb3Sn layer with d = 50 nm and  = 3nm: Hc1 = 1.4T is much higher than H0 A single layer coating more than doubles the breakdown field with no vortex penetration, enabling Eacc 100 MV/m

  12. Global surface resistance layer bulk Nb3Sn coating of thickness L = 50 nm, RNb3Sn(2K)  0.1RNb Screen the surface of Nb cavities using multilayers with lower surface resistance

  13. Why is Nb3Sn on Nb cavity much better than Nb3Sn on Cu cavity? vortices H(t) H(t) H Nb Cu w Nb3Sn/Nb cavity is much better protected against small transverse field components than Nb3Sn/Cu cavity Meissner state is destroyed for small H < (d/w)Hc1(Nb3Sn) << Hc1(Nb3Sn) due to large demagnetization factor w/d 103-105 Meissner state persists up to H < Hc1(Nb)

  14. Vortex penetration in a screen Dynamic equation for a vortex Vortex flight time and energy release For a 30 nm Nb3Sn film,  10-12 s, much shorter than the rf period  10-9 s Maximum rf field at which the surface barrier disappears: Nb3Sn coating more than doubles vortex penetration field for Nb

  15. T Tm coolant H(t) Ts T0 x d Analytical thermal breakdown model Equations for Tm and Ts Thermal runaway due to exponential increase of Rs(T) Kapitza thermal flux: q = (T,T0)(T – T0) Fora general case of thermal quench, see Gurevich and Mints, Reviews of Modern Physics 59, 941 (1987),

  16. Maximum temperature BCS + residual surface resistance Ri Since Tm – T0 << T0 even Hb, we may take  and h at T = T0, andobtain the equation for H(Tm):

  17. Thermal feedback stability for multilayers Breakdown field as a function of the total overlayer thickness L Here s = exp(-2L/), r = R0/Rb,  = 0/b • 50 nm Nb3Sn overlayer triples Q at low field • 100 nm overlayer more than doubles the thermal breakdown field

  18. Conclusions: • Multilayer S-I-S-I-S coating could make it possible to take advantage of superconductors with much higher Hc, than those for Nb without the penalty of lower Hc1 • Strong increase of Hc1 in films allows using rf fields > Hc of Nb, but lower than those at which flux penetration in grain boundaries may become a problem • Strong reduction of BCS resistance because of using SC layers with higher  (Nb3Sn, NbN, etc) • The significant performance gain may justify the extra cost.

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