MIT Lincoln Laboratory 1 Magnesium Diboride Films for SRF Cavity Applications Y. D. Agassi Naval Surface Warfare Center, Carderock Division, Bethesda MD D. E. Oates MIT Lincoln Laboratory, Lexington MA B. H. Moeckly STI, Inc., Santa Barbara CA Sponsor: Office of Naval Research (SRF2011, CHICAGO, JULY ‘11)

MIT Lincoln Laboratory 2 Outline I. Introduction and Approach: The Need to Go Beyond Bulk-Nb Cavities The T > 4 K Approach: MgB 2 /{Cu, Nb} Films; Thickness ~2 Properties, Potential Payoffs, Challenges II. II. The Case for MgB 2 : ► Reactive Evaporation High-quality Thin-film Deposition ► R s (T, H) Data Power Dependence (Low, High) ► Film Passivation Film stability ► System Simulations Thermal Management ► Energy Gap Data (IMD, (T < 5K)) and Theory of  Energy-Gap Symmetry III. Summary, Outlook

MIT Lincoln Laboratory 3 5K  T OPERATION  20 K ● New Superconductor (2001); T C = 39.5 K. Advantage: ● Payoffs: ► Simplified Cryogenics: Gaseous He vs. Liquid He for Nb ► Reduction in Size, Weight, Power (SWP) ► Enhanced Reliability The T > 4 K Approach: MgB 2 Films of Thickness O(2 ) Prime Power (kW) Operating Temperature (K) Bulk Nb MgB 2 Prime Power: MgB 2 vs. Nb REF: AES, Inc. ' ,000 10, PERCENT CARNOT 3 PERCENT CARNOT 1 PERCENT CARNOT 1 WATT CRYOCOOLERS INPUT POWER (WATTS) INPUT POWER vs. T OPER FOR 1 WATT CRYOCOOLER T OPER (K)

MIT Lincoln Laboratory 4 Property Nb (Type I/II) MgB 2 (Type II) Implications Critical Fields (T=0K) (Tesla) H C = 0.2 H C > 1.55 (.16x15.) (1/2) ● Potential for High Gradient Operating Temperature (K) ● Payoffs: Big Cost, SWP Savings ● Enhanced Reliability ξ ab (T=0) (nm) (Coherence Length) 38 ~ 4 - 8● Polycrystalline Films: OK Oxide Structure Many Oxides Some are Magnetic Only MgO Only B 2 O 3 ● Reduced Potential for Corrosion Crystal Structure BCC AlB 2 (Hexagonal), ● Stable, Simple Classification Single Element Metal Two Elements Ceramic ● Challenge: Quality SRF Cavity Coating The MgB 2 Alternative Nb vs. MgB 2 Material Properties

MIT Lincoln Laboratory 5 Outline I Introduction And Approach: The Need to Go Beyond Bulk-Nb Cavities The T > 4 K Approach: MgB 2 /{Cu, Nb} Films; Thickness ~2 Properties, Potential Payoffs, Challenges II. II. The Case for MgB 2 : ► Reactive Evaporation High-quality Thin-film Deposition ► R s (t, H) Data Power Dependence (Low, High) ► Film Passivation Film stability ► System Simulations Thermal Management ► Energy Gap Data (IMD, (T < 5K)) and Theory of  -Gap Symmetry III. Summary, Outlook

MIT Lincoln Laboratory 6 Advantages: Localized source of high-pressure Mg vapor Mg and substrate temperatures: Different Films: T C  39K,  T C  1K, Resistivity(T c )  2  Ω 2” Wafers RMS roughness = 4.4 nm Rotating blackbody heater Film Deposition: Reactive Evaporation Ref.: B. H. Moeckly et al., Supercond. Sci. Technol. 19, L21 (2006) Solves MgB 2 Film-growth Difficulties: Mg Volatility, Oxidation

MIT Lincoln Laboratory 7 Low Power R S (T) MgB 2 vs. Nb Films ● R S (MgB 2 ; T) 2 K ● R S (MgB 2 /Nb (Polished)): Comparable to MgB 2 /Sapphire REF.: D. E. Oates, Y. D. Agassi and B. H. Moeckly, Supercond. Sci. Tech. 23, (2010) ● Nb-Bulk: R S (2K, f = 2. 2GHz )  R RESIDUAL (2K)   Ω ● T c (Nb) = 9.2 K, T c (MgB 2 ) = 39 K Stripline Resonator: Films on Dielectric Substrate

MIT Lincoln Laboratory 8 H C1 ~ 300 Oe ● R s (NL Onset)/Sapp. at H ~ 800 Oe (  E ACC ~ 20 MV/m): Material Limited? ● R S (NL Onset)/Nb at H > 200 Oe Flat! Equipment Limited ● Sample Variability REF.: D. E. Oates, Y. D. Agassi and B. H. Moeckly, Supercond. Sci. Tech. 23, (2010) Higher Power R S (T) MgB 2 /(Nb, Sapphire) Dielectric Resonator Films on Metallic or Dielectric Substrates

MIT Lincoln Laboratory 9 Passivation Success at Film-Stabilization MgB 2 Degrades in Air Passivation with 5 X (2.5 nm Al 2 O 3 and 2.5 nm ZrO 2 ) by ALD Over 6 Months & 5 Temperature Cycles R s Unchanged. Q (f = 1. GHz)  1.  10 8 (Measured in a 2” Dielectric Resonator.) R S (1 GHz ) = 2 x Ω

MIT Lincoln Laboratory 10 He(T, P) = (30K, 3 Atm) (Q =.608E 9 f = MHz) System Simulations Thermal Management, Power (AES, Inc.) Based on Our R S Data  Simulations Confirmed Feasibility Two Thermal-management Issues: 1. Gaseous He Cooling : MgB 2 /Cu Five-Cavity Array 2. Resonance-Frequency Shift : Due to Thermal Expansion Cooling Load: Not a Problem Thermal Expansion: Relatively Small Feasible Worst Case Scenario Prime Power (kW) Operating Temperature (K) Bulk Nb MgB 2 Prime Power: MgB 2 vs. Nb REF: AES, Inc.

MIT Lincoln Laboratory 11 Energy-Gap (  -Gap) Symmetry ● YBCO Work  IMD-Power: Nonlinear Probe of Energy-Gap Symmetry ● Surprise: Low-T IMD-power Upturn: Inconsistent with s-wave  -Gap REF.: D. Agassi and D. E. Oates, Phys. Rev. B 72, (2005); Phys. Rev. B 80, (2009) Theory ● Impact of  -Gap ℓ=6 Symmetry: Surface-Resistance Variation, NL, at Low-T YBCO MgB 2 Reduced Temperature T - 2

MIT Lincoln Laboratory 12 Accomplishments Materials Science (Reactive Evaporation Deposition): ► High-quality, Flat, Ultra Smooth MgB 2 /Sapphire, MgB 2 /Nb Films ► Passivation Success with ZrO-AlO Coating Data/Characterization and Analysis: ► R S Database: at Low, High Power of Flat MgB2/(SAPPHIRE, Nb) Films ► Thermal Management Simulation  OK ►  Energy-Gap ℓ=6 Symmetry: Data and Theory Outlook: E ACC : ● CURRENT RECORD ~ 20MV/m ● LIKELY that E ACC IS HIGHER Summary The 2- Thick MgB 2 -Coated Cavity Proposition: Promising

MIT Lincoln Laboratory 13 Future Challenges ► High-Quality Film Deposition on Curved Metallic Surfaces ► Demonstrating Low R S in those films ► Demonstrating a High Field Gradient in those Films ► Theory ► Make a MgB 2 -based RF Cavity with High Field Gradient

MIT Lincoln Laboratory 14 End Presentation 14

MIT Lincoln Laboratory 15 Backup Slides 15

MIT Lincoln Laboratory 16 (II) RESONATORS Stripline vs. Dielectric Stripline Resonator: Side View Conductor Cross Section |J(x)/J max | Position from Center (  m) J max ~ 1.6 x 10 8 A/cm  m rf magnetic field directions Dielectric Resonator: Top View Position from Center (mm) |J(x)/J max | J max ~ 4.0 x 10 6 A/cm 2 Cross section DIELECTRIC SUBSTRATESMETALLIC SUBSTRATES

MIT Lincoln Laboratory 17 MgB Crystal structure: AlB 2 M ► MGB: TWO, WEAKLY COUPLED BAND PAIRS (σ 2, π 2 ) TWO GAPS L UNIQUE 7.1 meV 2.3 meV BASIC MgB 2 STRUCTURES P. Canfield et al., Physics Today, March 2003, p.34; M. Iavarone et al., Phy. Rev. Lett. 89, (2002)   Gap structure Fermi-Surface structure

MIT Lincoln Laboratory 18 P IMD (dBm) T (K) (Log Scale) P IMD (T) (II) P IMD (T) and  -GAP SYMMETRY on LaAlO 2 on Sapphire data ℓ = 6 s-wave ● GAP-SYMMETRY is CONSTRAINED by CRYSTAL SYMMETRY ● PARAMETER-FREE FIT with ℓ = 6 GAP SYMMETRY Y. D. Agassi, D. E. Oates and B. H. Moeckly, Phys. Rev. B 80, (2009) Significant Impact on the Low-T Surface-resistance

MIT Lincoln Laboratory 19 REF.: Y. D. Agassi, D. E. Oates and B. H. Moeckly, unpublished (II) CORROBORATING DATA: Penetration Depth Upturn (T < 4K) ● Origin: Andreev Surface States. As in the Cuprates ● Gap Nodal Lines  Surface-Attached States for Particular Surface Orientations ℓ = 6 MgB 2 ℓ = 2 Cuprates