HEAT TRANSFER MEASUREMENTS OF NIOBIUM FOR SRF CAVITIES

1. HEAT TRANSFER MEASUREMENTS OF NIOBIUM FOR SRF CAVITIES S.K. Chandrasekaran, A. Aizaz, C. Compton, T.L. Grimm, N.T. Wright Michigan State University East Lansing, MI 48824 May 23, 2007 This research is funded by the Fermi National Accelerator Laboratory

2. Heat Generation In SRF Cavities

3. Motivation • Greater B greater accelerating gradient and • Cooler Ts smaller Rs reduced reduced cryogenic load Thermo-magnetic interactions in defect free cavities Cavity Parameters Ro = 5 n e = 3 mm Tb = 2 K f = 1.3 GHz = 230

4. Ts Ti Tb e Nb He II e 1 k hk Ts Ti Tb Heat Transfer Problem • Assumptions: • Steady state • No internal heat generation (surface penetration of B is in the order of nanometers)

5. Samples: • Two cylindrical (RRR 232) – Tokyo Denkai • Two rectangular flat plates (RRR 390) – FermiLab/Wah Chang • One single crystal and one bi crystal sample (RRR 280) – JLab/CBMM Treatments: • Flat plate samples • 750 °C heat treatment • Titanification • RRR measurement • Single/bi crystal samples (EDM cut) • Baseline measurements • Cylindrical sample 1 • 3% strain • Titanification • Cylindrical sample 2 • Surface deformation (S.I.>3) • 750 °C heat treatment • Titanification For all samples, • 750 °C heat treatment – FermiLab • Titanification – Cornell University Outline of Experiments

6. Experimental Apparatus Heater Heater C2 C1 C3 Temperature Sensor Single Crystal Niobium Flat plate Nb sample Heat Sink Conflat flange Adapted from Aizaz, 2006

7. 1 0.1 As received k (W/cm/K) 3% strained After titanification 0.01 0 1 2 3 4 5 Avg. Temp. (K) 1 K) 2 (W/cm 0.1 Bousson k As recd. h 3% strain Aft. Ti 0.01 1.5 1.7 1.9 2.1 T (K) b Results Cylindrical sample 1 Adapted from Aizaz, 2006

8. 1 Bousson As recd. Sur. Strain Low HT k (W/cm/K) Aft. Ti hk (W/cm2/K) 0.1 0.01 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 Avg. Temp. (K) Tb (K) 1 As received Surface strained After titanification 0.1 0 1 2 3 4 5 Results (cont.) Cylindrical sample 2 Adapted from Aizaz, 2006

9. Results (cont.) Flat plate samples Adapted from Aizaz, 2006 • Post-titanification thermal conductivity measurements suggest RRR ~ 80 • Post-titanification expectation of RRR ~ 600 • Measured RRR in samples cut from same plate indicate RRR 67 (+/- 20%) • (Blind measurements by Fermilab)

10. 1 S1(1-2) S1(2-3) k (W/cm/K) S2(1-2) S2(2-3) 0.1 1 10 Avg. Temp. (K) Results (cont.) Single/Bi crystal samples • Reduced phonon contribution observed in either sample • Both samples to be annealed and re-tested

11. Discussion • Plastic deformation caused the phonon peak to disappear • Thermal conductivity of Nb decreased by ~80% at 2 K • Annealing at ~750 °C for 2 hrs insufficient to recover the phonon peak • Annealing at ~1200 – 1300 °C for 6 hrs during titanification recovers the phonon peak • Kapitza conductance increased ~300% due to annealing and titanification • Post titanification RRR of flat sample 67 (+/- 20%)

12. Future Steps • Purification of flat plate sample • Low and high temperature annealing of single and bi crystal samples