Surface Resistance of a bulk-like Nb Film Sarah Aull, Anne-Marie Valente-Feliciano, Tobias Junginger and Jens Knobloch

Sample Preparation OFHC copper substrate: mechanically polished Electron beam welded to Nb ring (EBW 1) 12 μm electro polishing Rinsing with ultra pure water at 6 bar Shipped to Jefferson Lab for coating Shipped back to CERN, EBW to support structure (EBW 2) Mounted in the Quadrupole Resonator EBW 1 EBW 2 sarah.aull@cern.ch

Deposition Conditions Cu substrate Final sulfamic acid rinse for cu passivation Deposition Conditions ECR Bake & coating temperature: 360 °C Total coating time: 60’ Dual ion energy: 184 eV for nucleation/early growth 64 eV for subsequent growth Hetero-epitaxial film Nb on OFHC Cu Typical Cu substrate valente@jlab.org

Film characterization Witness sample Nb/(11-20) Al2O3 Tc= 9.36 ± 0.12 K RRR = 179 Diffraction on Nb/Cu witness sample: EBSD IPF map and XRD pole figure show very good crystallinity and grain sizes in the range of the typical Cu substrate valente@jlab.org

Penetration Depth and R(T) Measurement λ(0K) [nm] Rres [nΩ] Δ [K] 400 MHz 40 ± 2 46.6 ± 0.8 14.2 ± 0.3 800 MHz 38 ± 1 79 ± 2 14.8 ± 0.2 1200 MHz 156 ± 11 15.1 ± 1 ℓ* [nm] RRR 144 ± 20 53 ± 7 * with λL = 32 nm and ξ0 = 39 nm Bulk-like film in the clean limit sarah.aull@cern.ch

RRR is unlikely the cause for the strong Q-slope of Nb films. Q-Slope: film vs. bulk 2.5 K Q-Slope of Nb film is linear for B > 5 mT for temperatures up to 4 K. Q-Slope of the Nb film is significantly stronger than for bulk Nb (1 order of magnitude) RRR is unlikely the cause for the strong Q-slope of Nb films. 4 K sarah.aull@cern.ch

Thermal cycling does not affect the (low field) BCS contribution. Thermal cycling: warm up the sample to the normal conducting state and cool down under different conditions. Thermal cycling does not affect the (low field) BCS contribution. sarah.aull@cern.ch

Influence of the Cooling Conditions Influence on the surface resistance: Slow uniform cooling increases RS by more than a factor 2. Nb Film, 400 MHz, 2K, 5 mT Bulk Nb, 400 MHz, 2.5K, 15 mT sarah.aull@cern.ch

Influence of the Cooling Conditions 400 MHz, 2 K Thermal cycling acts on the Q-slope: The faster the cooling the flatter the slope. sarah.aull@cern.ch

Conclusions for the ECR film This bulk-like Nb film shows significantly different behaviour than bulk Nb with the same RRR: In contrary to bulk Nb: cooling fast and with a high temperature gradient leads to lower surface resistance. Lowest surface resistance was achieved by quenching. The Q-Slope of the film is much more severe than the one of bulk Nb. Therefore the RRR is unlikely the cause for strong Q-slopes in Nb film cavities. The cooling conditions act on the Q-Slope, leading to better performance after fast cooling. sarah.aull@cern.ch

Comparison with HIPIMS coating Single cell 1.3 GHz Cu cavity + EP Coating by Giovanni Terenziani RF Cold test by Tobias Junginger For more RF results of this cavity, see: HIPIMS Development for Superconducting Cavities, G. Terenziani et al. (TFSRF14) Lower RS for fast cooling and smaller temperature gradient. Thermal cycling influences the Q-Slope as well. sarah.aull@cern.ch

Comparison with HIE Isolde Quarterwave, 100 MHz For more RF results, see The influence of cooldown conditions at transition temperature on the quality factor of niobium sputtered quarter-wave resonators, P. Zhang et al. (TFSRF14) Lower surface resistance for small temperature gradients. Cooling rate has no significant influence on RS. Courtesy of Pei Zhang

Comparison between QPR, 1.3 GHz and HIE Isolde RRR Geometry Cooling Grain size Quadrupole Resonator: ECR Lower RS for fast cooling with T gradient 53 disc conduction tens of microns 1.3 GHz: HIPIMS Lower RS for fast cooling with small T gradient 21 elliptical Bath cooled 30 nm HIE Isolde: Diode sputtering Lower RS for small T gradients 15 QWR 200 nm – 1 μm depending on thickness Unknown Influence of grain size Influence of geometry Thermal currents Influence of stress Oxidation Roughness … sarah.aull@cern.ch

Conclusions for Nb films As for bulk Nb: The cooling conditions, speed and/or spatial gradient, influence the RF performance. Different film projects are difficult to compare due to different coating techniques and geometries. Optimum cooling procedure to minimize the low field RS is accompanied by a flattened Q-Slope. Further conclusions require dedicated experiments, where spatial and temporal gradients and thermal currents can be controlled independently. sarah.aull@cern.ch

Questions for the TTC meeting For bulk Nb cavities which perform better after fast cooling: How do the cavities behave after quench? For bulk Nb cavities with regular Q-slope: Do you see an effect on the slope depending on the cooling conditions? Concerning the mechanism: If the Q change with cooling conditions is due to flux expulsion, why is the effect in the same order of magnitude for films although they are much less sensitive to trapped flux? (0.1∗ nΩ μT vs. 3.5 nΩ μT ) *Benvenuti et al, SRF97B05 (recent HIE Isolde results agree well after scaling to 100 MHz) sarah.aull@cern.ch