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The atomic scale investigation of Nb for superconducting RF cavity

The atomic scale investigation of Nb for superconducting RF cavity. Kevin Yoon, David N. Seidman – Northwestern University Claire Antoine, Chris Boffo Fermi National Accelerator Laboratory May 23-24, 2007 Fermilab. Atom-probe tomography (APT).

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The atomic scale investigation of Nb for superconducting RF cavity

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  1. The atomic scale investigation of Nb for superconducting RF cavity Kevin Yoon, David N. Seidman – Northwestern University Claire Antoine, Chris Boffo Fermi National Accelerator Laboratory May 23-24, 2007 Fermilab

  2. Atom-probe tomography (APT)

  3. Coordinates of ions(x, y, and z): permits the three-dimensional reconstruction of the lattice in real space • Times-of-flight:Mass-to-charge state ratio yields identification of the elements and their isotopes • Atomic resolution: depth resolution is equal to the interplanar {hkl} spacing (< 0.1 nm); lateral resolution is ca. 0.3 to 0.5 nm in a given {hkl} plane.

  4. Laser pulsing system A pulsed-laser assisted LEAP tomograph involves replacing electric pulses with picosecond laser pulses, indicated by incident red wave in left-hand figure. Simulation of the electric field (E) at a tip caused by the E-field of the laser pulse, see right-hand figure. (Prof. Tamar Seideman, Chemistry Department, Northwestern University)

  5. Field evaporation mechanism using laser pulses

  6. Sudden change in tip radius; from oxide to bulk Nb Nb-pink, O-blue 51 x 53 x 14 nm3, 300 K atoms Nb-pink, O-blue 37 x 38 x 79 nm3, 1.6M atoms

  7. Before LEAP SEM

  8. Nb-pink, O-cyan 87 x 89 x 11 nm3, 480 K atoms Baked at 120 ºC for 2 days

  9. Before LEAP experiment, SEM micrograph of tip show NO sudden change in radius. • Sudden increase in radius from surface oxide to bulk Nb present in three-dimensional reconstruction of LEAP result • Maybe the indication of premature tip fracture

  10. Laser-assisted LEAP tomographic experiments • To avoid tip fracture • Pico-second laser • Pulse rate: 100 kHz • Pulse energy: 0.5~1.5 nJ • Temperature of specimen: 80~100 K • Typical parameters used at Northwestern for studying metallic alloys: 250 kHz, 1.5 nJ, 40 K

  11. 3D reconstruction (1) Analysis direction Specimen temperature: 80 K Pulse energy: 0.5 ~ 1.3 nJ 18 x 18 x 116 nm3, 500 K atom data set Nb: pink, O: cyan

  12. Proximity Histogram

  13. 3D reconstruction (2) Analysis direction Specimen temperature: 100 K Pulse energy: 1.5 nJ 62 x 60 x 106 nm3 1.5 M atom

  14. Nb Oxide Bulk Nb Difference in required field for evaporation

  15. Conclusions and Next steps • Sudden increase in tip radius: Fracture • Use laser pulsing system • Perfecting optimum parameters • Variation in oxide thickness with HF rinsing • 3~15 nm • Oxygen in bulk Nb: ~100 at. ppm • No Ta detected: it must be <100 at. ppm

  16. DOE High Energy Physics for funding Fermilab for funding NSF and ONR for LEAP tomograph ONR for laser upgrade Acknowledgements

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