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. Nb-pink, O-blue 51 x 53 x 14 nm 3, 300 K atoms Nb-pink, O-blue 37 x 38 x 79 nm 3, 1.6M atoms Sudden change in tip radius; from oxide to bulk Nb

7. Before LEAP SEM

8. Nb-pink, O-cyan 87 x 89 x 11 nm 3, 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. Analysis direction Specimen temperature: 80 K Pulse energy: 0.5 ~ 1.3 nJ 18 x 18 x 116 nm 3, 500 K atom data set Nb: pink, O: cyan 3D reconstruction (1)

12. Proximity Histogram

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

14. Bulk Nb Nb Oxide 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. Acknowledgements DOE High Energy Physics for funding Fermilab for funding NSF and ONR for LEAP tomograph ONR for laser upgrade