Effect of low temperature baking in nitrogen on the performance of a niobium superconducting radio frequency cavity

We report the rf performance of a single cell superconducting radiofrequency cavity after low temperature baking in a nitrogen environment. A significant increase in quality factor has been observed when the cavity was heat treated in the temperature range of $120–160°\mathrm{C}$ with a nitrogen partial pressure of $\sim 25\mathrm{m}\mathrm{Torr}$. This increase in quality factor as well as the $Q$-rise phenomenon (anti-$Q$-slope) is similar to those previously obtained with high temperature nitrogen doping as well as titanium doping. In this study, a cavity ${\mathrm{N}}_2$-treated at $120°\mathrm{C}$ and at $140°\mathrm{C}$ showed no degradation in accelerating gradient, however the accelerating gradient was reduced by $\sim 25\%$ with a $160°\mathrm{C}$ ${\mathrm{N}}_2$ treatment, compared to the baseline tests after electropolishing. Sample coupons treated in the same conditions as the cavity were analyzed by scanning electron microscope, x-ray photoelectron spectroscopy and secondary ion mass spectroscopy revealed a complex surface composition of ${\mathrm{Nb}}_2{\mathrm{O}}_5$, NbO and ${\mathrm{NbN}}_{(1-x)}{\mathrm{O}}_x$ within the rf penetration depth. Furthermore, magnetization measurements showed no significant change on bulk superconducting properties.

Figure 1

Typical temperature and partial pressure of the gas measured during the heat treatment process. All partial pressures during the heat treatment were below ${10}^{-8}\mathrm{torr}$. The residual gas analyzer was turned off before ${\mathrm{N}}_2$ injection.

Figure 2

Material parameters obtained from the fit of $R_s(T)$ from 4.3 to $\sim 1.5\mathrm{K}$. The error bars on some $R_{\mathrm{res}}$ values are within the size of the marker.

Figure 3

Radio frequency measurement taken at 2.0 K for (a) the baseline measurements and (b) after different surface treatments.

Figure 4

Representative low magnification BSE images coupon sample surfaces with grain boundaries (GB) after different heat treatments, (a) $800°\mathrm{C}/3\mathrm{h}$ with no nitrogen infusion and (b)–(d) are low temperature nitrogen infused at 120, 140 and $160°\mathrm{C}$, respectively.

Figure 5

Representative high magnification BSE image coupon sample surfaces with grain boundaries (GB) after different heat treatments, (a) $800°\mathrm{C}/3\mathrm{h}$ with no nitrogen infusion and (b)–(d) are low temperature nitrogen infused at 120, 140 and $160°\mathrm{C}$, respectively.

Figure 6

(a) Background subtracted XPS spectra on sample F9 ($800°\mathrm{C}/3\mathrm{hrs}$) and F11 ($800°\mathrm{C}/3\mathrm{hrs}+140°\mathrm{C}/48\mathrm{hrs}$ @ 25 mTorr ${\mathrm{N}}_2$) at 45° takeoff angle. (b) Angle resolved XPS on sample F11.

Figure 7

SIMS depth profile of (a) carbon, (b) oxygen and (c) nitrogen measured in samples F9–F12.

Figure 8

${\mathrm{H}}^{-}/{\mathrm{Nb}}^{-}$ as a function of depth measured by TOF-SIMS in Nb samples F9–F12.

Figure 9

Isothermal dc magnetic hysteresis of samples M8–M12 measured at 4.2 K. The inset shows the $M(H)$ close to $H_{c2}$ showing an enhanced $H_{c2}$ for samples M11 and M12.

Figure 10

$R_s/R_s(B_p\sim 10\mathrm{mT})$ vs ${\mathrm{B}}_p$ calculated using the model in Ref. [33] along with the experimental data for cavity heat treated at 140 and $160°\mathrm{C}$, respectively.