Understanding Quality Factor Degradation in Superconducting Niobium Cavities at Low Microwave Field Amplitudes

In niobium superconducting radio frequency (SRF) cavities for particle acceleration, a decrease of the quality factor at lower fields—a so-called low field $Q$ slope or LFQS—has been a long-standing unexplained effect. By extending the high $Q$ measurement techniques to ultralow fields, we discover two previously unknown features of the effect: (i) saturation at rf fields lower than $E_{\mathrm{acc}}\sim 0.1\mathrm{MV}/\mathrm{m}$; (ii) strong degradation enhancement by growing thicker niobium pentoxide. Our findings suggest that the LFQS may be caused by the two level systems in the natural niobium oxide on the inner cavity surface, thereby identifying a new source of residual resistance and providing guidance for potential nonaccelerator low-field applications of SRF cavities.

Figure 1

Decay of cavity output power upon turning the RF source off. At each moment in time $t_0$, time decay can be characterized by the “instantaneous” time constant $\tau (t_0)$, from which the loaded quality factor $Q_L$ is obtained. Dashed lines reflect the noise floor in the respective configurations.

Figure 2

$Q(E)$ in the full field range for electropolished 1-cell cavity obtained by concatenating overlapping CW and single shot decay measurements. $\mathrm{RBW}=10\mathrm{kHz}$ has been used for the red curve to compensate for the Lorentz force detuning at large rf fields. The fit to TLS model (Eq. (3) is shown as well.

Figure 3

Surface resistance as a function of the rf field amplitude measured at $T=1.5–1.6\mathrm{K}$. Removing the thick oxide by light electropolishing brings the residual resistance back down.

Figure 4

Additional surface resistance as compared with that found at $5\mathrm{MV}/\mathrm{m}$ as a function of the rf rield amplitude measured at $T=1.5–1.6\mathrm{K}$. Dashed lines are fits to TLS model (Eq. (3).

Figure 5

Volume electric field amplitude distribution for ${\mathrm{TM}}_{010}$ mode normalized to the stored energy of 1 J, which is used to calculate surface electric field participation.