Radio Frequency Magnetic Field Limits of Nb and ${\mathrm{Nb}}_3\mathrm{Sn}$

Superconducting radio frequency (srf) cavities, essential components of many large particle accelerators, rely on the metastable flux-free state of superconducting materials. In this Letter, we present results of experiments measuring the magnetic field limits of two srf materials, Nb and ${\mathrm{Nb}}_3\mathrm{Sn}$. Resonators made using these materials were probed using both high power rf pulses and dc magnetic fields. Nb, which is the current standard material for srf cavities in applications, was found to be limited by the superheating field $H_{\text{sh}}$ when prepared using methods to avoid excessive rf dissipation at high fields. ${\mathrm{Nb}}_3\mathrm{Sn}$, which is a promising alternative material that is still in the early stages of development for srf purposes, was found to be limited between the onset field of metastability $H_{c1}$ and $H_{\text{sh}}$. Analysis of the results shows that the limitation is consistent with nucleation of flux penetration at defects in the rf layer.

Figure 1

Measured quench field of Nb (left) and ${\mathrm{Nb}}_3\mathrm{Sn}$ (right) superconducting cavities under high power rf pulses as a function of temperature. For niobium, the cavity was tested both with bake to prevent limitation from HFQS and without it; the measurements that appear to be affected by HFQS are marked with white points at their centers. The dark and light shaded bands show the approximate temperature dependence of $H_{\text{sh}}$ and $H_{c1}$ extracted from CW measurements.

Figure 2

Goodness of fit statistics to pulsed quench field data. The two models represent different types of surfaces: ideal ones ($n=2$) and those with imperfections ($n=4$). ${\chi }_{\text{red}}^2$ of 1 indicates good agreement between data and model within the estimated variance of 5 mT. ${\chi }_{\text{red}}^2\gg 1$ indicates a poor fit.

Figure 3

Pulsed 4.2 K quench field in the ${\mathrm{Nb}}_3\mathrm{Sn}$ cavity as a function of the time it takes to raise the field until quench occurs with uniform forward power. The time to quench was varied by changing $P_f$ from the klystron. The dashed line shows a fit to the measurement using a simple thermal model.

Figure 4

Measured flux penetration field of Nb (left) and ${\mathrm{Nb}}_3\mathrm{Sn}$ (right) superconducting cavities under an external dc magnetic field as a function of temperature. The dark and light shaded bands show the approximate temperature dependence of $H_{\text{sh}}$ and $H_{c1}$ extracted from cw measurements.