Role of thermal resistance on the performance of superconducting radio frequency cavities

Thermal stability is an important parameter for the operation of the superconducting radio frequency (SRF) cavities used in particle accelerators. The rf power dissipated on the inner surface of the cavities is conducted to the helium bath cooling the outer cavity surface and the equilibrium temperature of the inner surface depends on the thermal resistance. In this manuscript, we present the results of direct measurements of thermal resistance on 1.3 GHz single cell SRF cavities made from high purity large-grain and fine-grain niobium as well as their rf performance for different treatments applied to outer cavity surface in order to investigate the role of the Kapitza resistance to the overall thermal resistance and to the SRF cavity performance. The results show no significant impact of the thermal resistance to the SRF cavity performance after chemical polishing, mechanical polishing or anodization of the outer cavity surface. Temperature maps taken during the rf test show nonuniform heating of the surface at medium rf fields. Calculations of $Q_0(B_p)$ curves using the thermal feedback model show good agreement with experimental data at 2 and 1.8 K when a pair-braking term is included in the calculation of the Bardeen-Cooper-Schrieffer surface resistance. These results indicate local intrinsic nonlinearities of the surface resistance, rather than purely thermal effects, to be the main cause for the observed field dependence of $Q_0(B_p)$.

Figure 1

Schematic of experimental setup to measure the thermal resistance.

Figure 2

Typical experimental data measured. (a) The increase in temperature of superfluid He inside the cavity as a function of applied power at 2.05 K and (b) the plot of $\mathrm{\Delta }T$ vs $q$ for cavity TD5. The slope of the fit gives the thermal resistance at 2.05 K.

Figure 3

Results of the thermal resistance measurement on ingot Nb cavity TD5.

Figure 4

Results of the thermal resistance measurement on fine-grain cavity RDT13.

Figure 5

$Q_0$ vs $B_p$ data for ingot Nb cavity TD5 at 2.0 K (solid symbols) and 1.6 K (empty symbols) after different outer surface modification. The rf measurement during tests 1 and 3 at 1.6 K was stopped at $B_p\sim 75$ in order to remain below the multipacting barrier.

Figure 6

$Q_0$ vs $B_p$ data for fine-grain cavity RDT13 at 2.0 K (solid symbols) and at 1.6 K (empty symbols) after different treatments applied to the outer surface.

Figure 7

“Unfolded” temperature maps at 1.6 K on cavity TD5 at (a) $B_p\sim 62\mathrm{mT}$ and (b) at $B_p\sim 116\mathrm{mT}$.

Figure 8

“Unfolded” temperature map at 2.0 K on cavity TD5 (a) just before quench ($B_p\sim 116\mathrm{mT}$) and (b) $\mathrm{\Delta }{T}_{\mathrm{out}}$ measured at three different locations highlighted with a white border in (a), during increasing rf power.

Figure 9

Temperature map taken during quench at 1.8 K at $B_p\sim 120\mathrm{mT}$.

Figure 10

Comparison between measured and calculated $Q_0(B_p)$ curves for cavity TD5 (a) and RDT13 (b) using the TFBM with the measured thermal resistance for the cases of linear (--) and nonlinear BCS resistance (-) as described in the text.