Ultralow Surface Resistance via Vacuum Heat Treatment of Superconducting Radio-Frequency Cavities

Abstract

We report on an effort to improve the performance of superconducting radiofrequency cavities by the use of heat treatment in a temperature range sufficient to dissociate the natural surface oxide. We find that the residual resistance is significantly decreased, and we find an unexpected reduction in the BCS resistance. Together these result in extremely high quality factor values at relatively large accelerating fields Eacc ~20 MV/m: Q0 of 3-4´10 11 at <1.5 K and Q0 ~5´10 10 at 2.0 K. In one cavity, measurements of surface resistance versus temperature showed an extremely small residual resistance of just 0.63 ± 0.06 nW at 16 MV/m. SIMS measurements confirm that the oxide was significantly dissociated, but they also show the presence of nitrogen after heat treatment. We also present studies of surface oxidation via exposure to air and to water, as well as the effects of very light surface removal via HF rinse. The possibilities for applications and the planned future development are discussed.

Quality Factor Enhancement of 650 MHz Superconducting Radio-Frequency Cavity for CEPC

“…R BCS was 2 nΩ constantly at low fields, and slightly increased as a function of accelerating gradient above 20 MV/m in Figure 5. The decrease in R BCS as a function of accelerating gradient for 1.3 GHz cavity [1][2][3] did not appear for 650 MHz cavity. It confirms that mid-T furnace baking could not reverse R BCS at the frequency of 650 MHz.…”

“…There was no field emission during the test, and the cavity quenched at 31.2 MV/m (B peak = 131 mT) finally. Furthermore, there was no anti-Q-slope behavior, which is usual for 1.3 GHz cavities after mid-T baking [1][2][3].…”

“…There was no high-temperature annealing for hydrogen outgassing before or after EP, because we wanted to simplify the recipe of mid-T furnace baking, which combines high-temperature annealing and mid-T furnace baking at a later stage. Therefore, the 650 MHz cavity received high-pressure rinsing (HPR), assembly, low-temperature baking (120 • C for 48 h) and vertical test directly after bulk EP [1]. In the vertical test of EP baseline (blue icon), the 650 MHz cavity quenched at a very high gradient of 37.5 MV/m (B peak = 158 mT) with Q 0 of 1.4 × 10 10 in Figure 1, which indicated smooth and defect-free surfaces of the cavity.…”

“…Here, R S is the electrical resistance of surface layer to current, and inversely proportional to Q 0 of SRF cavity. Recently, the technology of medium-temperature (mid-T, 250-400 • C) baking is used to increase Q 0 of 1.3 GHz cavities successfully, which dissolves the oxides in the surfaces of niobium [1][2][3]. It is more convenient and easier than nitrogen doping/infusion [4][5][6].…”

Effects of nonmagnetic impurities and subgap states on the kinetic inductance, complex conductivity, quality factor and depairing current density

“…Reduction of R s (or improvement of Q) has been the primary interest of researchers of resonant cavities over the last decades. Vortex-free Nb cavities [50][51][52][53][54][55] exhibit huge quality factor Q ∼ 10 10 -10 12 at T < 2 K [56][57][58][59] even under the strong rf current [60][61][62][63] close to the depairing current density. The depairing current density J d is related to the bias current of SSPD and is the maximum current that SRF cavities and superconducting cables can support.…”