High field Q slope and the effect of low-temperature baking at 3 GHz

Abstract

A strong degradation of the unloaded quality factor with field, called high field Q slope, is commonly observed above B p ≅ 100 mT in elliptical superconducting niobium cavities at 1.3 and 1.5 GHz. In the present experiments several 3 GHz niobium cavities were measured up to and above B p ≅ 100 mT. The measurements show that a high field Q slope phenomenon limits the field reach at this frequency, that the high field Q slope onset field depends weakly on the frequency, and that the high field Q slope can be removed by the typical empirical solution of electropolishing followed by heating to 120°C for 48 hrs. In addition, one of the cavities reached a quench field of 174 mT and its field dependence of the quality factor was compared against global heating predicted by a thermal feedback model.

Electron Tunneling and X-Ray Photoelectron Spectroscopy Studies of the Superconducting Properties of Nitrogen-Doped Niobium Resonator Cavities

“…These discoveries of the microwave reduction of R s (H 0 ) have triggered intense investigations of its microscopic mecha) iavarone@temple.edu anisms, as well as the materials modifications [27][28][29][30][31][32] , by which it can be further enhanced and extended to higher RF fields of prime importance for the SRF cavity applications. It turned out that the behavior of the nonlinear surface resistance at high RF fields is controlled by the physics of nonequilibrium superconductivity and nonlinear RF current pairbreaking interconnected with subtle materials features and impurity distributions of the first few nm at the surface.…”

Q-factor optimization for high-beta 650 MHz cavities for PIP-II

“…One interesting characteristic is that the onset of the exponential growth of the residual resistance responsible for the HFQS in the Q-factor versus accelerating field curve appears at approximately 80 mT, while for 1.3 GHz cavities it is usually observed at approximately 100 mT 13 . A frequency dependence of the HFQS onset is not observed when examining a wider range of frequencies 14 . Therefore the different onset of the residual resistance exponential growth may be due, for example, to a different intake of hydrogen during the electropolishing treatment instead of being fundamentally linked to the frequency.…”

A White Paper on Design and Fabrication of SRF Deflecting Cavities for Elletra-2

“…The latter has been demonstrated in cryomodules developed for LCLS-II using magnetic shielding around the accelerating cavity [11]. According to the tests of the 3-GHz elliptical cavity performed at JLab [12], one can expect to achieve a surface resistance of Nb at 2 K and 3 GHz in the range 35-70 nΩ. However, we also consider that the cavity string assembly is much longer than its transverse dimension, and it can be difficult to perform surface treatment procedures (electropolishing and high-pressure rinsing) to achieve the best performance, so we take a very conservative approach in our estimation of the surface resistance of materials.…”