Abstract
Superconducting niobium film cavities deposited on copper substrates ($\mathrm{Nb}/\mathrm{Cu}$) have suffered from strong field-dependent surface resistance, often referred to as the $Q$-slope problem, since their invention. We argue that the $Q$-slope may not be an intrinsic problem, but rather originates from a combination of factors which can be revealed in appropriate environmental conditions. In this study, extrinsic effects were carefully minimized in a series of experiments on a seamless cavity. The origin of the $Q$-slope in low frequency cavities is traced back to two contributions with different temperature and magnetic field dependences. The first component of $Q$-slope, affecting the residual resistance, is caused by trapped magnetic flux which is normally suppressed by a magnetic shield for bulk niobium cavities. The second, temperature dependent component of $Q$-slope, is similar to the medium-field $Q$-slope which is well known in bulk niobium cavities. These results are compared with theoretical models and possible future studies are proposed.
Highlights
Superconducting radio frequency (SRF) cavities are becoming more and more attractive as a core component of modern accelerators
We argue that the Q-slope may not be an intrinsic problem, but rather originates from a combination of factors which can be revealed in appropriate environmental conditions
Nb=Cu cavities were adopted for the Large Electron Positron collider (LEP-II), Acceleratore Lineare Per Ioni (ALPI), Large Hadron Collider (LHC), and High Intensity and Energy Isotope Separator On Line DEvice (HIE-ISOLDE) accelerators [2]
Summary
Superconducting radio frequency (SRF) cavities are becoming more and more attractive as a core component of modern accelerators. The Nb=Cu technology reduces production costs, and increases thermal stability thanks to the high thermal conductivity of the film substrate. Nb=Cu cavities were adopted for the Large Electron Positron collider (LEP-II), Acceleratore Lineare Per Ioni (ALPI), Large Hadron Collider (LHC), and High Intensity and Energy Isotope Separator On Line DEvice (HIE-ISOLDE) accelerators [2]. All these projects had relatively low field requirements. The gradual increase of surface resistance at high rf field, the so-called Q problem, prevents the adoption of this technology for very high-gradient accelerators, such as the International Linear Collider (ILC). We experimentally studied Qs in a Nb=Cu cavity by changing the environmental conditions
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