Abstract
A strong degradation of the unloaded quality factor with field, called high field $Q$ slope, is commonly observed above ${\mathrm{B}}_{p}\ensuremath{\cong}100\text{ }\text{ }\mathrm{mT}$ 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 ${\mathrm{B}}_{p}\ensuremath{\cong}100\text{ }\text{ }\mathrm{mT}$. 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\ifmmode^\circ\else\textdegree\fi{}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.
Highlights
Radio-frequency surface resistance of superconducting niobium exhibits field dependence, which changes drastically with surface treatment and preparation
The high field Q slope was identified in the 1990s [1], when advances with high pressure rinsing allowed for field emission free cavities reaching peak magnetic fields (Bp) above 100 mT
The results show that accelerating gradients above 30 MV=m, and in one case up to ∼41 MV=m, can be reached even at these frequencies with the correct treatment
Summary
Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA and Department of Physics, Center for Accelerator Science, Old Dominion University, Norfolk, Virginia 23529, USA. Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA (Received 17 October 2017; published 29 January 2018). A strong degradation of the unloaded quality factor with field, called high field Q slope, is commonly observed above Bp ≅ 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 Bp ≅ 100 mT. 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
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