Medium Field Q-slope Studies in Quarter Wave Cavities

The quality factor of superconducting radio-frequency niobium cavities decreases with the applied RF field in the medium field range. The medium field Q-slope effect has been investigated by many authors but models, most commonly including thermal feedback, do not fully explain experimental evidence. In this contribution we analyze medium field Q-slope data measured on ISAC-II low beta quarter wave cavities. The investigation takes the direction of testing the thermal dependence of the medium field Q-slope. Two surface heaters are added on the LHe side of the cavity in the high magnetic field region and Q-curves are acquired at different heater power levels. The data is then compared with a model that, in addition to thermal feedback of the surface temperature, takes into account the reduction of the critical temperature with the applied magnetic field. We then draw conclusions concerning the thermal feedback mechanism and on the relationship between the critical temperature field dependence and the Q-slope. INTRODUCTION A typical plot of a cavity quality factor as a function of the peak magnetic field Bp shows a degradation in the range 20 – 100 mT known as ‘Medium field Q-slope’. Understanding the origin of this phenomenon is important, in particular to future CW applications where cryogenic costs dictate the permissible cavity power and the Q-value determines the gradient. Previous studies on medium field Q-slope have been conducted by many authors, and models include hysteresis losses due to ‘‘strong-links’’ formed on the niobium surface during oxidation [1], energy gap decreasing due to superfluid motion [2], and mainly “global thermal instability”[3], [4], [5]. Here the heat created at the interior surface of the cavity, if not efficiently conducted to the low temperature bath, can increase the temperature of the rf surface. The surface resistance, which depends exponentially on the temperature, will rise increasing the power deposition leading to a further increase of the surface temperature. This effect strongly depends on the properties of the niobium and of the interface to the He bath, usually modelled through the RF surface resistance Rs, thermal conductivity k and kapitza conductance Hk. Medium field Q slope can be, in this thermal instability case, represented by a dimensionless parameter introduced by Halbritter [3], defined via an expansion of the surface resistance Rs in even powers of the peak surface magnetic field B: