Very powerful RF cavities are now being developed for future large-scale particle
accelerators using niobium superconductor. Today’s prototype cavities operate in RF
surface magnetic fields of up to 180 mT. This is the result of a successful worldwide
technology development effort over the last decades.
The basic model for Q-slope in SRF cavities, i.e. the reduction of the cavity quality
factor with increasing operating electric and magnetic fields, is the so-called thermal
feedback model. The exponential dependence of the BCS surface resistance on
temperature, in feedback with the dependence of the RF power dissipation on the surface
resistance ultimately leads to thermal runaway (thermal quench) of the RF exposed
surface. Before investigating further the high field surface resistance it is important to
understand better the basic Q slope (or surface resistance increase with applied RF field
amplitude) due to thermal feedback.
The main purpose of this note is to compare calculations of Q-slope on the basis of the
BCS resistance and the thermal feedback model with experimental data from cavities.
The discussion encompasses a wide variety of cavities from DESY, CEA-Saclay, J-Lab
and Fermilab. This comparison also includes the non-linear correction to the BCS
resistance as recently proposed by A. Gurevich. |
