  Here, we study the paramagnetic ions behavior in presence of a strong
microwave electromagnetic field sustained inside a cryogenic sapphire
whispering gallery mode resonator. The high frequency measurement resolution
that can be now achieved by comparing two CSOs permit for the first time to
observe clearly the non-linearity of the resonator power sensitivity. These
observations that in turn allow us to optimize the CSO operation, are well
explained by the Electron Spin Resonance (ESR) saturation of the paramagnetic
impurities contained in the sapphire crystal.
