Physics of the sapphire whispering-gallery-mode solid-state MASER oscillator Mohamad Mrad, Ahmad Tarhini, Pierre-Yves Bourgeois and Vincent Giordano Eur. Phys. J. Appl. Phys., 91 3 (2020) 31001 Published online: 22 September 2020 DOI: 10.1051/epjap/2020200107