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We report the first observation of above-threshold maser oscillation in awhispering-gallery(WG)-mode resonator, whose quasi-transverse-magnetic, 17thazimuthal-order WG mode, at a frequency of approx. 12.038 GHz, with a loaded Qof several hundred million, is supported on a cylinder of mono-crystallinesapphire. An electron spin resonance (ESR) associated with Fe3+ ions, that aresubstitutively included within the sapphire at a concentration of a few partsper billion, coincides in frequency with that of the (considerably narrower) WGmode. By applying a c.w. `pump' to the resonator at a frequency of approx.31.34 GHz, with no applied d.c. magnetic field, the WG (`signal') mode isenergized through a three-level maser scheme. Preliminary measurementsdemonstrate a frequency stability (Allan deviation) of a few times 1e-14 forsampling intervals up to 100 s.Comment: REVTeX v.4, 3 pages, with a separate .bbl file and 3 .eps figure

Maser oscillation in a whispering-gallery-mode microwave resonator