  We theoretically investigate an ultrastrongly-coupled micromaser based on
Rydberg atoms interacting with a superconducting LC resonator, where the common
rotating-wave approximation and slowly-varying-envelope approximation are no
longer applicable. The effect of counter-rotating terms on the masing dynamics
is studied in detail. We find that the intraresonator electric energy declines
and the microwave oscillation frequency shifts significantly in the regime of
ultrastrong coupling. Additionally, the micromaser phase fluctuation is
suppressed, resulting in a reduced spectral linewidth.
