= Maser cavity
The maser cavity stores a resonant <electromagnetic wave> so that an arriving <molecule> encounters a field left by earlier <molecules>. An output coupling lets part of that stored <energy> reach a detector. The same coupling also removes <energy>, so increasing extraction can prevent startup. The cavity provides feedback; the selected <molecular beam> supplies the useful internal <energy>.
Let $U$ be stored field <energy>, $\omega=2\pi\nu_0$, and $\kappa=\kappa_{\mathrm{int}}+\kappa_{\mathrm{out}}$ the total passive energy decay rate. Internal dissipation and output extraction contribute $\kappa_{\mathrm{int}}U$ and $\kappa_{\mathrm{out}}U$ to lost <power>. With this energy convention, the loaded <quality factor> is
$$
Q_{\mathrm L}=\frac{\omega U}{\kappa U}=\frac{\omega}{\kappa},
\qquad
\frac{1}{Q_{\mathrm L}}=\frac{1}{Q_{\mathrm{int}}}+\frac{1}{Q_{\mathrm{out}}}.
$$
The field amplitude decays at $\kappa/2$; confusing amplitude and <energy> decay introduces a factor of two into a <maser oscillation threshold> calculation.
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