Lumped circuit element analogy for a resonant cavity

A resonant cavity is a volume enclosed by metal walls that supports an electromagnetic oscillation. In accelerator applications, the oscillating electric fields accelerate charged particles while the oscillating magnetic fields provide inductive isolation. A harmonic voltage generator with output $V(t)=V_0 e^{i\omega t}$ drives a parallel combination of a resistor, capacitor, and inductor. When $\omega=1/(\sqrt{LC})$ the impedance of the combined capacitor and inductor becomes infinite. This condition is called resonance; the quantity $\omega_0$ is the resonant frequency. At resonance, the net current from the generator is minimized for a given voltage. This is the optimum condition for energy transfer if the generator has nonzero output impedance. For a cavity with resistive losses, power must be supplied continuously to support oscillations. A circuit is in resonance when large reactive currents flow in response to input from a harmonic power generator. In other words, the amplitude of electromagnetic oscillations is high.