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Cavity resonance effect is a useful concept in beam-wave interaction for enhancing the conversion efficiency. It is shown that an interplay between the growth of field energy inside the cavity and its extraction can be used judiciously to extract a larger power after the initial transients. Cavity parameters in a coaxial virtual-cathode oscillator have thus been optimized to allow field energy inside the cavity to grow initially. This growth is limited by the energy that leaves the cavity through the extraction region so that the field strength inside the cavity saturates optimally at a maximum value. The energy lost by the beam to the field can then be directly extracted at a high conversion efficiency. The peak cycle-averaged power efficiency obtained thus in particle-in-cell simulation exceeds 9%, while the efficiency over several cycles is found to be 7.5%.