A quasi-optical electron cyclotron maser operating at 28 GHz is studied for applications in heating fusion plasmas. Large spherical mirrors with a small axial aperture and coupling mirror form the open resonator. In the experiment, both the large mirror and coupling mirror are adjusted to select a preferential mode of operation. This is found to improve the efficiency of interaction. Maximum efficiency was observed with a 2.5-A, 60-kV electron beam, with efficiency declining at higher currents. Water calorimetry was used to measure an efficiency of 10%. A photon-drag detector indicated higher peak power levels than those measured with calorimetry. The high-efficiency mode was due to the overlap of two cavity eigenmodes TEMn00 and TEM(n−1)10 (cylindrical notation) and to beam trapping effects that caused a better match between the beam footprint and the electric field profile than in other configurations tested.