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A plasma-coupled methane-air flame is produced at atmospheric pressure by a microwave plasma source utilizing a tunable waveguide. Laser diagnostics are used to examine the direct-coupled, plasma-ignited, and sustained flame, for multiple flame types and nozzle geometries. OH radical number densities are quantified using planar laser-induced fluorescence and temperature measured by Rayleigh scattering thermometry. Premixed and nonpremixed flames are studied using both solid and hollow inner conductors in the plasma-applicating nozzle. The plasma source is powered by a continuous 2.45-GHz magnetron producing 360 W of power. Plasma power is controlled by adjusting the reflected microwave power, measured at a dummy load attached to a circulator. Maximum OH radical number densities were quantified as approximately (3 - 5) × 1016 cm-3 for plasma powers around 100 W, with small variation between configurations. The maximum temperatures occurred in the nonpremixed flame, where the plasma is generated in air, reaching values of 3500 K. Temperatures are lower, peaking at 2000 K, when the plasma is generated at the air-fuel boundary or the air-premixed boundary through use of the hollow inner conductor. Additional parameters are adjusted, including flow rates, power level, and equivalence ratio, and the effects are discussed. Nonpremixed configurations are ill suited for flame enhancement, whereas a premixed flow through the hollow electrode best demonstrates nonthermal plasma-assisted combustion.