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A 2469 MHz discharge was generated at an open end of a coaxial waveguide, through which atmospheric‐pressure gas flowed. Argon served as a plasma‐forming gas, hydrogen and calcium as probe elements for the measurement of electron density and gas temperature, respectively, and sodium as a seed for increasing electron density. The density of excited hydrogen atoms, the excitation temperature, the electron density, the ionization temperature, and the gas temperature were measured at operating powers of 50, 200, and 400 W. The plasma in the close vicinity of the electrode (0 to 0.5 mm from the terminus of the inner conductor) was not in the local thermal equilibrium at any power level investigated, independently of the sodium seeding. At a distance between 1 and 3 mm from the electrode, on the contrary, the nonseeded plasma approached the local thermal equilibrium when the power was increased, while the seeded plasma exhibited a deviation from the local thermal equilibrium. The deviation from the local thermal equilibrium in the nonseeded plasma can be interpreted to be due to inhomogeneity of the plasma. In the outer zone of the seeded plasma, the excitation temperature was equal to the gas temperature but not to the ionization temperature. A separation of plasma components occurred which was depressed by the sodium seeding.