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This paper reports a theoretical and experimental study on the spatial structure of a large-scale, slot-antenna excited plasma source operating in Ar. The overdense plasma (with a plasma frequency ωpl higher than stimulating frequency ω) close to the wave energy source sustained by the TM330 surface mode is analyzed. A self-consistent theoretical model based on a set of coupled equations is developed, which includes the electron Boltzmann equation, the rate balance equations for the most important excited species and charged particles, the gas thermal balance equation, and the wave mode electrodynamic equations. The principal collisional and radiative processes that determine the populations in the Ar(3p54s) and Ar(3p54p) levels are accounted for. The model determines the three-dimensional discharge structure, i.e., the radial, azimuthal and axial variations of the main discharge quantities. An experimental validation of the model predictions is achieved using probe techniques and radiophysics methods. Strong correlations are shown to exist between the density distributions of plasma electrons, positive ions, and electronically excited states of Ar atoms and the electric field intensity distribution in the discharge zone of the plasma source.