Surface dielectric barrier discharges (DBDs) in CO2 from atmospheric pressure up to supercritical conditions generated using 10 kHz ac excitation are investigated experimentally. Using current - voltage and charge - voltage measurements, imaging, optical emission spectroscopy, and spontaneous Raman spectroscopy, we identify and characterize a field-emitting Townsend discharge regime that emerges above 0.7 MPa. An electrical model enables the calculation of the discharge-induced capacitances of the plasma and the dielectric, as well as the space-averaged values of the surface potential and the potential drop across the discharge. The space-averaged Laplacian field is accounted for in the circuit model by including the capacitance due to the fringe electric field from the electrode edge. The electrical characteristics are demonstrated to fit the description of atmospheric-pressure Townsend DBDs (Naudé et al 2005 J. Phys. D: Appl. Phys. 38 530-8), i.e. self-sustained DBDs with minimal space-charge effects. The purely continuum emission spectrum is due to electron - neutral bremsstrahlung corresponding to an average electron temperature of 2600 K. Raman spectra of CO2 near the critical point demonstrate that the average gas temperature increases by less than 1 K.
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