Surface dielectric barrier discharges (DBDs) are investigated experimentally in CO2 from atmospheric pressure up to supercritical conditions. Two discharge regimes are generated using 10 kHz ac excitation. The 'standard' regime is similar to previously studied surface DBDs in terms of onset voltage as a function of pressure, as well as electrical and optical emission characteristics. However, a 'field-emitting' regime emerges starting from 0.7 MPa that exhibits constant onset voltage up to 7.9 MPa, purely continuum emission spectra in the visible/near-infrared range and current waveforms similar to an atmospheric-pressure Townsend discharge with ionization instabilities. The maximum amount of negative charge deposited as a function of the applied voltage amplitude is consistent with the Fowler-Nordheim equation, which demonstrates the presence of field emission. Furthermore, this behavior cannot be attributed to the Townsend or streamer ionization mechanisms, secondary electron emission or non-discharge processes. No field-emitting structures are specially added to the electrodes. The onset voltage of the field-emitting regime does not follow the modified Paschen's law for field emission-assisted breakdown.
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