Transitions Between Various Diffuse Discharge Modes in Atmospheric-Pressure Helium in the Medium-Frequency Range
Résumé
In this paper, we investigate DBDs in the medium frequency range (MF, 0.3–3 MHz). More precisely, for a 2 inter-dielectric gap in helium at atmospheric pressure, the frequency is varied from 1.0 to 2.7 MHz. The generated discharge shows similarities with both the low-frequency atmospheric-pressure glow discharge (APGD) and the atmospheric pressure capacitively coupled radio-frequency (CCRF) discharge. In the frequency range under investigation, two diffuse discharge modes can be observed depending on the voltage applied between the electrodes. At low applied voltage, the discharge emissions are barely visible and are concentrated in the center of the gas gap similarly to CCRF discharges in the $\Omega $ mode where the electron density is concentrated in the bulk. Ohmic heating is the main power transfer mechanism. At higher applied voltage, the discharge emissions are 10 times more intense and are closer to the dielectric surfaces similarly to the more common radio-frequency α mode. These two discharge modes can be observed in the same experimental conditions with the amplitude of the applied voltage as sole control parameter. The gas temperature obtained from N2 impurities rotational spectrum increases from room temperature to about 500 K while the power density rises from 10−1 to 101 W cm−3 when the applied voltage is increased. In addition, when the discharge transits back and forth from the $\Omega $ to the α mode, a hysteresis is observed. The transition from the $\Omega $ to the α mode occurs abruptly with a large RMS current increase while the transition from the α to the $\Omega $ mode is rather smooth with no significant discontinuity in the RMS current.