Discharge mode transition in a high-pressure RF capacitive discharge

$\alpha $ and $\gamma $ mode of a RF helium capacitive discharge were investigated at higher than 5 torr up to atmospheric pressure. The discharge source consisted of two parallel electrodes of same diameters of 60 mm for avoid the self-bias voltage. The discharge gap was fixed as 1 cm at (5 -- 200) torr and varied from 0.5 mm to 5 mm at atmospheric pressure. $\alpha $ and $\gamma $ modes and the mode transition were observed with a nearly 40{\%} voltage drop and a 55{\%} V-I phase angle decrease. The relation between the mode transition voltage and the multiplication of pressure and distance (\textit{pd}) looked similar to the Paschen curve. At atmospheric pressure, the mode transition occurred abruptly with an instantaneous arc generation, different from a smooth transition at lower pressures. At less than 3 mm gap, an abnormal glow discharge occurred, showing a linear current increase with respect to the voltage. At 3 mm gap, $\alpha $ mode excited as a normal glow discharge with a constant current density (17 mA/cm$^{2})$. At over 5 mm gap, either $\gamma $ mode was excited or the discharge was extinguished. It means there is a critical (\textit{pd})$_{cr}$ value for $\alpha $-mode generation at atmospheric pressure, like at lower pressures. From the experimental result and a simple electrical circuit model, we conclude that the transition between two modes resulted from the $\alpha $-sheath breakdown.