Electron energy dynamics in a RF discharge with trapezoidal driving voltage: comparison of experiment and particle-in-cell Monte Carlo collision simulation

We report on the properties of a special RF discharge characterized by a trapezoidal shape of the driving voltage pulse. This driving mode offers an advantage in comparison to the common sinusoidal voltage—in allowing the separation in time of effects which are connected with the time derivative of the driving voltage (displacement current, α-mechanism) from effects connected with the amplitude of the driving voltage (production of electrons at the electrodes, γ -mechanism). The energy gain and loss of electrons in this RF discharge are studied in a collision-dominated helium discharge with special emphasis on the α-mechanism. Experimental results of spaceand time-resolved plasma-induced emission spectroscopy are compared and combined with the results of a particle-in-cell Monte Carlo collision simulation. It is demonstrated that the electrons gain energy during the temporal change of the discharge voltage at high neutral particle pressure (400 Pa) in the vicinity of both sheaths and that they dissipate energy in these regions by inelastic collisions. At a lower neutral particle pressure (100 Pa) the electrons gain energy in the vicinity of the expanding sheath and lose energy at the contracting sheath. In this case the energy loss by inelastic collisions takes place in the bulk plasma.