Observation of the generation of multiple electron beams during a single sheath expansion phase in capacitive RF plasmas

The fundamental investigation of different electron heating modes is important in order to fully understand the generation of plasmas, as well as to optimize their technological applications. In this study, a capacitively coupled radio-frequency discharge is operated at its limit of comparably low plasma density. Phase resolved optical emission spectroscopy provides insights into the electron dynamics on a nanosecond time scale under these conditions. At low applied voltage amplitudes, it is observed that more than one electron beam is generated within a single phase of sheath expansion at a given electrode. When the voltage amplitude is increased these beams merge in time to a single electron beam. This effect has been predicted by particle in cell/MonteCarlo collision simulations before and contradicts existing models that assume the generation of a single beam per sheath expansion phase by stochastic heating (Wilczek et al 2015 Plasma Sources Sci. Technol. 24 024002; Wilczek et al 2016 Phys. Plasmas 23 063514). In this study, results from a systematic experimental study of the effect are presented, which support the theoretically predicted phenomenon.