Comprehensive understanding of the ignition process of a pulsed capacitively coupled radio frequency discharge: the effect of power-off duration

The effect of the pulse-off duration on the time evolution of the plasma and electrical parameters during the ignition phase in a pulsed capacitively coupled radio frequency argon discharge operated at 450 mTorr and 12.5 MHz is investigated synergistically by multifold experimental diagnostics, particle-in-cell/Monte Carlo collision simulations and an analytical model. In the experiment, the electron density is measured time-resolved by a hairpin probe, the spatio-temporal distribution of the electron impact excitation dynamics is studied by phase resolved optical emission spectroscopy, and the amplitudes and the relative phase, phi (vi), of the discharge voltage and current are determined based on the waveforms measured by a voltage and a current probe. The experimental results show that the plasma and electrical parameters during the ignition process depend strongly on the duration of the afterglow period, T (off), primarily because of the dependence of the remaining charge density on this parameter. Computed values of phi (vi) show a similar time-dependence compared to the experiment, if the simulations are initialized with specific initial charged particle densities, n (ini). This allows us to further understand the time evolution of phi (vi) for different values of T (off) based on the simulation results together with an analytical model. In particular, the optical emission intensity is found to change with time in the same fashion as the power deposition into the system at T (off) > 100 mu s, suggesting that the power is primarily absorbed by the electrons, which dissipate their energy via inelastic collisions. The system goes through different mode transitions of electron power absorption during the ignition phase depending on T (off). Specifically, for short T (off) (high n (ini)), the alpha mode dominates during the entire ignition process, as the electric field is largely shielded by the abundant charge located in the interelectrode space. For intermediate values of T (off) (moderate n (ini)), another excitation pattern caused by an enhanced drift electric field at the center of the gap is observed, since a large fraction of the externally applied potential can penetrate into the central region in the absence of high charged particle densities. For longer T (off) (very low n (ini)), the ignition of the pulsed plasma behaves like a gas breakdown.