Striations in dual-frequency capacitively coupled CF\textunderscore 4 plasmas*

Striations in dual-frequency (8/40 MHz) capacitively coupled CF\textunderscore 4 plasmas have been investigated by Phase Resolved Optical Emission Spectroscopy (PROES) and via particle in cell/Monte Carlo collision (PIC/MCC) simulations. The properties of the striated structures as a function of the high-frequency voltage amplitude, $\phi $\textunderscore H, have been analyzed. The measured spatiotemporal electronic excitation patterns at different $\phi $\textunderscore \textunderscore H showed good agreement with the simulation results. As $\phi $\textunderscore H increased, the width of each ion density peak increased, while the striation gap (i.e., the distance between adjacent ion density peaks) remained nearly unchanged, leading to a decrease in the number of striations and, finally, to their disappearance at a certain $\phi $\textunderscore H. In the presence of striations, the maximum ion density inside the bulk was found to increase slightly with $\phi $\textunderscore H, while the minimum ion density was almost independent of $\phi $\textunderscore H, as it is primarily determined by the lower excitation frequency, and is almost unaffected by $\phi $\textunderscore H. A hysteresis associated with the striated - DA (drift ambipolar) mode transition induced by increasing and decreasing $\phi $\textunderscore H has been observed both in the experiments and in the simulations.