Sheath dynamics of a planar atmospheric pressure RF plasma

The dynamic process of plasma formation and propagation is important to deepen understanding of plasma physics and to exploit further applications. In this contribution, the dynamics of a planar plasma generated by dielectric barrier discharge in He gas is investigated by time resolved spectroscopy and numerical simulation. The plasma source is powered by a 13.56MHz power supply, and operates as uniform alfa-mode discharge with power below 35W and He flow rates of 4 SLM. Time resolved spectroscopy and fast imaging techniques are used to study the discharge behavior. An overview spectrum of He discharge ranging from 250-800 nm is obtained and analyzed. The gas temperature in the center of the plasma region in between the electrodes is estimated around 375±50 K. The RF plasma discharge sustains the sheath regions near both electrodes with 100μm width during the whole voltage cycle. The variation in the sheath region size and intensity is observed at different timing of T+25ns, T+35ns, T+45ns, T+60ns during the RF cycle by means of imaging techniques with 5ns resolution as presented for typical conditions and time frames corresponding to maximum and minimum of the applied voltage. The results show an apparent intensity change between upper and lower electrode. At the positive cycle the upper electrode (virtual cathode) has a higher intensity of the discharge nearby it, whereas in the last image an opposite situation is clearly visible corresponding to the bottom electrode as the cathode. It was found that images obtained with OH radicals emission filter with a center wavelength at 309 nm do not show similar phenomena. The OH(A) emission does not follow the RF voltage waveform. In our opinion, it is explained by the fact that among most important reactions of OH(A) productions, the electron impact and ion impact reactions of OH(A) generation are likely less important than reactions with He metastables. 3D fluid modeling of the RF discharge is performed with COMSOL. It is found that the sheath changes are very similar to the experiment observations. Simulation results indicate that the electron temperature in the sheath and the plasma bulk are about 6 eV and 0.3 eV, respectively. Besides, electron density in the sheath and the plasma bulk are around 1.5×10^18 m^-3 and 1.0×10^18 m^-3. The research results suggest that there is a strong correlation between the mechanism of sheath formation and electron density as well as electron temperature variation