Discharge evolution mechanism and spectral diagnostic study of loop dot-matrix concentric-roll pattern

Dielectric barrier discharge (DBD) can produce abundant discharge patterns. It is one of the most interesting nonlinear systems for studying pattern formation. In this work, circular boundaries with different radii are utilized and superimposed to form a narrow and wide combined discharge gap. The pressure is set to 25 kPa for the experiment, and the frequency is fixed at 58 kHz. By varying the applied voltage, concentric-roll pattern, loop dot-matrix concentric-roll pattern, target-wave pattern and honeycomb pattern are obtained. The electrical and optical properties of several types of patterns are analyzed. This study focuses on the spatiotemporal evolution of the loop dot-matrix concentric-roll patterns by using an intensified charge-coupled device (ICCD), and theoretically analyzes the formation mechanism of these patterns. The results show that the discharge pattern has a radial development with a gradual breakdown process from the outside to the inside. It is related to the pre-ionization effect of the narrow gap on the discharge. The emission spectra of different discharged filaments in the radial direction of loop dot-matrix concentric-roll pattern are measured and analyzed. A spatially resolved diagnosis of plasma parameters is performed. It is found that the molecular vibrational temperature, electron density, and electron temperature are much larger in narrow gap than those in wide gap. In the wide gap, the molecular vibration temperature, electron density, and electron temperature gradually increase along the radial direction from the inside to the outside, but the changes are relatively small. In the narrow gap, the parameters such as the molecular vibration temperature, electron density, and electron temperature far from the center of the circle are smaller than those near the center of the circle. This is related to the micro-change of the electric field.