Wave-like emission propagation and fine structures at the contact points of adjacent dielectric pellets in packed bed plasma reactors (PBPRs) operated in helium

Packed bed plasma reactors (PBPRs) inherently have complex geometries where the volume between the electrodes is filled with dielectric/catalytic pellets to form a large array of voids. While the dimension of the plasma region can be several centimeters, the size of a single void at the edges and pores of dielectrics/pellets can reach micrometer dimensions. The understanding of plasma propagation on these diverse length scales is essential for optimizing and controlling plasma processes performed in such discharges. It is known that plasmas are generated in PBPRs as multiple pulses due to cathode-directed positive streamers in the volume, surface ionization waves, or surface streamers over the dielectric surface and stationary microdischarges at the contact points of adjacent dielectrics. In this work, we have investigated the discharge formation and propagation as a function of applied voltage in simplified PBPRs with a single layer of hexagonally arranged hemispherical pellets, operated in helium, using phase and space resolved optical emission spectroscopy. Despite similar discharge conditions at multiple positions, the emission intensity during each pulse spreads like a wave from the center to the edges in the whole discharge cell. The emission due to surface ionization waves is significantly reduced compared to earlier works. These observations could be explained by possible interactions between adjacent microdischarges, already known in other arrays of microdischarges or adjacent streamers. Higher resolution images of the contact points show that the discharge has fine structures with stronger emission at the edges of the contact points; this effect is enhanced as a function of the driving voltage amplitude. This is possibly the consequence of non-uniform electric field distribution at the contact points due to the polarization of dielectrics.