Time-resolved evolution of micro-discharges, surface ionization waves and plasma propagation in a two-dimensional packed bed reactor

Plasma packed bed reactors (PBRs) are being investigated for applications ranging from pollution remediation to chemical synthesis, including plasma catalysis. Plasma PBRs typically operate as dielectric barrier discharges where the plasma propagating through the PBR strongly interacts with the dielectric packing media and gas in the interstitial spaces. The nature of plasma propagation through this macroscopically porous-like medium is not well understood. Plasma formation in PBRs is a function of many parameters, including dielectric media composition and surface morphology, dielectric constant, packing fraction, pressure, and the applied voltage waveform. Imaging the plasma propagation through the complex three-dimensional geometry of the packing media and interstitial spaces that make up the PBR is difficult to experimentally execute. In this regard, a two-dimensional PBR composed of dielectric disks was developed to enable optical imaging of plasma formation and propagation. The mode of plasma propagation and the sensitivity of discharge formation to material dielectric constant, applied voltage, and pressure were experimentally and computationally investigated. We found that higher dielectric constants of the packing material produced more intense, localized filamentary micro-discharges between disks. In general, plasma propagation through the PBR at 1 atm is initiated by localized micro-discharges between adjacent dielectric disks, which in turn give rise to surface ionization waves (SIWs) that propagate along the dielectric surface. At pressures below 1 atm, the discharge was more diffuse regardless of the dielectric media, filling the interstitial space instead of forming SIWs.