  Packed bed plasma reactors (PBPRs) inherently have complex geometries where
the volume between the electrodes is filled with dielectric/catalytic pellets
to form an array of voids and cavities. This design generates multiple length
scales i.e. the dimension of the plasma region (of the order of centimeters),
of the dielectric/catalytic pellets (of the order of millimeters) and of the
void between the dielectrics (or cavities) and/or surface pores that can reach
micrometer dimensions. The plasma is formed as an array of connected or
segmented microdischarges. The understanding of plasma propagation on these
diverse length scales is important in optimizing and controlling this process.
In this work, we investigate the discharge formation and propagation at
different length scales as a function of applied voltage in a simplified design
of PBPRs, a patterned dielectric barrier discharge (p-DBD), operated in helium
at atmospheric pressure, using phase and space resolved optical emission
spectroscopy (PROES). The p-DBD is based on an array of plasma facing glass
hemispheres implemented into one of two electrodes and regularly arranged in a
hexagonal pattern. The plasma is initiated as filamentary microdischarges at
the positions of minimum gap followed by radial acceleration of electrons over
the dielectric surface as surface ionization waves to finally form surface
microdischarges at the voids between the pellets, i.e. at the contact points.
With the increase of the voltage amplitude, subsequent pulses of surface
microdischarges are generated at the contact points. The plasma emission of
each of the surface micro-discharges (S-MD) is found to consist of microscopic
structures whose emission intensity increases as a function of the driving
voltage amplitude. Adjacent microdischarges interact to generate a wave-like
emission intensity propagation from the center of the array to the edges.
