  Plasma technologies are a promising way of addressing a number of challenges
in combustion, ranging from stabilization and flame-holding for hypersonic
vehicles, to enabling low-emissions propulsion and power. Whereas most works in
the literature have centered on the forward problem: the impact of the plasma
on different combustion metrics, including static and dynamic flame
stabilization; fewer studies have been devoted to the backward problem: or the
significance of having a strongly inhomogeneous environment, often unsteady in
time, on the discharge characteristics. In this perspectives article, the
backward problem is addressed in a hierarchical manner. First, the implications
of the discharge proceeding in an inhomogeneous background gas, with gradients
in density and composition, and nonuniform flows, are discussed. Next, these
foundation experiments are used as elementary building blocks to explain
experiments of nanosecond pulsed discharges in dynamic combustion environments,
including a transient laminar flame and a combustor undergoing dynamics.
Finally, the strong coupling between the forward and backward problem is
illustrated with an imaging study, using transparent electrodes, of a mesoscale
premixed flame and a nanosecond pulsed dielectric barrier discharge. The
experiments demonstrate the importance of considering the pulse-to-pulse
evolution due to changes in the background environment when using plasma
assistance in unsteady combustion processes, including active control of
combustion dynamics.
