Electromagnetic wave propagation in pulsed surface wave sustained plasmas at atmospheric pressure

Abstract In this work, a long surface wave plasma column is generated using high power pulse-modulated microwave power in argon at atmospheric pressure. The temporal evolutions of the electron density and temperature are diagnosed by optical emission spectroscopy. It is found that the emission intensity peaks correspond to the nodes of standing surface waves where the local electric field is reduced, rather than the antinodes, which is in contrast with that in low pressure discharges. The reasons for this behavior are discussed by considering the excitation balance of the excited levels of Ar I in the plasma. A standing surface wave pattern propagating with the movement of the ionization front in the plasma column, which plays the role of a discontinuity, is observed by means of microsecond time-resolved imaging. Another standing wave at the location of the launcher is also described which indicates that the region below the gap also acts as a discontinuity for the wave propagation. The formation of the propagating standing wave is discussed with respect to the wave propagation characteristics by using an electromagnetic (EM) model based on the propagation of the surface wave. This study underlines the fundamental differences in the EM wave/plasma interactions between continuous and pulsed surface wave discharges and provides new insights into the importance of the microwave applicator segment for a complete description of the plasma column formation.