Axial light emission and Ar metastable densities in a parallel plate dc microdischarge in the steady state and transient regimes

Axial emission profiles in a parallel plate dc microdischarge (feedgas: argon; discharge gap d = 1 mm; pressure p = 10 Torr) were studied by means of time-resolved imaging with a fast ICCD camera. Additionally, volt-ampere (V-A) characteristics were recorded and Ar* metastable densities were measured by tunable diode laser absorption spectroscopy (TDLAS). Axial emission profiles in the steady-state regime are similar to corresponding profiles in standard size discharges (d approximate to 1 cm, p approximate to 1 Torr). For some discharge conditions relaxation oscillations are present when the microdischarge switches periodically between the low current Townsend-like mode and the normal glow. At the same time the axial emission profile shows transient behavior, starting with peak distribution at the anode, which gradually moves toward the cathode during the normal glow. The development of argon metastable densities highly correlates with the oscillating discharge current. Gas temperatures in the low current Townsend-like mode (T-g = 320-400 K) and the high current glow mode (T-g = 469-526 K) were determined by the broadening of the recorded spectral profiles as a function of the discharge current.