Methane dissociation in pulsed dc discharges at high reduced electric field

High-speed discharge imaging, current/voltage characteristic, and time-resolved emission spectroscopic measurements were performed on fast-rise (20 ns), short-pulsed (400 ns), high-voltage (4-8 kV) dc discharges into CH4 and pure H-2 flows at 50 torr as functions of discharge gap (0.25-1.00 cm). Additionally, the discharges were spectrally imaged using plasma emissions from the C-2 bands near 516 nm and the CH band at 431 nm. The dependence of discharge homogeneity on discharge gap at constant pressure and discharge gas preionization are documented. Current/voltage measurements with variable gap give a sheath voltage drop of 300 V and an average E/N of 80 Townsends during the steady-state portion of the discharges, and up to 300 Td during the early part of the discharge pulse (5-kV drive voltage). Under conditions that enhance discharge inhomogeneity, C-2 emissions are substantially brighter (similar tofactor of 7) than when the discharge is diffuse. This suggests that the buildup of carbon clusters on the discharge electrodes plays a role in the formation of highly inhomogeneous discharge geometries. Time-resolved measurements of H-alpha radiation in CH4 and H-2 discharges suggest significant H-atom production (similar to10(14) H atoms/cm(3)) during a single discharge pulse into CH4 with 5.6-mJ (0.70-mJ/cm(3)) pulse energy.