Mode coexistence in radio frequency capacitive discharge of flowing argon at atmospheric pressure with bare perforated electrodes

Three different modes (alpha, gamma and their coexistence) of radio frequency (RF) capacitive discharge are achieved in flowing argon gap at atmospheric pressure with parallel bare metal electrodes perforated with honeycomb arrays of holes. The holes in the electrodes permit active species produced in discharges exiting out for applications such as surface modification and coating deposition. In this paper, pure argon is used as working gas for only discharge characterization. The gas is fed through via the electrode holes. With electrical measurements of discharge voltage, current and their phase angles, alpha and gamma modes are identified and characterized. However, the measurements fail to distinguish the coexistence mode from gamma mode since the I-V slopes of the coexistence mode and gamma mode are analogous to each other, while the discharge photography can reveal all the three modes. Therefore, the OES (optical emission spectroscopy) of the discharges is measured not only to evaluate the discharge gas temperatures but also to distinguish the three modes. The 2nd positive bands of N-2 and the violet bands of OH are simulated to determine the rotational temperature of N-2 and OH, which is believe strongly correlated with gas temperature. The temperature trends vs discharge power can discriminate the three modes better than electrical measurement. By the way, the rotational temperature of OH is well consistent with that of nitrogen, which means that the neutral species are in thermal equilibrium for the three discharge modes. (C) 2012 Elsevier Ltd. All rights reserved.