Ion Energy Distributions in the Cathode Sheath of an Argon DC Microdischarge

Summary form only given. The cathode sheath serves many important functions in a discharge. It is responsible for the creation of energetic ions which subsequently create secondary electrons upon striking the cathode. These secondary electrons are themselves accelerated gaining sufficient energy to participate in ionization. Seminal work for ion energy distribution (IED) measurements with theoretical analysis for macroscale direct-current (DC) discharges was done by Davis and Vanderslice. According to their theory, the energy distribution is a function of the ratio of the sheath thickness to the charge exchange mean free path (S/lambda_i). A DC microdischarge has a relatively high ratio of cathode surface area to volume. Understanding the structure of the sheath and its associated physical phenomena is even more critical to the understanding of microdischarges. In this study, the IED in the cathode sheath of an Ar DC microdischarge was measured by energy-resolved molecular beam mass spectroscopy. The pressure-gap distance product (pd) was fixed at 1 cm torr with environmental pressure from 2 torr to 20 torr. The measured IED was analyzed by the expanded theory of Davis and Vanderslice. With a constant pressure normalized current density (Jp^-2=0.082plusmn0.006 mA cm^-2 torr^-2), a higher pressure yields a lower S/lambda_i ratio. The results in the constant Jp^-2 case indicate that a scaling law of Jp^-2 is no longer applicable. The background gaseous temperatures expected from IED and the collisional Child law have reasonably increased with increased J for constant Jp^-2 and constant pressure (10 torr). For example, the expected temperature was elevated to 700 K at 20 torr in the constant Jp ^-2 case. Moreover, the expected temperature shows good agreement with the one estimated by Doppler broadening via laser absorption spectroscopy. Given this result, it is concluded that t- e expanded theory might accurately describe microdischarges at least at this study´s conditions