Ion energy distribution and gas heating in the cathode fall of a direct-current microdischarge

This paper reports on measurements of the ion energy distribution (IED) at the cathode of an argon dc microdischarge using energy-resolved molecular beam mass spectrometry. The measurements are conducted at a fixed pressure-electrode separation product $(pd)$ of 10.3em0excm0.2em0exTorr with a maximum discharge pressure of 200.3em0exTorr. The measured IED is compared to the theory of Davis and Vanderslice [W. D. Davis and T. A. Vanderslice, Phys. Rev. 131, 219 (1963)]. A higher pressure in a case of almost constant normalized current densities by pressure $(J{p}^{{-}2}=0.080±{}0.006{{0.3em}{0ex}}mA{{\;}}{cm}^{{-}2}{{0.2em}{0ex}}{Torr}^{{-}2})$ yields a lower ratio of the ion mean free path to the sheath thickness. The results in almost constant $J{p}^{{-}2}$ case then indicate that a scaling law of $J{p}^{{-}2}$ is no longer applicable for IED of microdischarge. Expected background gaseous temperatures from IEDs with the collisional Child law have reasonable increasing with increased current density $(J)$ in both cases of almost constant $J{p}^{{-}2}$ and a constant pressure of $10{{0.3em}{0ex}}Torr$. Supported by temperature measurement by laser absorption spectroscopy, it is demonstrated that the expanded theory might be applicable also to microdischarges $(Ar{~}20{{0.3em}{0ex}}Torr)$ with temperature adjusting.