Combined plasma diagnostic approaches for characterization of pulsed-DC driven hollow cathode discharge in a metal tube based on current-voltage characteristics, Langmuir probe, and optical emission spectroscopy

Abstract

Pulsed vacuum discharge is widely used in the deposition of films on the inner walls of tubes for lubrication and corrosion prevention. However, the mechanisms by which plasma impacts this process keep ambiguous due to the limited knowledge about plasma characteristics such as spatial-temporal evolution and discharge in tubes of various diameter and length. In this study, high-voltage pulsed-DC driven hollow cathode discharge (HCD) is performed on a metal tube system for the deposition of anti-corrosive films on the inner surface. The current-voltage (CV) characteristics are analyzed in addition to Langmuir probe diagnostics and optical emission spectroscopy (OES). A brief voltage breakdown occurs at the beginning of the discharge, lasting approximately 3–4 μs, but the discharge quickly stabilizes, followed by a steady voltage state. The voltage cathode drop (Uc) is sufficiently high to sustain the discharge, thereby generating secondary electrons (γ-electrons) for excitation and ionization. And, a full HCD is established with a sufficient quantity of γ-electrons at higher discharge voltages (e.g., magnitude lager than 4000 V in our case), indicating that substantial voltage levels are necessary to develop adhesive, high-quality films. The discharge persists even after reaching the peak discharge current, as indicated by the temporal distribution of electron density and emission line ratios obtained from the Langmuir probe and OES measurements, respectively. In conjunction with the Langmuir probe measurements, the effective prediction of electron density and temperature using OES, based on the collision radiative (CR) model, reveals a more suitable method for evaluating plasma properties within tubes, enhancing the understanding of plasma-assisted film growth. Overall, the discharge characteristics, especially the pulse-driven evolution, can be better characterized by CV and Langmuir probe measurements. These reveal the evolution mechanisms of discharge voltage, density, and γ-electrons, which impact film properties such as brittleness induced by insufficient HCD. OES-CR provides a convenient, non-intrusive means to determine plasma radicals and densities, evaluating their effects on film formation, despite limitations in small-diameter tubes which are difficult for intrusive measurements. This comparative investigation thus elucidates the discharge characteristics and feasible methods for revealing the underlying mechanisms associated with pulse-driven discharges and film deposition in diverse tube configurations.