Discharge characteristics and operational modes in an inertial electrostatic confinement plasma source

Inertial Electrostatic Confinement (IEC) systems exhibit diverse discharge behaviors that have not been quantitatively standardized, and no common reference exists for their characteristic current–voltage relationships. Inconsistent terminology in prior work further limits cross-comparison between studies. This study presents a systematic experimental investigation of breakdown behavior, discharge modes, and visible plasma structure in an argon-fueled IEC device. The influence of cathode geometry parameters, such as diameter, wire gauge, structure, and material, were examined over a pressure range of 2–12 mTorr. Comprehensive current–voltage measurements and high-resolution imaging were used to correlate discharge characteristics with mode transitions and to identify the input parameters governing each regime. Breakdown voltage analysis showed that for pressure and electrode gap products above approximately 0.19 Torr · cm, all cathodes exhibited comparable values independent of geometry, while geometry-dependent deviations up to 76% (for cathode diameter) and 14%–28% (for other geometric parameters) appeared below 0.06 Torr · cm. At a pressure and electrode gap product of about 0.15 Torr · cm, breakdown voltages across all geometries converged within 10% of −1 kV. These results revealed systematic departures from the existing cathode scaling law, leading to a revised formulation that incorporates both anode and cathode diameters and improves predictive accuracy across configurations. Two dominant discharge regimes, jet and spray, were consistently observed; their onset and transitions were quantitatively correlated with the measured current–voltage behavior rather than visual morphology alone. The clarified two-mode classification and improved scaling relationship establish a quantitative framework for predicting and controlling IEC discharge modes, enabling reproducible comparison between systems of differing geometry and scale.