Reduced electric field in unconfined pulsed glow discharge at low pressure under still air and hypersonic flow conditions

This study investigates the fundamental principles governing non-intrusive hypersonic flow diagnostics utilizing glow discharges by evaluating the hypothesis that the reduced electric field within these discharges remains uniform in hypersonic environments. The electric field of an unconfined pulsed glow discharge is measured in both low-pressure stationary air and Mach 10 flow conditions using the floating probe method. The feasibility of the probe technique for measuring transient plasmas is also assessed. The time histories of the floating potential demonstrate that the probe method possesses a sufficiently rapid temporal response to resolve the quasi-steady phase of the glow discharge. In stationary air, the axial electric field is uniform and scales linearly with the gas number density, maintaining a constant reduced electric field E/N of approximately 1 × 10 −20 Vm 2 . Synchronized optical and electrical diagnostics reveal that luminous spots at the probe tips—likely driven by localized double layers—introduce disturbances to the plasma, resulting in an overestimation of the measured floating potential. The frequent emergence of these spots under high-pressure or hypersonic flow conditions poses a challenge to utilizing this technique for electric field measurements. To mitigate the influence of these localized perturbations on the acquired data, a double-probe setup was implemented under the assumption that the potential shifts induced by comparable luminous spots on both probes are largely canceled out. Based on this configuration, tentative results indicate that the reduced electric field remains constant along the positive column interacting with the shock wave in the hypersonic flow. The constancy of the reduced electric field across both stationary and hypersonic flows simplifies the theoretical model by eliminating the need for simultaneous electric field measurements, thereby validating the practical feasibility of the glow discharge method as a non-intrusive diagnostic technique.