Resolving transient discharge cycle behaviour in modulated inductive plasmas

The further development of inductive plasma generators (IPGs) for space propulsion applications has driven the need for the development of non-intrusive experimental techniques that can resolve transient discharge cycle behaviour and accommodate a variety of propellant gases and gas-mixtures. In this work, a new approach is presented that simultaneously measures the antenna current, radially-resolved axial magnetic field strength and the radially-resolved emission in the visible range, at a sampling frequency sufficient to capture the driving and discharge cycle frequencies. Experiments were conducted using IPG7: a helical antenna IPG, driven nominally at 586 kHz, with 300 Hz discharge cycles. Nitrogen, argon-nitrogen, and argon-oxygen propellants were tested at capacitive and inductive conditions with input powers ranging from 4 kW to 36 kW. The propellants could be sourced from the space environment under in-situ resource utilisation. Analysis of the mean discharge cycle has made it possible to identify different coupling modes occurring within the cycle. These findings underscore the need to model such plasmas at the timescale of individual discharge cycles and demonstrate the value of the measured data for developing such models.