Real-time electron density measurement technique using a microwave resonant probe for low-pressure plasmas

Abstract This paper presents a technique to perform a time-resolved electron density measurement in a plasma using a microwave resonant probe named curling probe and a Voltage Controlled Oscillator fast-swept circuit. The technique is an interesting alternative to the Vector Network Analyzer used in on-point mode on repetitive triggered discharges as it also works for non-repeatable events or in cases where trigger signals are unavailable. First, the temporal resolution of the presented setup is assessed to be 5 µ s with an uncertainty on the electron density of 5 × 10 7 cm −3 . Second, the accuracy of the diagnostic is successfully demonstrated by measuring similar electron density for different measurement rates and by measuring forced plasma oscillations at 25 kHz in a magnetic nozzle thruster. Third, the ability of the diagnostic to perform real-time measurements is demonstrated by capturing the transient dynamic of the electron density during the switch-off of the thruster, with a fast plasma density variation of the order of 6 × 10 7 cm −3 / µ s being measured during the first tens of microseconds after plasma extinction.