Ignition of an ionic liquid dual-mode monopropellant using a microwave plasma torch

The blend of hydroxylammonium nitrate (HAN) and imidazole-based ionic liquid has shown good feasibility in chemical and electric dual-mode green space propulsion. As conventional catalyst-driven ignition faces challenges arising from long preheat delay and catalyst failure, this work designs a novel ignition actuator using a microwave plasma torch. The precise tuning of the ¼-λ resonant cavity and improvement in the quality factor (QF) of the microwave igniter make the device efficient and thus allow the propellant to be ignited at a power of approximately 100 W. In the present reduced test rig, the maximum fuel flow rate corresponds to 0.1 N chemical thrusters, and the plasma-assisted ignition is mainly attributed to thermal and kinetic effects. The temperature of the actuator and the plasma torch is investigated by infrared thermometry and spectral fitting of excited molecule nitrogen, respectively. First, the hot electrode tube helps evaporate water molecules and accelerate the liquid jet through rapid gas expansion. Then, the ionic liquid quickly decomposes to small species when the flow reaches the torch with a nitrogen vibrational temperature above 4000 K and rotational temperature above 2000 K. Meanwhile, the nonequilibrium plasma-excited species enhance the combustion of gaseous intermediates by direct impact dissociation, where the OH radical profile is visualized using the planar laser-induced fluorescence technique. In addition, the copper atoms released by plasma erosion are expected to exhibit a significant catalytic effect on ionic liquid decomposition, which is a critical step in controlling the overall reaction rate. It is also noted that such mild erosion does not affect the multi-start operations during the firing test that lasts for at least 30 min.