A Supersonic Bidirectional Vortex Enhanced Radio-Frequency Inductively Coupled Plasma Torch for Electrothermal Propulsion Applications.
Abstract
Radio-frequency (RF) inductively coupled plasma (ICP) torches have numerous applications in material processing, analytical chemistry, and testing of thermal protection materials. When combined with a supersonic nozzle they also offer applications for electrothermal space propulsion. However, due to historical thermal efficiency limitations, they have not been able to provide the required performance for propulsion. This thesis investigates a supersonic RF ICP torch for propulsion applications that utilises a novel gas injection configuration to overcome such limitations and enhance performance. There are three key objectives: to investigate the influence of gas injection, study the influence of nozzle geometry and obtain stable operation at high pressures.
The feed gas injection systems of RF ICP torches play an important role in stabilisation, gas heating and thermal management. Therefore, two gas injection systems are explored. The first is the conventional forward vortex where gas is injected into four tangential inlets at the RF ICP torch top end resulting in a vortex swirl towards the nozzle. The second is the bidirectional vortex, which is formed by injecting gas into four tangential inlets located at the downstream end of the source tube near the nozzle, resulting in two counter-propagating vortices. To enable safe operation conditions, the RF ICP source tubes are water-cooled, resulting in the cooler outer vortex propagating away from the nozzle, while the hotter inner vortex propagates downstream towards the nozzle. The second configuration is the bidirectional vortex, created by injecting gas through four tangential inlets located at the downstream end of the source tube, near the nozzle. This setup generates two counter-propagating vortices. To maintain safe operating conditions, the RF ICP source tubes are water-cooled, which causes the cooler outer vortex to move away from the nozzle, while the hotter inner vortex travels downstream toward it.
The vortex flow field structure of the bidirectional vortex is expected to play a significant role in gas heating through a reduction of conductive heat losses to the walls. Additionally, nozzle geometry significantly affects the gas exit velocity and is expected to influence both the bidirectional vortex flow field structure and the overall torch performance. In this thesis. The influence of bidirectional and forward vortex gas injection methods are investigated alongside a parametric analysis of nozzle throat diameters ranging from 1.5 mm to 4.0 mm. Their impact on the performance of a supersonic RF ICP torch is investigated across RF power levels from 200 to 1000 W and argon mass flow rates between 0 to 400 mgs$^{-1}$. A torch instability prevents the RF ICP torch from operating at higher pressures. As a result, a preliminary characterisation of the instability is conducted, and its possible origins and causes are discussed.
The results demonstrate that the bidirectional vortex gas injection provides a significant improvement over the forward vortex, including increased gas heating, greater thermal efficiency, and superior thruster performance. Nozzle geometry is found to have an important influence on electron density, excitation temperature and thermal efficiency. An initial characterisation of the torch instability reveals a low-frequency oscillation which could be the result of neutral depletion, however further work is required to better understand the instability. The supersonic vortex-enhanced RF ICP torch displays considerable potential for optimisation across a wide range of operating conditions, offering a variety of prospective industry applications.