Cylindrical shaped Hall thrusters (CHTs) lend themselves to miniaturisation and have a smaller surface-to-volume ratio that prevents erosion of the thruster channel. Researchers at the Harbin Institute of Technology in China have developed an inlet design for CHTs that significantly increases thrust.
CHTs are designed for low-power operations. However, low propellant flow density can cause inadequate ionisation, a key step in the creation of the plasma and the generation of thrust. In general, increasing the gas density in the discharge channel while lowering its axial velocity (the speed perpendicular to the thrust direction) will improve the thruster's performance.
Liqiu Wei, lead researcher, said: “The most practical way to alter the neutral flow dynamics in the discharge channel is by changing the gas injection method or the geometric morphology of the discharge channel.”
The investigators tested a simple design change. The propellant is injected into the cylindrical chamber of the thruster by several nozzles that usually point straight in, toward the centre of the cylinder. When the angle of the inlet nozzles is changed slightly, the propellant is sent into a rapid circular motion, creating a vortex in the channel.
Wei and his co-workers simulated the motion of the plasma in the channel for both nozzle angles using modelling and analysis software that uses a finite element approach to modelling molecular flow. The results showed that the gas density near the periphery of the channel is higher when the nozzles are tilted and the thruster is run in vortex mode. In this mode, gas density is significantly higher and more uniform, which also helps improve thruster performance.
The investigators verified their simulation's predictions experimentally, and the vortex inlet mode successfully produced higher thrust values, especially when a low discharge voltage was used. In particular, the specific impulse of the thruster increased by 1.1 to 53.5% when the discharge voltage was in the range of 100 to 200V.
Wei said his team still need to study the effect of nozzle angle, diameter, the ratio of depth to diameter and the length of the discharge channel He went on to predict that the vortex design will be tested in flight-type HTs soon and may eventually be used in spaceflight.
