It takes very powerful propellant and powerful engines to raise payloads out of the Earth’s deep gravity well. But once you are in space, there are much less powerful propulsion systems that can provide low thrust over time which can add up to substantial acceleration for station keeping in orbit and getting around in the solar system. Hall thrusters show great promise for this application.
Hall thrusters use xenon gas. The xenon is turned into a plasma and electrical fields accelerate the ions, expelling them at seventy thousand miles per hour. The impulse imparted to the craft is very small, but it is constant.
One popular configuration for Hall thrusters is a cylinder (CHT). This configuration can be miniaturized and the small surface to volume ratio helps to prevent erosion of the surface of the sides of the thruster which is a major problem with Hall thruster. Scientists at the Harbin Institute of Technology in China have developed the design for a new inlet for CHTs. This new inlet increases thrust significantly. Their research is in this week’s edition of the journal Physics of Plasmas.
CHT are specifically designed for low-power operations. Unfortunately, when the flow density of the propellant is low, there can be inadequate ionization of the xenon gas. Full ionization is important for the proper creation of the plasma and the generation of thrust. The new inlet allows the increase of the gas density in the discharge channel while the speed of the gas perpendicular to the direction of thrust is reduced. This leads to an improvement in thruster performance.
One of the author’s of the Chinese paper 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 new inlet design required one simple change in the original design. In the original design, the xenon gas is injected into the cylindrical chamber from a bunch of nozzles that point straight in toward the center of the cylinder. The Chinese researchers changed the orientation of the nozzles so that they are pointing in at an angle. The result of this change is to send the gas into a rapid circular motion which creates a vortex in the chamber.
The Chinese researchers used COMSOL modeling software to simulate both of the injection angles. The new design with tilted nozzles caused the density of the gas near the periphery of the chamber to increase. Gas density is significantly higher and more uniform. This also helps to improve thruster performance. The researchers verified their simulation with experiments with physical hardware. The new nozzle configurations produced higher thrust values especially with a lower discharge voltage. When the discharge voltage was between one hundred and two hundred volts, the specific impulse rose between one and fifty percent.
One of the papers authors said, “The work we report here only verified the practicability of this gas inlet design. We still need to study the effect of nozzle angle, diameter, the ratio of depth to diameter and the length of the discharge channel.” He also said that the vortex design will be flight-tested in CHTs and might be eventually used in actual spacecraft.
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