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Improved ion thruster

Researchers from the Harbin Institute of Technology in China have created a new inlet design for Cylindrical shaped Hall thrusters (CHTs) that may significantly increase the thrust and allows spaceships to travel greater distances.

Xenon is often used as a propellant for Hall Thrusters. This is accelerated by an electric field which strips electrons from neutral xenon atoms, creating a plasma. Plasma ejected from the exhaust end of the thruster can deliver great speeds, typically around 70,000 mph.

CHTs are designed for low-power operations while low propellant flow density can cause inadequate ionization—a crucial step in the creation of the plasma and the generation of thrust. A thruster’s performance improves by increasing the gas density in the discharge channel, while lowering its axial velocity—the speed perpendicular to the thrust direction.

Physics of Plasmas – Effect of vortex inlet mode on low-power cylindrical Hall thruster

a new propellant inlet mode for a low-power cylindrical Hall thruster called the vortex inlet mode. This new mode makes propellant gas diffuse in the form of a circumferential vortex in the discharge channel of the thruster. Simulation and experimental results show that the neutral gas density in the discharge channel increases upon the application of the vortex inlet mode, effectively extending the dwell time of the propellant gas in the channel. According to the experimental results, the vortex inlet increases the propellant utilization of the thruster by 3.12%–8.81%, thrust by 1.1%–53.5%, specific impulse by 1.1%–53.5%, thrust-to-power ratio by 10%–63%, and anode efficiency by 1.6%–7.3%, greatly improving the thruster performance.

Low-power Hall thrusters (HTs) have recently received increased attention as one of the most promising electric propulsion systems for space applications, particularly in conjunction with advanced space missions such as formation flying and micro-spacecraft constellation. Hall thrusters (HTs) have been developed to have a relatively high efficiency of 45%–55% in the power range of 0.5 to 5 kW.2 However, scaling down the HT to a low power range has several challenges owing to its large surface-to-volume ratio and difficulty in miniaturizing the inner magnetic pole, which would aggravate the channel erosion and decrease the thruster lifetime. A cylindrical Hall thruster (CHT) is a type of Hall thruster designed for low power operations. Unlike conventional annular Hall thrusters, the CHT has a smaller surface-to-volume ratio that makes it more convenient for miniaturization and ensures reduced plasma-wall interactions, leading to erosion of the thruster channel. Its performance is comparable with that of a conventional coaxial Hall thruster of the same size.

Neutral flow dynamics is a basic physical process that has a very important effect on the ionization and plasma motion of a Hall thruster. Neutral flow dynamics can be described by distributions of neutral density and velocity, where the density and velocity of the neutral gas directly affect the plasma density in the discharge channel and the residence time of the neutral gas in the discharge channel. Generally, higher neutral gas density in the discharge channel and lower axial velocity will improve the ionization rate of the thruster and thus improve the overall performance of the thruster.

The most practical way to alter the neutral flow dynamics in the discharge channel is by changing the gas injection method or the geometrical morphology of the discharge channel. Vial et al. and Kim et al. tested a variety of anode geometries and concluded that the divergence of the plume was decreased due to geometry changes that increased the neutral residence time and caused ionization improvements. They pointed out that an optimal injection method must maintain an azimuthally and radially uniform neutral flow, which will maximize the neutral residence time. The neutral velocity can be controlled by changing the anode injection method or by directly cooling the anode to reduce the neutral thermal velocity. They cooled the propellant to reduce the velocity of the flow, leading to lower discharge oscillations and hence increased thruster stability. Another method to study neutral flow dynamics is changing the profile of the discharge channel. Raitses et al. diminished the discharge channel cross-section at the ionization zone. The ionization efficiency was increased by increasing the neutral density in this region.

In general, reducing the axial velocity of the neutral gas in the discharge channel is an effective means of increasing the ionization rate of the thruster.

They recently proposed a magnetically insulated anode structure for the low-power CHT, which effectively improves the life and performance of the thruster by changing the distribution of the magnetic field lines in the discharge channel. In this paper, a circumferential vortex inlet mode is proposed on the basis of the low-power CHT with magnetic induction. The influence of the vortex inlet mode on the low-power CHT performance is studied via computer simulations as well as experimentally.

“The work we report here only verified the practicability of this gas inlet design,” Wei said. “We still need to study the effect of nozzle angle, diameter, the ratio of depth to diameter and the length of the discharge channel.”

According to Wei, the vortex design will be tested in flight-type HTs in the near future and could potentially be used in spaceflights.

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