Elsevier

Acta Astronautica

Volume 171, June 2020, Pages 290-299
Acta Astronautica

Performance optimization of a krypton Hall thruster with a rotating propellant supply

https://doi.org/10.1016/j.actaastro.2020.03.021Get rights and content

Highlights

  • Rotating propellant supply is achieved by a particular gas distributor design.

  • Rotating propellant supply slows down krypton neutrals inside the channel and improves ionization efficiency.

  • Performance increment under rotating propellant supply increases with both discharge voltage and krypton flow rate.

Abstract

The use of krypton in the field of electric propulsion has become a key research topic owing to its high specific impulse and high reserves. However, because krypton has features such as a small ionization cross section and high ionization energy, its ionization performance is relatively low, thereby increasing the actual specific impulse loss of the krypton Hall thruster in comparison with its theoretical value. Therefore, to use of krypton as the propellant of the Hall thruster instead of xenon, it is necessary to study the optimization of its ionization process. In this study, a gas distributor with a rotating propellant supply mode is designed, considering factors such as the velocity and density of the velocity of neutral atoms, which affects the ionization process. The distribution of neutral atom parameters in the discharge channel is changed by the rotation; and its influences on the propellant ionization and other performance parameters of the Hall thruster are experimentally analyzed with respect to the magneto-ampere, volt-ampere, and flow rate-ampere characteristics. The results demonstrate that the rotating propellant supply reduces the axial velocity of neutral atoms, increases the density and circumferential motion distance, and thus improves the ionization efficiency of the krypton Hall thruster. Furthermore, the performance parameters, such as thrust and efficiency, are also improved, and the plume divergence angle is reduced. When the discharge voltage is 450 V and the anode flow rate is 60 sccm, the rotating propellant supply can increase the anode efficiency of the axial propellant supply mode from 46.2% to 53.2%. However, the plume divergence half-angle reduces from 39.2° to 35.5°. In addition, an increase in the anode flow rate and discharge voltage continuously increases the performance promotion in the rotating propellant supply mode.

Introduction

The enormous achievements brought by the rapid development of aerospace technology has greatly affected human lifestyles [1]. However, owing to the continuous deep exploration of outer space, higher requirements have been placed on spacecraft technology [2]; highly advanced structures, materials [[3], [4], [5]], and control technologies [6,7] therefore need to be studied. In particular, space power technology, such as electric propulsion technology, requires continuous innovation and development.

The electric propulsion, used on satellites and spacecraft, has a history of more than 110 years [8]. Among various types of electric thrusters [9,10], the Hall thruster is one of the most widely used types owing to its simple structure, high reliability, and long service life. Xenon is commonly used as the propellant of the Hall thruster because of its low ionization energy and large atomic mass [11,12]. In 2019, low-orbit satellite network used by SpaceX's Starlink adopted the krypton Hall thruster for orbit keeping and partial orbit transfer; thus, the Hall thruster, operating with krypton propellant, has officially entered the aerospace application [13]. SpaceX's latest launch plan comprises 42,000 satellites for networking. Moreover, the high-power and large-total impulse electric propulsion with a huge propellant consumption has become a development trend at present [[14], [15], [16]]. All these potential aerospace applications place enormous challenges and undue costs on propellant reserves. Xenon content in the atmosphere is extremely low and its extraction cost is high; however, the reserves of krypton are 10 times those of xenon, and its current market price is less than one-eighteenth that of xenon (USD 472/m3 for krypton and USD 8750/m3 for xenon). Therefore, researchers are increasingly interested [[17], [18], [19], [20], [21], [22], [23], [24], [25]] in the use of krypton as the propellant for electric propulsion.

In comparison with xenon, krypton has a lower atomic mass but higher ionization energy (the relative atomic mass of xenon is 131.3 and that of krypton is 83.8, while the first ionization energy of xenon is 12.1 eV and that of krypton is 14 eV) [17]. Theoretically, the specific impulse of krypton propellant is 25% higher than that of xenon propellant [20]. This extra specific impulse has significant advantages [24,25] in performing space missions, such as maintaining spacecraft orbit.

In 2006, Linnell et al. of Michigan State University in America, used the NASA-173Mv1 Hall thruster to conduct optimization experimental research [19] on the working efficiency and performance of a krypton Hall thruster by adjusting the anode flow rate and discharge voltage. It was found that the propellant utilization efficiency could be improved by adjusting the mass flow rate, channel length, and electron temperature; however, the improved effect was limited. In the same year, Linnell summarized the results [20] of krypton propellant experiments performed on the P5 Hall thruster of the Air Force Research Laboratory in his academic dissertation. The relative changes in the discharge and performance parameters of the Hall thruster, when using krypton and xenon propellants separately, are compared. When the specific impulses of krypton and xenon are the same, the thrust generated by the krypton propellant at the same power is much smaller than that by the xenon propellant, indicating that the power demand of the thrusters with krypton propellant is large. However, the specific impulses of krypton and xenon propellants under various working conditions were tested, and it was found that although the theoretical specific impulse of krypton is higher than that of xenon, the actual specific impulse of krypton is over 1000 s lower than the theoretical value. In 2003, the NASA Glenn Research Center replaced the xenon propellant with krypton in the 50-kW Hall thruster for experimental research [18]. It was found that at the highest anode mass flow rate and discharge voltage, the specific impulse generated was close to the theoretical value, and the efficiencies of krypton and xenon at high power were close. In 2011, they used the NASA-300 M Hall thruster to repeat the test, at powers of 2.5–20 kW [22]. The experimental results demonstrated that under different power and discharge voltages, all performances of the thruster with the krypton propellant were lower than those with the xenon propellant. At the early stage, Moscow State Technical University of Radio Engineering (MIREA) and Kurchatov Research Center in Russia used the SPT A-3 Hall thruster to perform a comparative experiment of xenon and krypton for differences [23] in the overall characteristics. The Institute of Plasma Physics and Laser Microfusion (IPPLM) in Poland in 2018 and the Plasma Sources and Applications Center of Nan-yang Technological University in Singapore in 2019 both conducted related research on low-power Hall thrusters with krypton propellant. The experimental results demonstrated that the performance of the krypton propellant at low power was relatively poor [24,25]. In 2012, the Shanghai Space Propulsion Research Institute in China also conducted experimental tests on the properties related to krypton propellant and determined some factors that affect the performances of krypton propellant, such as the magnetic field configuration, as well as improvements [26] for the following application research on krypton. These results revealed that the krypton propellant performs well at high power; however, its performance is relatively poor at medium and low powers, and the low ionization efficiency utilization of the krypton is one of the main reasons for the poor performance of the krypton Hall thruster. Corresponding optimizations of the input discharge parameters, such as the magnetic field, anode flow rate, and discharge voltage, were proposed; however, an increase in discharge voltage and anode flow can easily cause thruster thermal instability, which obscures the promotion of krypton ionization efficiency. Furthermore, the involved mechanisms have not been studied in depth; therefore, the optimization on the ionization efficiency of krypton propellant are, hitherto, unsatisfactory; additional efforts must be paid on this issue of ionization efficiency.

In this study, a gas distributor structure with a rotating propellant supply mode is proposed for the optimization of the ionization performance of krypton propellant. The experimental study on the working characteristics of the propellant, in comparison with the conventional gas distributor with an axial supply mode, is conducted for a 2.5 kW high-voltage Hall thruster. The conventional gas distributor axially sprays the internally homogenized propellant gas into the discharge channel through the axial direct hole, whereas the rotating gas distributor drives the propellant gas to have circumferential rotating motions through the gas supply hole before the propellant gas is injected into the discharge channel, transforming the partial axial motion momentum of the gas in the axial supply mode of the propellant into the circumferential rotating motion momentum. This reduces the axial motion velocity of the neutral atoms in the discharge channel, while increasing the distance of its motion in the discharge channel. Finally, the purpose of increasing the ionization efficiency of the neutral atoms is achieved. The experiment primarily measures the magneto-ampere, volt-ampere, and flow rate-ampere characteristics of the thrusters for two types of gas distributor structures; it also measures the variation characteristics of the performances and ionization efficiency, among other parameters. Furthermore, the axial dynamic pressure distribution data of the neutral gas in the discharge channel is measured to assist the analysis of the experimental results.

The remainder of the paper is organized as follows. Section 2 describes the experimental design and apparatus. Section 3 presents and analyzes the experimental results. Section 4 summarizes the main conclusions.

Section snippets

Structural scheme with rotating propellant supply mode

For a specific propellant, such as xenon or krypton, when the electron density ne neenters the discharge channel, the discharge voltage UA and the magnetic field configuration remain unchanged, according to the ionization mechanism of the Hall thruster; thus, reducing the motion velocity of the neutral atoms va in the channel and increasing the number density naof neutral atoms na can increase the collision probability between electrons and neutral atoms, which in turn can increase the

Experimental results and analysis

From previous experimental results on a Hall thruster using krypton as propellant in the Plasma Propulsion Laboratory of the Harbin Institute of Technology, it was found that the flow density boundary of fully ionized (good ionization performance) for the krypton propellant was 0.089 mg s−1 cm−2 [33]. For the HET-100 thruster in the experiment, this corresponds to a volume flow rate of approximately 57 sccm. Dr. Linnell of Michigan University conducted a statistical analysis of the experimental

Conclusion

The use of krypton propellant has become a highlighted research topic owing to its unique advantages. However, its poor ionization characteristic is an urgent problem that needs to be solved. In this study, a gas distributor with a rotating propellant supply mode was designed and was used to convert most of the axial motion momenta of the propellant gas into circumferential rotating motion momenta. In addition, it reduces the axial movement velocity of neutral atoms, increases the movement path

Declaration of competing interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgments

The work described in this study is funded by the National Natural Science Foundation of China (Grant Nos. 51736003and 51777045).

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