Development of a highly throttlable, bipropellant (Xe-Kr) Hall thruster and investigations into the newly discovered oscillation modes in magnetically shielded thrusters

NASA has expressed the need for a high-power Hall thruster to serve as the primary mode of propulsion for interplanetary robotic/cargo missions. This demands a long-life Hall thruster capable of both high specific impulse and high thrust. The effort to increase the lifetime of Hall thrusters led to magnetic shielding technology, which has proven itself to be a necessity for long-duration missions by eliminating chamber wall erosion as a failure mechanism. Conventionally, increasing the specific impulse of any Hall thruster requires increasing the discharge voltage and thus, the operating condition. However, limitations to increasing the discharge voltage include the limited power available to propulsion subsystems via on-board power processing units, as well as the newly discovered high frequency oscillations modes that have thwarted attempts to operate at discharge voltages greater than 800 V. In tandem, these difficulties have limited the specific impulse of state-of-the-art Hall thrusters to ~3000 seconds. In order to best satisfy the needs of a Hall thruster suitable for interplanetary missions, this research proposes to develop a bipropellant Hall thruster, based on the magnetically shielded H9, that can operate in two distinct modes: a high-thrust mode (suitable for orbital insertion/ejection maneuvers) in which the thruster will operate on xenon, and a high-specific impulse mode (suitable for interplanetary cruise phases) in which the thruster will operate on krypton. Investigations into the performance of krypton have suggested theoretical improvements to the specific impulse of 20-25% at the expense of a degradation in the mass utilization efficiency of ~10%. This research intends to directly address this degradation by delving into the performance of a thermally controlled anode injector and investigating the high frequency oscillatory modes at high voltages. The innovation of this research is the marriage of xenon and krypton propellants with a magnetically shielded thruster to be utilized as an individual bipropellant system. The hypothesized benefits of this new breed of thruster are given as follows. Firstly, a bipropellant Hall thruster expands the versatility and thus the operational envelope capable of a single thruster by harmonizing the benefits of both xenon and krypton propellants. Secondly, a bipropellant Hall thruster allows for an increase in specific impulse without changing the operating point—that is, without increasing the discharge voltage or mass flowrate, but by merely increasing the ratio of krypton in the propellant. Thirdly, considering both that the cost of xenon is highly volatile and roughly ten times the cost of krypton, a bipropellant thruster could significantly reduce both the qualification and on-orbit operational costs. If successful, a thruster of this type would contribute to the necessary advancements in Hall thruster technology for missions requiring very large total impulses and high thrusts, such as deep-space exploration or manned missions to Mars.

The innovation of this research is the marriage of xenon and krypton propellants with a magnetically shielded thruster to be utilized as an individual bipropellant system. The hypothesized benefits of this new breed of thruster are given as follows. Firstly, a bipropellant Hall thruster expands the versatility and thus the operational envelope capable of a single thruster by harmonizing the benefits of both xenon and krypton propellants. Secondly, a bipropellant Hall thruster allows for an increase in specific impulse without changing the operating point—that is, without increasing the discharge voltage or mass flowrate, but by merely increasing the ratio of krypton in the propellant. Thirdly, considering both that the cost of xenon is highly volatile and roughly ten times the cost of krypton, a bipropellant thruster could significantly reduce both the qualification and on-orbit operational costs. If successful, a thruster of this type would contribute to the necessary advancements in Hall thruster technology for missions requiring very large total impulses and high thrusts, such as deep-space exploration or manned missions to Mars.

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