Near-Surface Plasma Characterization of the 12.5-kW NASA TDU1 Hall Thruster

Abstract

To advance the state-of-the-art in Hall thruster technology, NASA is developing a 12.5-kW, high-specific-impulse, high-throughput thruster for the Solar Electric Propulsion Technology Demonstration Mission. In order to meet the demanding lifetime requirements of potential missions such as the Asteroid Redirect Robotic Mission, magnetic shielding was incorporated into the thruster design. Two units of the resulting thruster, called the Hall Effect Rocket with Magnetic Shielding (HERMeS), were fabricated and are presently being characterized. The first of these units, designated the Technology Development Unit 1 (TDU1), has undergone extensive performance and thermal characterization at NASA Glenn Research Center. A preliminary lifetime assessment was conducted by characterizing the degree of magnetic shielding within the thruster. This characterization was accomplished by placing eight flush-mounted Langmuir probes within each discharge channel wall and measuring the local plasma potential and electron temperature at various axial locations. Measured properties indicate a high degree of magnetic shielding across the throttle table, with plasma potential variations along each channel wall being ≤ 5 V and electron temperatures being maintained at ≤ 5 eV, even at 800 V discharge voltage near the thruster exit plane. These properties indicate that ion impact energies within the HERMeS will not exceed 26 eV, which is below the expected sputtering threshold energy for boron nitride. Parametric studies that varied the facility backpressure and magnetic field strength at 300 V, 9.4 kW, illustrate that the plasma potential and electron temperature are insensitive to these parameters, with shielding being maintained at facility pressures 3X higher and magnetic field strengths 2.5X higher than nominal conditions. Overall, the preliminary lifetime assessment indicates a high degree of shielding within the HERMeS TDU1, effectively mitigating discharge channel erosion as a life-limiting mechanism.

Ion Velocity in the Discharge Channel and Near-Field of the HERMeS Hall Thruster

“…6 Figure 1 shows a diagram of the testing on the HERMeS TDUs thus far as well as tests that are planned. Testing on the TDU1 included the propellant uniformity test, 7 magnetic shielding characterization test, 8 performance characterization test, [9][10][11] thermal characterization test, 12,13 facility effect characterization test, 9,11,14 and the first wear test campaign. The performance, thermal, and facility effect characterization tests were performed with a single test setup.…”

Performance, Facility Pressure Effects, and Stability Characterization Tests of NASA’s 12.5-kW Hall Effect Rocket with Magnetic Shielding Thruster

“…The thruster's magnetic shielding was confirmed using Langmuir probes embedded in the discharge channel walls; a plasma potential of 800 V was measured along the entire length of the inner and outer discharge channel walls. 14 The magnetic shielding test on TDU-1 verified that channel erosion was no longer expected to limit the life of the thruster. Pressure effects testing on the TDU-1 thruster found that the thruster performance was mostly invariant to increased facility background pressure.…”

Optimization of the Magnetic Field Topology in the Hall Effect Rocket with Magnetic Shielding

“…This was validated by the plasma wall probe test and wear tests that indicated no measureable erosion on the discharge channel. [27] During the plasma wall probe test campaign, anode potentials were measured at the downstream chamfer edge of the discharge channel, this indicated that the thruster was magnetically shielded. However, while the HERMeS TDU-1 and TDU-3 thrusters wear test campaigns found that discharge channel erosion rates were undetectable, erosion of the front pole covers was observed at measureable levels with high sputter resistant material rendering the front pole covers as the next life-limiting mechanism.…”