A known issue with ground testing of EP devices is that their performance on-orbit is different than their performance during ground testing. This poses a significant risk for developing this technology, raising questions about the validity of the current standard practice of using ground test results for qualifying these systems for flight. The prevailing consensus is that the difference in performance largely can be attributed to so-called facility effects, i.e. differences in the test environment related to the presence of confining walls and limited pumping capacity as compared to the space environment. With this in mind, there was a concerted effort in the 1990s and early 2000s to develop standard practices for trying to replicate the space environment as closely as possible in ground test facilities. These best practices mostly centered on the problem of finite facility pressure. The goal was to establish an acceptable facility pressure for testing. These standards were informed by detailed pressure studies on the then most-flown Hall thruster, the SPT-100, a 1.5 kW device. It was thought that neutral gas ingestion due to finite facility pressures accounted for changes in performance. In order to ensure that neutral gas ingestion did not affect performance measurements, a facility pressure limit of 3×10−5 torr was decided upon. This was the pressure at which the ingested gas was on the order of the uncertainty in the mass flow measurements.

Subsequent investigations have shown that the canonical presentation of pressure effects did not fully capture the impact. Moreover, these effects appears in all types of configurations including thrusters with externally mounted cathodes, with internally mounted cathodes, and magnetically shielded thrusters. Due to this extensibility, these effects are critical to understand. While the classical theory suggested that neutral ingestion was the dominant effect resulting in increased performance, there appear to be more nuanced but insidious effects that can occur even below the pressure level suggested by Randolph that can not be explained by a simple ingestion model. Parameters known to be affected are the location of the acceleration region, thrust and cathode coupling voltage. In general, the acceleration region moves axially downstream, the thrust decreases, and the cathode-coupling voltage increases in magnitude with decreasing background pressure. Accordingly, this affects the efficiency and specific impulse of each of the thrusters in ways not predicted by the simple ingestion model. Additionally, thruster stability is impacted by pressure effects.

A one-dimensional quasi-linear model was developed to capture the susceptibility of the cathode coupling voltage to the background facility pressure. Experimental evidence has shown that the magnitude of this parameter increases with decreasing background pressure. The underlying hypothesis is that the onset of ion acoustic turbulence as the background pressure decreases takes energy away from the electrons and makes it harder for them to couple to the Hall thruster channel. Alternatively, the model states that the damping of the ion acoustic modes leads to decreased electron resistivity and coupling voltage magnitude with increasing background pressure. The model agrees well with experimental data from the SPT-100. The results show up to a 2% decrease in voltage utilization by decreasing the pressure. Even though the cathode coupling voltage only represents a small portion of the total performance, this is a promising first-principles model of the effect. Open-questions remain including the extensibility to other thrusters and the implications of the cathode-coupling voltage changes on the boundary condition for the acceleration region.