In radiated EMI testing of electric thrusters the basic nature of the thrusters often precludes feasible corrections to remove limit violations. In addition, the constraint of operation in vacuum forces deviations from conventional (e.g. MIL-461) test practices. This introduces ambiguities and unquantified uncertainties into procedures and test results, respectively, thereby calling for some effort towards development and refinement of test methods. Even then, simple compliance tests (such as MIL-461) alone are inadequate to fully characterize the nature of critical emissions. Furthermore, the conditions, mechanisms and causes of thruster discharge emission usually have not been experimentally investigated and modeled. This can inhibit meaningful analysis of compatibility issues if they arise. Inadequate enforcement of complementary EMI susceptibility test requirements (specified by ICDs) on other sensitive payloads can exacerbate the resolution or risk assessment of potential problems.

In the past, radiated EMI test were performed to characterize radiated emissions against MIL-STD-461 limits (from 10 kHz to 18 GHz) and allow comparison of results between various thrusters using a repeatable setup in a designated testbed. A test methodology, adapted from MIL-461/2, was devised in order to accommodate the constraints of a metal vacuum chamber and plasma environment. These factors, not typical for conventional EMI testing, were understood to have a significant effect on results of the measurements and their interpretation. Changes of setup and/or venue for EMI tests of other thruster types or development versions would be expected to further confuse any desired comparisons.

The purpose of this work is to assess the impact of reflections and possible chamber resonances on EMI emission testing/diagnostics of EP thrusters inside a metal vacuum chamber ground test environment. Because EP technologies (e.g., ion, Hall, MPD, PPT) typically behave as broadband emitters of radiated EMI at frequencies between ~0.2-10 GHz, they pose degrees of risk regarding EMI in spaceflight applications. Ground test characterization of thruster emissions before flight is therefore important for risk evaluation and mitigation. Due to the propellant flow rates and vacuum level requirements associated with operation of many EP thrusters, this often necessitates the use of large (usually metal-walled) vacuum chambers for any such EMI testing. Unfortunately, the reflective environment presented to the EMI source (thruster) and measurement antennas complicates interpretation of and introduces additional un-quantified uncertainty into EMI test results. (This has proven to be a recurrent question/issue in the spacecraft community regarding EMI results obtained on Hall thrusters in GRC VF5 and VF6). Anechoic materials used in conventional EMI test facilities to reduce effects of wall reflection tend to be expensive, not vacuum compatible, and vulnerable to the sputter/contamination from thruster operation. Without e.g., a dedicated anechoic EMI test facility surrounding an RF transparent (e.g., fiberglass) appendage to a large (high pumping speed) vacuum chamber, this means it will usually only be practical to perform such measurements in the available metal chambers. Characterization of the metal walled facility impact on measurement of emissions from a controlled broadband source (simulating a thruster) performed interior to the chamber with that obtained in a more open environment external to the chamber would therefore be of value in providing a quantitative basis for assessing facility effects.