Spatiotemporally Resolved Ion Velocity Distribution Measurements in the 12.5 kW HERMeS Hall Thruster

Non-invasive measurements of the ion velocity distribution function (IVDF) obtained using laser-induced fluorescence (LIF) are playing a critical role in the life qualification of NASA’s 12.5 kW Hall Effect Rocket with Magnetic Shielding (HERMeS), which will be accomplished through a combination of limited duration wear testing and computational modeling validated by experiments.  Previous LIF measurements on HERMeS have revealed bimodal time-averaged IVDFs in the acceleration region of the thruster, suggestive of oscillations in the acceleration region’s position, as well as time-averaged velocity vectors that are difficult to reproduce in simulations.  In order to understand these phenomena in more detail, we are making time-resolved LIF measurements using the transfer function averaging technique, which employs phase-sensitive detection and averaging in Fourier space to enable measurements resolving both periodic and aperiodic oscillations at typically Hall thruster breathing mode frequencies (10-60 kHz).  The first time-resolved IVDFs measured in HERMeS will be presented, along with time-averaged 2D velocity vector maps spanning a finer spatial mesh than in previous studies.  Implications for performance and life modeling of HERMeS using the Hall2De code will be discussed.