Mary F Konopliv, Vernon H Chaplin, Lee K Johnson, Richard E Wirz, “Accuracy of using metastable state measurements in laser-induced fluorescence diagnostics of xenon ion velocity in Hall thrusters,” Plasma Sources Science and Technology, Volume 32 (2023); pdf

Laser-induced fluorescence measurements of singly-charged xenon ion velocities in Hall thrusters typically target metastable states due to lack of available laser technology for exciting the ground state. The measured velocity distribution of these metastable ions are assumed to reflect the ground state ion behavior. However, this assumption has not been experimentally verified. To investigate the accuracy of this assumption, a recently developed xenon ion (Xe II) collisional-radiative model is combined with a 1D fluid model for ions, using plasma parameters from higher fidelity simulations of each thruster, to calculate the metastable and ground state ion velocities as a function of position along the channel centerline. For the HERMeS and SPT-100 thruster channel centerlines, differences up to 0.5 km s−1 were observed between the metastable and ground state ion velocities. For the HERMeS thruster, the difference between the metastable and ground state velocities is less than 150 m s−1 within one channel length of the channel exit, but increases thereafter due to charge exchange (CEX) that reduces the mean velocity of the ground state ions. While both the ground state ions and metastable state ions experience the same acceleration by the electric field, these small velocity differences arise because ionization and CEX directly into these states from the slower neutral ground state can reduce their mean velocities by different amounts. Therefore, the velocity discrepancy may be larger for thrusters with lower propellant utilization efficiency and higher neutral density. For example, differences up to 1.7 km s−1 were calculated on the HET-P70 thruster channel centerline. Note that although the creation of slow ions can influence the mean velocity, the most probable velocity should be unaffected by these processes. Keywords: metastable, diagnostics, electric prop

Uchizono, N., Wright, P., Collins, A., et al., “Emission spectra of glows produced by ionic liquid ion sources,” Appl. Phys. Lett. 121, 154101 (2022); https://doi.org/10.1063/5.0096595

Electrospray devices, such as ionic liquid ion sources, often exhibit glows during operation in high vacuum facilities. The lack of electron excitation mechanisms during electrospray operation prompts the question: “What causes glow in an electrospray device?” Our optical emission spectroscopy results show that electrospray glow exhibits a broad spectral response between 350 and 800 nm with emission lines corresponding to atomic metal constituents of impinged surfaces, neutral and ionized atomic constituents of the ionic liquid propellant, and molecular line shapes that may also be dissociation products of the ionic liquid. We have previously defined secondary species emission to describe the many complex interfacial phenomena that occur when electrosprayed species impact surfaces downstream of the emitter. Our analysis of the optical emission spectra shows that these glows are only possible in the presence of secondary species emission. Therefore, the answer to the proposed question: high-velocity impacts that generate secondary species are the root cause of glow for electrosprays in high vacuum facilities.

Konopliv, M., Wirz, R., Johnson, L., “Fast Optical Emission Spectroscopy Measurements of Time-Resolved Electron Temperature in a Hall Thruster,” AIAA SCITECH 2024; PDF, https://doi.org/10.2514/6.2024-2165

Time-dependent behavior of electrons can be critically important to Hall thruster discharge behavior. Fast Optical Emission Spectroscopy (FastOES) is a non-invasive diagnostic capable of measuring the time-resolved electron temperature of the plasma. The FastOES approach splits the collected light and isolates two distinct wavelengths that are detected by photomultiplier tubes, and amplified to a measurable signal. These measured emission lines are paired with a collisional-radiative model to determine time-resolved electron temperature. Using this technique, the FastOES results show that the electron temperature exhibits both temporal and spatial variations within the Hall thruster. There is a dynamic relationship between the electron temperature and the discharge current, with the electron temperature consistently exhibiting an out-of-phase relationship in correlation with the discharge current. Additionally, a loose scaling relationship was identified, providing valuable insights into the complex interplay within Hall thruster plasmas.