Plume divergence characterization for electric propulsion via standard deviation and emittance.

Electric propulsion systems require careful consideration of plume divergence and evolution over a range of operating conditions and environments. Existing means of describing plume divergence such as outlines, plume profiles, and snapshots of the plume are dominated by outlier particles and do not provide reliable or quantitative insight. Proposed herein are two novel methods for describing plume divergence using standard deviation and emittance to provide quantitative insight of the collective behavior of plume species. Furthermore, the emittance metric from the particle accelerator community is shown to accurately describe plume evolution in a two-dimensional position and momentum angle space. Cross-sectional emittance measurements are used to display the presence of non-Hamiltonian forces in plume evolution, namely stochastic Coulomb collisions between neighboring particles. Finally, full-plume emittance diagrams are demonstrated as a means of identifying when an electric propulsion plume has reached steady state.