Let’s check on Journal of Propulsion and Power for May (vol. 41 #3):
Ezuka, F. Koizumi, H. Nakano, M. et al. Effect of Nonuniform Plasma on Grid Erosion of the Water Ion Thruster pp. 313 1.B39004
Kim, J. W. Bhosale, V. K. Yoon, W. et al. Reducing Preheating Requirements for 1-N-Scale Thruster with Ammonium Dinitramide-H202 Monopropellant pp. 400 1.B39599
Ion thrusters: one of the enabling technologies expanding our reach through the Solar System. Because asteroids (plus comets) have low mass and density, their gravities are thusly low: a few percent of Earth’s gravity (in the case of Ceres, the largest asteroid) to a few percent of that, for more typical small bodies. This low gravity allows low accelerations, by low-thrust engines. But nontrivial missions require continuous firing of these engines, for months to maybe a year or two. Erosion of engine parts is a concern, and one that’s difficult to prove or disprove to the assurance level of a space mission. Things like Ezuka et al.’s paper are desired to assure mission planners.
Short of a true electric thruster, a monopropellant thruster is commonly used on space missions. Particularly for small, inexpensive missions, monoprop systems are good for trajectory corrections, and housekeeping functions like momentum-wheel desaturation cycles. But the traditional monoprop fuel, hydrazine, is an irritant, a toxin, and a carcinogen. Highly-trained staff need lots of equipment to deal with hydrazine systems and their threats. What if that all went away? What if the fuel was, mostly, benign to humans? Missions would save time, risk, and therefore money; programs would retain more staffers, more often, for longer careers, again saving money. It is this search for better propellants that leads to ADN (ammonium dinitramide). Kim et al. further refine the case for ADN-based systems.