A fully three-dimensional computer particle simulation model for ion optics is developed. This model allows multiple apertures to be included explicitly in the simulation domain and determines the upstream sheath and downstream beam neutralization through simulations. Simulations are performed for the NSTAR ion-thruster optics, and results are compared with grid-erosion measurements obtained during NSTAR long-duration test. It is shown that the simulation not only predicts accurately all of the features in the measured erosion pattern but also gives excellent quantitative agreement with the measured erosion depth. Nomenclature E= electric eld n b , n n , n cex = number density for the beam ions, the neutrals, and the charge-exchangeions, respectively n 0 ,n 1 = discharge plasma and downstream plasma density, respectively R = depth erosion rate T e0 , T e1 = electron temperature in the upstream and the downstream plasma, respectively T w = discharge chamber wall temperature Y = sputter yielḑ D0 = Debye length in the discharge plasma ¾ cex = charge-exchangecollision cross section 8 = electric potential 8 0 , 8 1 = upstream and downstream plasma potential, respectively

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NASA's goal for human spaceflight is to expand permanent human presence beyond low Earth orbit (LEO). NASA is identifying potential missions and technologies needed to achieve this goal. Mission options include crewed destinations to LEO and the International Space Station; high Earth orbit and geosynchronous orbit; cis-lunar space, lunar orbit, and the surface of the Moon; near-Earth objects; and the moons of Mars, Mars orbit, and the surface of Mars. NASA generated a series of design reference missions to drive out required functions and capabilities for these destinations, focusing first on a piloted mission to a nearEarth asteroid. One conclusion from this exercise was that a solar electric propulsion stage could reduce mission cost by reducing the required number of heavy lift launches and could increase mission reliability by providing a robust architecture for the long-duration crewed mission. Similarly, solar electric vehicles were identified as critical for missions to Mars, including orbiting Mars, landing on its surface, and visiting its moons. This paper describes the parameterized assessment of power and propulsion technologies for a piloted solar electric vehicle to a near-Earth asteroid. The objective of the assessment was to determine technology drivers to advance the state of the art of electric propulsion systems for human exploration. Sensitivity analyses on the performance characteristics of the propulsion and power systems were done to determine potential system-level impacts of improved technology. Starting with a "reasonable vehicle configuration" bounded by an assumed launch date, we introduced technology improvements to determine the system-level benefits (if any) that those technologies might provide. The results of this assessment are discussed and recommendations for future work are described.

High specific power (power per mass) ultralight solar arrays made of perovskite solar cells (PSCs) are being considered to power spacecraft in deep space conditions as far as Neptune (30...