1. Iain D. Boyd University of Michigan Modeling of Ion Sputtering and Product Transport

2. Background and Motivation Absalamov, S. K. et al., AIAA 92-3156 Hall thruster channel wall erosion: –Key mechanism limiting life of Hall thrusters below mission requirements –As ions impinge on the walls, the wall material is gradually sputtered away –Eventually the erosion will expose the magnetic circuits to the plasma flow and cause thruster failure –Thruster life testing is EXTREMELY time- consuming and expensive Transport of sputtered material: –Re-deposition on thruster walls, affecting operation and decreasing erosion –Deposition on external spacecraft surfaces Modeling can offer unique insight as compliment to experimental studies (Gallimore)

3. Modeling of Ion Sputtering and Product Transport Objectives of Research Develop models for ion sputtering of Hall thruster insulator materials: –Start with xenon sputtering of boron nitride, consider other propellants and materials later –Generate sputter yields as a function of energy and impact angle –Generate probability density functions (pdf’s) of properties of sputtered materials, e.g. number and energy flux as a function of angle –Assess (hopefully validate) model using experimental data Analyze transport of sputtered material within thruster: –Use sputter yields and product pdf’s as boundary conditions within Hall thruster discharge models (both hydrodynamic and kinetic) Analyze transport of sputtered material in plume created by thruster: –Use thruster analysis to develop boundary conditions at thruster exit for sputter products –Input into existing PIC code for plume analysis

4. Modeling of Ion Sputtering and Product Transport Technical Approach: 1. Ion Sputtering Molecular dynamics (MD): –Simulate individual sputtering events by integrating Newton’s laws of motion –Key inputs are the inter-atomic potentials: – modified form of Tersoff potential for B - N (Albe) – shielded Moliere potential for Xe - BN interaction (Yim) –Consider hexagonal BN structure

5. Modeling of Ion Sputtering and Product Transport Technical Approach: 1. Illustrative MD Results Yim, J.T., PhD, University of Michigan, 2008

6. Modeling of Ion Sputtering and Product Transport Technical Approach: 2. Thruster Wall Erosion Hydrodynamic analysis of Hall thruster discharge and wall erosion: –Simultaneous solution of neutral / ion/ electron conservation equations –Sputtering and erosion then determined using MD sputter yields –MD also supplies properties of sputtered products –Re-deposition on walls of sputtered material –Virtual life-test can be simulated in minutes-to-hours Particle (PIC) simulation code of Hall thruster discharge also available: –Advantages: – more physically accurate for the nonequilibrium plasma – more natural treatment of sputtered products and their transport –Disadvantage: – three orders of magnitude slower than hydrodynamic approach!

7. Modeling of Ion Sputtering and Product Transport 2. Thruster Wall Erosion: Illustrative Results Hydrodynamic analysis of eroding Hall thruster Yim, J.T., PhD, University of Michigan, 2008

8. Modeling of Ion Sputtering and Product Transport Technical Approach: 3. Sputter Product Transport in Thruster Plume Discharge / erosion models used to determine thruster exit conditions: –Plasma properties –Number and energy flux distributions of sputter products Used as inputs into existing plasma plume models: –Ion and neutral transport treated using Particle-In-Cell (PIC) –Collisions (CEX) treated using direct simulation Monte Carlo (DSMC) –Electrons treated using a detailed hydrodynamic approach –Code already validated for many EP thrusters

9. Modeling of Ion Sputtering and Product Transport 3. Thruster Plume Analysis: Illustrative Results Ion energy distribution in plume backflow region of Hall thruster employed in space on SMART-1 3D simulation of the plasma density in the plume of a cluster of four Hall thrusters Cai, C.-P., PhD, University of Michigan, 2005 Boyd, I.D., IEEE Trans. Pl. Sci., 2006

10. Modeling of Ion Sputtering and Product Transport Anticipated Results Detailed database for ion sputtering of Hall thruster insulators: –Sputter yields as a function of energy and impact angle –Probability density functions (pdf’s) of properties of sputtered materials, e.g. number and energy flux as a function of angle –Data validated using experimental data Transport of sputtered material within a Hall thruster: –Fraction of sputtered material re-deposited on walls –Fraction escaping from thruster exit and their properties Transport of sputtered material in a Hall thruster plume: –Angular distributions of number and energy fluxes of sputter products –Compare with measurements to be obtained by Gallimore –Assessment of associated spacecraft contamination potential