Transcript Slide 1

Modeling of Ion Sputtering
and Product Transport
Iain D. Boyd
University of Michigan
Modeling of Ion Sputtering and
Product Transport
Background and Motivation
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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 timeconsuming and expensive
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Transport of sputtered material:
– Re-deposition on thruster walls, affecting
operation and decreasing erosion
– Deposition on external spacecraft surfaces
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Absalamov, S. K. et al., AIAA 92-3156
Modeling can offer unique insight as
compliment to experimental studies (Gallimore)
Modeling of Ion Sputtering and
Product Transport
Objectives of Research
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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
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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)
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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
Modeling of Ion Sputtering and
Product Transport
Technical Approach:
1. Ion Sputtering
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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
Modeling of Ion Sputtering and
Product Transport
Technical Approach:
1. Illustrative MD Results
Yim, J.T., PhD, University of Michigan, 2008
Modeling of Ion Sputtering and
Product Transport
Technical Approach:
2. Thruster Wall Erosion
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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
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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!
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
Modeling of Ion Sputtering and
Product Transport
Technical Approach:
3. Sputter Product Transport in Thruster Plume
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Discharge / erosion models used to determine thruster exit conditions:
– Plasma properties
– Number and energy flux distributions of sputter products
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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
Modeling of Ion Sputtering and
Product Transport
3. Thruster Plume Analysis:
Illustrative Results
3D simulation of the plasma
density in the plume of a cluster
of four Hall thrusters
Cai, C.-P., PhD, University of Michigan, 2005
Ion energy distribution in plume
backflow region of Hall thruster
employed in space on SMART-1
Boyd, I.D., IEEE Trans. Pl. Sci., 2006
Modeling of Ion Sputtering and
Product Transport
Anticipated Results
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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
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Transport of sputtered material within a Hall thruster:
– Fraction of sputtered material re-deposited on walls
– Fraction escaping from thruster exit and their properties
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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