Flagship research program
VIRTUAL ION-OPTICS TESTBED
A multiscale assessment of carbon-nanomaterial accelerator-grid coatings for charge-exchange (CEX) erosion mitigation in the NASA NSTAR ion thruster.
Inside the work
From a plasma to a surface.
Positive beam ions are extracted through screen and accelerator grids; a charge-exchange ion is accelerated back to the downstream accelerator-grid face. Conceptual illustration; not simulation output.
Accelerate
Electric fields guide xenon ions through the grid aperture and into the beam.
Exchange
A beam ion can exchange charge with a neutral xenon atom. The resulting slow CEX ion can be pulled back toward the negatively biased accelerator grid.
Resolve the impact
CEX ions can sputter the accelerator-grid surface; the surface response connects impacts to coating loss and grid erosion.
The Problem
The dominant end-of-life mechanism.
Gridded ion thrusters accelerate Xe⁺ electrostatically. Fast beam ions undergo resonant charge exchange with slow neutrals, creating slow CEX ions born inside the accelerating field. These are pulled into the negatively biased accelerator grid, sputtering it.
Our research asks: Can thin nanostructured carbon coatings on a conventional Mo grid capture the sputter resistance of bulk carbon grids, without the structural, thermal, and manufacturing penalties of monolithic carbon optics?
Core Hypotheses
GNP and/or graphene coatings reduce accelerator-grid erosion.
GNP reduces erosion 1.39–4.23×. Graphene worsens it (0.37–0.85×, i.e. up to 2.7× more mass loss). Cross-run CV < 0.57% on all 12 cases.
Graphene "transparency" diminishes; penetrative energy transfer dominates.
Heavy Xe passes through the monolayer, depositing momentum in the Mo substrate; the low-thermal-coupling sheet cannot dissipate the cascade, ejecting C and sputtering Mo.
GNP nanoscale roughness geometrically shadows grazing-angle flux.
Reproduces the suppression of the 60–75° Mo yield peak via RustBCA + Gaussian facet-tilt convolution (σ=15°).
GNP edge-rich morphology elevates field-emission / vacuum-breakdown risk.
Onset field drops from 7.3 → 2.3 MV/m. Fowler–Nordheim Weibull model confirms this is potentially design-limiting.
Key Insight
The Graphene Paradox
Intuition suggests an ultra-hard 2D coating should protect the substrate. Crystalline MD shows the opposite. A heavy Xe ion at CEX energies passes through the monolayer, depositing momentum in the Mo substrate; the low-thermal-coupling sheet cannot dissipate the cascade, so it ejects several carbon atoms per impact and the substrate sputters through it.
The coating adds an erosion channel instead of removing one.