Theoretical Ion Sputtering Yields from Loose Powders using a Multiscale Monte Carlo Approach

TL;DR

This paper introduces a multiscale Monte Carlo model to simulate ion sputtering yields from loose powders, revealing distinct behaviors from flat surfaces and providing universal fitting functions based on porosity and incident angles.

Contribution

The study develops a novel multiscale Monte Carlo approach that accurately models sputtering from powders, capturing effects of porosity and geometry not addressed by previous voxel-based methods.

Findings

Sputtering from powders is dominated by backward ejecta at angles > 0°.

Yield peaks near the ion-beam origin for angles < 60°.

Angular distribution peaks are significantly lower than flat slabs.

Abstract

Ion sputtering from loose powders remains poorly understood despite its relevance to planetary science and industry. We developed a multiscale Monte Carlo model to simulate sputtering from powders, using a higher-fidelity approach for the target geometry compared to voxel-based methods. Simulating Kr+ ions impacting Cu powders and flat slabs, we show that sputtering from loose powders differs markedly from that of flat slabs or rough surfaces. The main differences are: (1) for incident angles a > 0 degree relative to the bulk normal, the escaping sputtering yield is dominated by backward-directed ejecta for all ion energies; (2) for a < 60 degrees, the yield peaks toward the ion-beam origin, similar to the opposition effect seen in optical observations of airless bodies; (3) the angular distribution peak is half or less than that of a flat slab; (4) as ion energy increases, no evolution occurs from primary to secondary knock-on sputtering in the ejecta angular distribution. We attribute these behaviors to the powders interconnected voids. Ions penetrate these voids and sputter underlying grains; the ejecta then preferentially escape toward the ion-beam origin, where shadowing is minimal. We derive two fitting functions: 1) relating the escaping sputtering yield of a powder to that of a flat surface, depending only on porosity, incident angle, mean local incidence angle, and the corresponding flat slab yield; 2) providing the double-differential angular distribution of the escaping ejecta for porosities > 0.49. These provide a potentially universal fitting function of the absolute doubly-differential escaping sputtering yield from loose powders.