Monte Carlo raytracing method for calculating secondary electron emission from micro-architected surfaces

Abstract

Secondary electron emission (SEE) from inner linings of plasma chambers in electric thrusters for space propulsion can have a disruptive effect on device performance and efficiency. SEE is typically calculated using elastic and inelastic electron scattering theory by way of Monte Carlo simulations of independent electron trajectories. However, in practice the method can only be applied for ideally smooth surfaces and thin films, not representative of real material surfaces. Recently, micro-architected surfaces with nanometric features have been proposed to mitigate SEE and ion-induced erosion in plasma-exposed thruster linings. In this paper, we propose an approach for calculating secondary electron yields from surfaces with arbitrarily-complex geometries using an extension of the ray tracing Monte Carlo (RTMC) technique. We study nanofoam structures with varying porosities as representative micro-architected surfaces, and use RTMC to generate primary electron trajectories and track secondary electrons until their escape from the outer surface. Actual surfaces are represented as a discrete finite element meshes obtained from X-ray tomography images of tungsten nanofoams. At the local level, primary rays impinging into surface elements produce daughter rays of secondary electrons whose number, energies and angular characteristics are set by pre-calculated tables of SEE yields and energies from ideally-flat surfaces. We find that these micro-architected geometries can reduce SEE by up to 50% with respect to flat surfaces depending on porosity and primary electron energy.

Monte Carlo modeling of low-energy electron-induced secondary electron emission yields in micro-architected boron nitride surfaces

“…As mentioned earlier, these data will be used in ray-tracing Monte Carlo simulations of SEE in arbitrary surface geometries. The details of the function fitting process can be found in our prior publications [17,18]. The final expressions for the total SEE yield and energy distributions are generated using machine learning software [35]:…”

“…Next we calculate SEE from microfoam structures with various porosities. The details of these structures are given in our past studies [17,18]. A finite element reconstruction of the material is used to extract surface elements that may 9 be intersected by electron trajectories.…”

“…The material considered here is BN (used in current thrusters thanks to its low mass density, low thermal expansion, and highly dielectric properties). The procedure is described in two papers from our group focused on W [17,18].…”

Effect of the Surface Morphology of Porous Coatings on Secondary Electron Yield of Metal Surface

“…Surface treatment on metals has long been researched, mainly including Alodine coatings [8,9], inert metal coatings, surface amorphous carbonation [10][11][12][13][14][15][16][17] and roughening [18][19][20][21][22][23][24][25]. In the field of rough surfaces, some aspects, such as calculations and mechanisms, have been well-defined [26,27] while experimental studies continue to prosper. Montero et al [16] introduced the roughness effect when studying the SEE properties of graphene nanoplatelets.…”

Efficient Combination of Surface Texturing and Functional Coating for Very Low Secondary Electron Yield Surfaces and Rough Nonevaporable Getter Films

“…A SEY reduction can be obtained by surface roughening, [39][40][41][42][43] machining of deep trenches or grooves, [44][45][46][47] and several approaches to create a micro-or nanostructured morphology. Microporous structures [48][49][50] as well as a sponge or foam topology [51][52][53][54] enable trapping of impinging and emitted electrons as efficiently as textured surfaces formed by etching [55] or nanowires created by different techniques. [56][57][58][59] Furthermore, plasma-based nanostructure formation [60] as well as laser-induced surface structuring on different length scales [61][62][63][64] have been validated to reduce secondary electron emission.…”