Dynamic coupling between particle-in-cell and atomistic simulations

Abstract

We propose a method to directly couple molecular dynamics, the finite element method, and particle-in-cell techniques to simulate metal surface response to high electric fields. We use this method to simulate the evolution of a field-emitting tip under thermal runaway by fully including the three-dimensional space-charge effects. We also present a comparison of the runaway process between two tip geometries of different widths. The results show with high statistical significance that in the case of sufficiently narrow field emitters, the thermal runaway occurs in cycles where intensive neutral evaporation alternates with cooling periods. The comparison with previous works shows that the evaporation rate in the regime of intensive evaporation is sufficient to ignite a plasma arc above the simulated field emitters.

Mechanism of Spontaneous Surface Modifications on Polycrystalline Cu Due to Electric Fields

“…In Equation ( 2), 𝐹 is the electric field strength at a surface point and 𝜀 is the vacuum permittivity. Although more elaborate hybrid Electrodynamic-MD models are available for including electric field effects in MD [25][26][27], for the purposes of this work it is sufficient to include 𝜎 directly in MD as an external force term on the surface atoms. This is valid under the assumptions of a uniform field strength above the surface, which is accurate as long as the local surface curvature is small.…”

General scaling laws of space charge effects in field emission

“…In this section we shall generalize the calculation of ω, using a numerical method applicable to any geometry. Our technique is based on the finite element code FEMOCS [38], which has been recently enhanced with PIC capabilities [13]. In this work we do not aim to reproduce the temporal evolution of the SC distribution, but only the steady state, which is reached in a sub-picosecond timescale [13].…”

“…Our technique is based on the finite element code FEMOCS [38], which has been recently enhanced with PIC capabilities [13]. In this work we do not aim to reproduce the temporal evolution of the SC distribution, but only the steady state, which is reached in a sub-picosecond timescale [13]. In the steady state, the electric field distribution is constant and all particles emitted from a given point follow the same path to the collector.…”

“…Then charged superparticles (SPs) are injected from the surface into the vacuum. Unlike our previous work [13], here we inject one SP at the center of each surface element and adjust the corresponding SP weight according to the locally emitted current. Thus the weight of the emitted SP is w sp = J f A f Δt/e, where J f is the emission current density on a certain face element, A f is its area, Δt is the path integration timestep, and e the elementary charge.…”

In-situ plasma treatment of Cu surfaces for reducing the generation of vacuum arc breakdowns

“…There are several indications that the breakdowns are linked to intrinsic effects, such as dislocations in the subsurface region [16][17][18] . Recent studies have shown how these mechanisms can lead to the formation of runaway processes, eventually creating a conducting plasma channel between the electrodes [19][20][21] . Plasma cleaning has been shown to improve the performance of a tandem particle accelerator functioning in the megavolt range 22 .…”