Speaker
Description
One of the most common hypotheses attempting to explain the ignition of a vacuum arc is the thermal runaway of field emitting tips residing on the metal electrodes. Here we use multi-scale simulations in order to explore this hypothesis, investigate the conditions under which a field emitter is driven to thermal runaway and how this can lead to the ignition of a vacuum arc.
Our simulation method concurrently couples Molecular Dynamics (MD) with the Finite Element Method (FEM) which is used to solve the electrostatic field and thermal diffusion equations. Furthermore, we combine the above with a novel method for the calculation of electron emission from sharp emitters in the general thermal-field regime and the Particle-In-Cell (PIC) method to include the space-charge effects.
We find that in case the emitter geometry is considered static, thermal runaway cannot appear if the tip does not have a minimum size of micrometer order, regardless of its aspect ratio. However, our simulation approach of dynamically coupling MD with FEM allows to follow the shape evolution of a nanotip at an atomistic level. These dynamic simulations show that the field-induced
force elongates and sharpens the hot apex of the nanotip, which initiates a positive feedback process and eventually causes evaporation of large fractions of the tip. We find that the total evaporation rate exceeds the one required to ignite arc plasma as found by recent PIC simulations of the plasma onset. Therefore the above mechanism can explain the ignition of a vacuum arc as an
intrinsic response of a nano-scale metal tip to the application of a high
electric field.