Review: Experiments and simulations for small-scale electrical discharges

Characteristics of electrical discharges in submicron gaps between platinum nanotip cathodes and plane gold anodes and their associated electrode modifications are reviewed. The breakdown field strengths in small gaps were large - several hundred volt/μm - but decreased when femtosecond laser pulses were used to trigger discharges. The average duration of current pulses in 500-nm gaps energized by a voltage source with no series resistance depended on applied potential, being approximately 5 ns at potentials of 25 V or less, and much longer at potentials greater than 40 V. The addition of a series resistance larger than 1.5 kΩ limited the discharge duration to 4 ns, regardless of applied potential. When a series resistance of 1 MΩ was used to minimize anode heat input, melt diameters ranged from approximately 1.2 to 0.07 μ.m as potential varied from 80 to 27.5 V. Experimentally, laser triggering of discharges was associated with minor ablation of material from the sharpened cathodes. Electrode vaporization was represented in particle-in-cell (PIC) simulations by distributions of ionized electrode material in the gap region. When ionized electrode materials were continuously added with a volumetric source, the predicted current pulse duration was controlled by the source duration. With preloaded electrode materials, the current pulses predicted by PIC simulations were considerably shorter than experiments, but had comparable peak values. When the simulated current pulse durations were normalized so that transferred charge was equal to experiments, the anode heat input distributions predicted by PIC simulations produced anode melt spot diameters comparable to experiments.