Studies of field emission current density and outgassing in copper and carbon fiber electrodes based on microscopic material models
Quantitative analysis of field emission current density and assessment of various factors possibly affecting the high current density electron sources are of significant interest to the pulsed power and high power microwave communities, and well as other fields such as panel displays, vacuum electronics, for charged particle generators, in lithography, and some medical therapies. Evaluation of field emission current from material emitters (single or in arrays) requires the inclusion of the internal potentials that shape the electronic wavefunctions and tunneling probabilities; details of the surface work function that are dependent on material quality and defects; and the role of the density of states (DOS) that influences the electronic supply. Here, these factors are collectively included using the first-principles calculations approach of the material physics to obtain predictions of field-dependent electron tunneling current densities. The Numerov algorithm has been employed for efficient numerical calculations. Current output results have been generated for three different orientations of copper, extended to copper with oxide surface layer, and also for tungsten emitters with cesium and cesium iodide coatings. Next, the molecular dynamics (MD) simulation approach has been used to probe the out-diffusion of hydrogen gas as it is a pertinent issue in high power systems. Outgassing can influence breakdown, cause surface flashover, pulse shortening and is typically the first stage of plasma formation. These MD simulations have been carried out with copper, carbon fiber, cesium coated carbon fiber, and carbon nanotubes, as representative electrode materials chosen based on their extensive use, durability, and robustness. In addition, sticking coefficients, outflow rate, temporal concentration profiles have been evaluated and presented. For completeness, the thermal conductivity of carbon fiber material was evaluated at nanoscales to probe the suitability of this material for high power applications from a thermal management standpoint.