Modeling the resupply, diffusion, and evaporation of cesium on the surface of controlled porosity dispenser photocathodes

Abstract

A controlled porosity dispenser (CPD) photocathode is currently being explored and developed to replace the Cs during operation and increase photocathode lifetime. Experimental results from cesium (Cs) emission of a sintered-wire tungsten CPD are presented and are used to inform a theoretical model of Cs resupply, diffusion, and evaporation on the surface of the photocathode. The evaporation of Cs from a tungsten surface is modeled using an effective one-dimensional potential well representation of the binding energy. The model accounts for both local and global interactions of Cs with the surface metal as well as with other Cs atoms. It is found that for typical activation temperatures within the range of 500 K–750 K, differences of less than 5% between the quantum efficiency (QE) maximum and minimum over ideal homogenous surfaces occur, even when variations to mimic surface non-uniformity due to pore blockage are included. The theoretical evaporation rates of sub-monolayer surface coverage of Cs compare well to the data of Taylor and Langmuir [I. Langmuir and J. B. Taylor, Phys. Rev. 40, 463–464 (1932)] and reproduce the nonlinear behavior of evaporation with varying coverage and temperature.

Cesium seeding for effective electron transpiration cooling in hypersonic flows

“…where σ is the flux of cesium atoms to the surface and ν is the rate of evaporation from the surface at the monolayer coverage (θ = 1). Analytical deduction of the rate of evaporation is more complex [19], and so we use an empirical formula for the Flow chart summarizing the analytical approach followed for determining total cooling flux from a porous tungsten surface through which cesium is transpired.…”

“…In this study, we perform a theoretical analysis of the nearsurface effects for the case of the liquid cesium injected into the air plasma through a porous tungsten surface at the leading edge, which will be referred to as the emitter surface. Cesium injection and diffusion through porous tungsten surface was briefly discussed in references [18,19], where authors studied cesium-based dispenser photocathodes. The mechanism of the cesium injection in hypersonic flight conditions and experimental proof of the proposed ETC concept are the subject of active research within the framework of the project, which will be reported in a future study.…”