Numerical estimation of the atomic fraction during laser-induced desorption of hydrogen from tungsten and beryllium

Abstract

Monitoring of the DT-fuel retention in plasma-facing components (PFCs) is vital for long-term operations of future fusion devices. Laser-induced desorption (LID) is a proposed technique for the systematic in-situ control of hydrogen isotope content in surface layers of PFCs. An accurate interpretation of LID measurements requires information on the composition of released particles. Under some conditions, both atoms and molecules can be desorbed. The present study aims to verify the role of hydrogen atomic release under nanosecond and millisecond laser pulses. After investigation of general trends, we carried out the detailed analysis of hydrogen desorption from clean tungsten and beryllium surfaces, assuming a uniform distribution of hydrogen in a 10 μm layer with 1 at.% of initial content. Calculations were based on a common reaction-diffusion model supplemented by rate equations of molecular and atomic desorption. As a result, the atomic fraction is estimated to reach ∼40% during ms-heating of tungsten to near-melting temperatures, and less than 10% in the ns-case. Regarding beryllium, molecular recombination is shown to be the main process of hydrogen desorption in both heating scenarios.