Effect of material properties on the laser-induced desorption of hydrogen from tungsten

Abstract

One of the priority tasks for sustainable operations of future fusion devices is the systematic control of hydrogen isotope content in plasma-facing components. A remote non-damaging analysis of the gas content can be realised by applying laser-induced desorption (LID). LID is based on local heating of the studied surface, followed by the detection of desorbed particles. Interpretation of the LID measurements relies on the definition of effective desorption region from which particles are being released. This region strongly depends on the properties of a laser pulse, an analysed material, and defect centres containing hydrogen isotopes. To investigate the role of these properties on LID, a series of numerical simulation was conducted for the case of hydrogen release from a 10 μm tungsten layer, initiated under nanosecond, microsecond, and millisecond heat loads. The simulations were performed using a common reaction-diffusion model with sequentially varied tungsten thermal conductivity, detrapping energy of hydrogen from traps, concentration of hydrogen traps, and their initial filling ratio. Additionally, the Soret effect was considered in order to estimate its impact on the LID efficiency. Findings reveal that the peak surface temperature reached during heating can characterise the LID efficiency under a particular heat pulse. The LID efficiency dependence on the peak surface temperature weakly varies with material thermal properties, but can alter with the change of trap parameters. The detrapping energy of hydrogen from traps considerably influences the desorption efficiency, while concentration and the initial filling ratio of traps have a moderate effect. The Soret effect is shown to noticeably affect the LID efficiency per a laser pulse only at near-melting temperatures of tungsten. Finally, results demonstrate that the highest desorption efficiency from thick layers can be reached using laser pulses with millisecond duration regardless of surface effects, when the release under short-term pulses can be limited by them.