Researchers have developed an updated thermal desorption model for Laser-induced-desorption quadrupole-mass-spectrometry (LID-QMS), a candidate diagnostic for monitoring tritium retention in the first wall of fusion devices. The new model, detailed in an arXiv preprint, specifically addresses the non-ideal, elliptical shape of the laser spot that occurs when the diagnostic laser irradiates tokamak tiles at non-normal angles. It formulates the heat transport and subsequent tritium removal from a solid tungsten sample, deriving a new analytical expression that describes the sample's temperature dynamics under these more realistic conditions. This work extends a previous analysis by the same authors published in Physica Scripta. Source: arXiv

The primary function of LID-QMS is to provide remote, in-situ measurements of the tritium inventory that accumulates in plasma-facing components. Managing this inventory is critical for both fuel cycle efficiency and operational safety in a device like ITER. The diagnostic works by firing a laser at the wall, which heats a small spot and causes trapped fuel isotopes to desorb. A quadrupole mass spectrometer then analyzes the released gas to quantify the retained fuel. Most prior modeling of this process assumed a perfectly circular Gaussian laser beam, an idealization that does not account for the geometric realities of positioning diagnostic hardware within a complex vacuum vessel. Source: arXiv

The primary function of LID-QMS is to provide remote, in-situ measurements of the [tritium](/glossary/tritium) inventory that accumulates in plasma-facing components.

The new contribution focuses on the impact of laser beam ellipticity on the desorption process. By modeling an elliptic Gaussian beam, the analysis shows how the shape of the heated area influences temperature distribution and the rate of gas release. The model was used to examine the specific case of tritium desorption from tungsten, a primary candidate material for the ITER divertor. The analytical framework allows for a more precise prediction of the measurement signal based on the actual beam parameters, improving the accuracy of the diagnostic and its interpretation. This is a crucial step in calibrating the LID-QMS system for reliable performance. Source: arXiv

A key outcome of the theoretical work is the definition of conditions under which an elliptical beam's effect on desorption can be reasonably approximated by that of a circular beam. This provides practical guidance for engineers and physicists operating the diagnostic, establishing a quantitative threshold for when the simplifying assumption is valid and when the more complex elliptical model is necessary. Such criteria are essential for robust data analysis and for minimizing systematic errors in the tritium inventory accounting for the entire first wall. The model's validation will depend on future experimental campaigns in test stands and eventually within fusion devices themselves. Source: arXiv