First measurement of the velocity slip coefficient and the tangential momentum accommodation coefficient (TMAC) of pure tritium

The tangential momentum accommodation coefficient (TMAC) describes to what degree the reflection of a molecule in a surface interaction is specular or diffuse, and is therefore an important quantity for gas dynamics simulations and tritium processing in future fusion power plants. To the best knowledge of the authors, up to now, no TMAC values for tritium can be found in literature: neither ab initio calculated, nor experimentally obtained. At the Tritium Laboratory Karlsruhe, with a license to handle up to 40g of tritium, we have set up the CryoViMA experiment aimed at measuring the viscosity of tritium. The CryoViMA setup is based on a spinning rotor gauge, with a stainless steel rotor. The measurement principle and accuracy has been verified using helium, hydrogen and deuterium. In this measurement, the deceleration rate (DCR) of the spinning rotor is measured in dependence of the pressure of the surrounding dilute gas. In addition to the viscosity, the analysis of the data acquired in this manner also allows access to the velocity slip coefficient, from which the TMAC can be extracted as described in [1]. The CryoViMA setup allows for cooling over a wide temperature range using evaporated liquid nitrogen. ... mehrWith slow thermal cycling, this system enables a high-resolution measurement of the temperature dependence of the TMAC between 110 K and 300 K. In this contribution we will present first experimental values for the velocity slip coefficient and the TMAC of tritium on stainless steel in a temperature range from 110 K to 300 K.