Incident Ion Angle and Redeposition Measurement on Plasma Facing Surface Using Micro-Engineered Target on Lithium Tokamak eXperiment-β*

A fully coated lithium wall in fusion devices enables a distinct plasma operation with low edge neutral density and high edge temperature, leading to increased fusion production volume and suppression of temperature-gradient-driven instabilities [1]. However, limited research has been conducted on plasma-wall interactions, which might determine the sustainability of such unique wall conditions. Here, we first report an experimental measurement of fundamental plasma-material-interaction parameters including incident ion angle and net lithium redeposition rate on the plasma-facing surface in the Lithium Tokamak eXperiment-β, which utilizes a full-coated lithium wall and exhibits nearly flat temperature profiles [2]. We expose Si(100) crystal targets fabricated with micro-trenches of 30×30×4 μm and 30×30×8 μm [JG1] [EJ2] to LTX-β plasmas[JG3] [EJ4] . The deposition pattern of impurities, likely consisting of Li, on the micro trenches, is clearly observed by scanning electron microscopy after exposures to 32 H plasma shots (with a maximum plasma current of around 120 kA and a pulse length of around 50 ms), providing data for the incident ion angle. In addition, X-ray photoelectron spectroscopy analysis of the Si crystal surface measures a net lithium redeposition thickness of 2 nm for solid lithium wall operation and 34 nm for liquid lithium wall operation with active heating.