Investigation of Incident Ion Angle Distributions on LTX-β Surfaces via a Micro-trench Method

LTX-β is a tokamak device that investigates the effects of using lithium-treated walls on plasma performance. Characterizing the incident ion angle distributions (IADs) at wall surfaces plays a significant role in advancing our understanding of plasma-material interactions (PMI) at the lithium surface and the collisionless scrape-off layer (SOL) potential structure. Measured IADs were used previously to verify the sheath potential structure at the DIII-D divertor surface [1]. Here, we fabricated micro-trenches on a silicon disc sample by using a focused ion beam (FIB) to measure the IADs. A sample manipulator including a holder attachment for the micro-trench sample was designed, fabricated, and installed on LTX-β. The micro-trench holder is equipped with a Langmuir probe situated near the sample surface for plasma parameter diagnostics, as well as a button heater and thermocouple to investigate temperature effects on liquid lithium PMI. Measured IADs were compared to ion trajectories calculated by an equation-of-motion model for different SOL and sheath potential assumptions. Using verified IADs, we then applied a Micro-Patterning and Roughness (MPR) code to calculate the erosion and ion shadowing effects on a 3D numerical surface of the LTX-β stainless steel wall obtained by confocal microscopy measurements [2].