Investigation of erosion properties of mixed tungsten and boron layers relevant for nuclear fusion devices

Abstract

Plasma-facing materials in fusion reactors are exposed to extremely harsh conditions. They have to withstand high temperatures and bombardment of energetic particles, escaping from the confined plasma. Tungsten was selected as first wallmaterial for ITER (International Thermonuclear Experimental Reactor) due toits favorable properties, such as high melting point and comparatively low sputteryield. One goal of ITER, like any fusion reactor, is to maximize energy output. To improve plasma performance and reduce impurities like oxygen, wall conditioning techniques such as boronization are employed. This process involves the injection of a gas containing boron into the vessel, followed by a glow discharge that decomposes the gas, resulting in the deposition of a thin layer of boron on the reactorwall. This layer modifies the erosion properties of the wall.This thesis investigates how the erosion behavior changes with varying boron concentration. To accomplish this, five different samples consisting of intermixed tungsten and boron at various concentration ratios were examined. This thesis focused on the angle dependent sputter yield of the desired materials under 2 keV Ar+ and2 keV D+2 irradiation. The experimental studies were conducted using a quartzcrystal microbalance (QCM), a device capable of detecting mass changes down to 10 pg cm−2 s−1. Additional binary collision approximation (BCA) simulations,utilizing the code SDTrimSP, were carried out to provide comparison between simulation and experiments. In addition to measuring the sputter yield, atomic force microscopy (AFM) was employed to provide information about possible changes to the surface when subjected to ion beam irradiation. The characterization of the surface before and after the irradiation was achieved by extracting parameters such as the root-mean-square roughness value (RMS) and the mean of the surface inclination angle distribution (SIAD) from the AFM images.This study showed that under the bombardment of 2 keV Ar+ and 2 keV D+2 the eroded mass decreased with increasing boron concentration of the target. Simul-taneously, the number of sputtered tungsten particles decreased while the number of sputtered boron particles increased, a beneficial outcome for fusion reactors, ashigh-Z impurities pose a significant risk to plasma performance.