Abstract In future fusion reactors such as ITER, erosion of first wall materials (FWMs) induced by neutral particles will increase rapidly with rising heating power and operating time. In tokamak devices, magnetic shadow areas (MSAs), such as the gaps between first wall modules, exhibit a reduced neutral particle flux due to their enclosed structure, which reduces erosion. To study the effect of gap size on neutral-induced FWMs erosion, a dedicated aluminum (Al)-coated sample exposure experiment