Studies on the impact of gap size on neutral-induced material erosion in EAST

In future fusion reactors such as the International Thermonuclear Experimental Reactor (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 FWM erosion, a dedicated exposure experiment on aluminum (Al)-coated samples was carried out on the Experimental Advanced Superconducting Tokamak (EAST). Two Al-coated samples were installed 50 mm behind the main limiter at a major radius of 2.4 m and shielded by molybdenum (Mo) sleeves with different heights. These sleeves mimic a gap-like region, restricting the incidence of neutral particles on the samples while protecting them from charged particle bombardment. These two samples were exposed to 491 discharges, and the neutral energy spectrum was measured using a low-energy neutral particle analyzer (LENPA). The thickness of the Al coating on each sample was quantified by Rutherford backscattering spectrometry (RBS) before and after exposure. For sample 1, the Mo sleeve with a length-to-inner-diameter ratio of 0.5 yielded a smaller solid angle (1.84 sr), resulting in an Al erosion rate of 8.1×10¹² atoms·cm⁻²·s⁻¹. For sample 2, the Mo sleeve with a length-to-inner-diameter ratio of 0.4 provided a larger solid angle (2.36 sr), with an Al erosion rate of 1.2×10¹³ atoms·cm⁻²·s⁻¹. Using the neutral energy spectrum from the LENPA as input, an estimate based on physical sputtering theory was used to compute the theoretical erosion rates of the samples. The computed Al erosion rates for samples 1 and 2 were 2.8×10¹³ atoms·cm⁻²·s⁻¹ and 3.6×10¹³ atoms·cm⁻²·s⁻¹, respectively. The calculated Al erosion rates are approximately 2.5 times those of the experimental results; this discrepancy may arise from impurity deposition on the sample surfaces, particularly during disruptions. Results indicate that increasing the solid angle from 1.84 sr to 2.36 sr enhances the experimental erosion rate by about 50%. This study experimentally demonstrated that the gap size can modulate neutral-induced FWM erosion by altering the solid angle, providing a reference for future research on the erosion and deposition processes of the FWMs in the MSA of fusion reactors.