Study on erosion and deposition processes of lithium in magnetic shadow region on EAST

Abstract The plasma–wall interaction in the magnetic shadow regions can influence phenomena such as fuel retention, though the role of neutral particle dynamics in the process remains scarcely quantified. Through a novel multi-diagnostic approach, this study investigates how neutral-induced erosion and impurity deposition modulates material migration behind limiters. By modifying and installing custom-designed Quartz crystal MicroBalances (QMBs) on the head of the Material and Plasma Evaluation System located at the equatorial port of sector H in EAST, the in-situ measurement of the erosion and deposition results of the lithium (Li) at different radial positions was performed. The average mass loss rates of the QMB crystal were −9.84 × 10 −1 ng cm −2 s −1 , 2.54 ng cm −2 s −1 , and 2.15 ng cm −2 s −1 at three locations with major radii of 2584 mm, 2784 mm, and 2734 mm, respectively. A clear transition from deposition to erosion was observed as the distance between QMB and plasma increased. Notably, the neutral-induced Li erosion rate was calculated using the 3D-GAPS code. The results prove that, in addition to the influence of plasma parameters on neutral-induced material erosion by varying the neutral energy spectrum, the radial position of the QMB also affects neutral-induced material erosion by changing the solid angle and the angle-dependent sputtering yield. This supports the existence of a limiter-adjacent impurity deposition zone, with material erosion by neutrals acting as a potential fuel retention mitigation mechanism. This work establishes the quantitative framework connecting neutral transport with wall material evolution, providing critical insights for next-step devices like ITER to optimize fuel retention amount.