The external Ne impurity seeding has been applied to reduce the heat flux to the divertor target as well as plasma temperature to prolong the lifetime of the EAST upper tungsten (W) target. The erosion of the W target during the Ne seeding in different divertor operation regimes has been assessed by using the semi-empirical formula, while the plasma background was provided by SOLPS modeling. The simulation results showed that the Ne impurity played a critical role in the W erosion, which depended strongly on the operation regimes. By increasing the Ne seeding rate, the discharge regime was varied from the attached to detached condition. During the attached regime, the insufficient seeded Ne impurity could reduce the heat flux to the target, but the erosion of the W target was obviously enhanced, which may shorten the target lifetime. By the sufficient impurity seeding, the detached condition could be achieved, and the W target erosion was obviously suppressed. The high-power discharge brings great challenge to the W target lifetime, and the erosion is mainly induced by Ne and D ions. The contribution of each species changes greatly with different Ne seeding rates. In addition, the lifetime of the EAST upper W divertor has also been evaluated in different operation regimes during high input power discharge.

To demonstrate the performance of tungsten (W) as the divertor target material and to solve the power handling problem during high power long-pulse discharge, the upgrade of EAST lower divertor is planned. In this work, the physical design of the W divertor is presented by using 2D edge plasma code SOLPS and Monte Carlo impurity transport code DIVIMP. The optimized divertor geometry is proposed after systematic examination of target shapes, target slant angles and the pump opening locations. The performance of the designed divertor is further assessed by impurity seeding. By comparing the medium and high power discharges with argon (Ar) seeding, the differences on the divertor power radiation and impurity core accumulation are distinguished. The simulated effective ion charge Z eff fits well the scaling law, which is based on multi-machine database. Ar seeding and neon (Ne) seeding scans are carried out separately. The simulation results indicate Ar has higher power radiation efficiency than that of Ne, thus promoting the achievement of plasma detachment. However, the core compatibility with Ar is worse than with Ne. The W target erosion and W impurity transport during impurity seeding are simulated by the DIVIMP–SOLPS coupled modeling. It illustrates that under the similar divertor plasma conditions, Ar seeding causes more serious W erosion and more severe core contamination by W impurity, than Ne seeding. Finally, the divertor in–out asymmetry is studied by considering electromagnetic drifts. The simulation results manifest that the designed open vertical inner target reduces in–out asymmetry due to that its weak power radiation capability is offset by the ion flow driven by the drifts. In addition, the designed divertor is compatible with the quasi snowflake magnetic configuration. These studies will improve the understanding of W target sputtering and W impurity transport control during the radiative divertor discharges for CFETR/DEMO.

The upgrade of experimental advanced superconducting tokamak lower divertor to use tungsten (W) target is undertaken, however the target erosion and W impurity accumulation in the core pose a great challenge during high power discharge with external impurity seeding. This paper aims to study the influence of argon (Ar) seeding on the target erosion and the W impurity transport on steady-state operation. The scrape-off layer/divertor plasma as well as W impurity are self-consistently simulated by using SOLPS-ITER code package. With the constant input power, the Ar seeding rate is varied to study the changes on divertor plasma, incident Ar flux, the target erosion and W impurity transport. The simulation results indicate that the external Ar impurity influences W target erosion through both reducing the plasma temperature and introducing heavier incident particles, which significantly complicates the erosion process. The eroded W flux has great impact on the W core accumulation, and the divertor plasma condition influences W impurity transport remarkably. A correlation between peak electron temperature (T e ) at the outer target and the W concentration (C W ) at the core-edge-interface is found. Moreover, the influence of the W impurity on the background boundary plasma is also evaluated.

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