One of the key challenges for future fusion research is to mitigate the high steady-state heat load on the divertor target plates, and divertor detachment offers a promising solution. EAST has developed several feedback control methods for divertor detachment. However, when an off-normal event momentarily disturbs the main plasma, impurity seeding may still be conducted by these methods for detachment, which probably drives the main plasma further away from its stable equilibrium or even causes the disruption. These off-normal events include excessive impurity seeding, loss of heating and dust droplets, which are not rare in present tokamak experiments, especially in long-pulse operation. To compensate the drawback of these methods, we propose and develop a module of stored-energy monitoring to ensure stable plasmas in long-pulse operation. The stored energy usually decreases when the main plasma is away from its stable equilibrium, which is suitable to monitor the state of the main plasma. Once the stored energy falls below a certain threshold, the module actively switches off the impurity seeding system. Without impurity seeding, the main plasma can recover with the increase of the stored energy. Only when the stored energy exceeds another threshold, does the module switch on the impurity seeding to continue the detachment operation. The module function has been verified during the EAST radiative divertor experiments in the newly-upgraded lower tungsten divertor. A typical ~20 s discharge in grassy-ELM H-mode regime with ~5 MW source heating power is demonstrated with divertor partial detachment and a good energy confinement by active impurity seeding (50% neon, 50% D2). The energy confinement factor is maintained at a high level, i.e., H_(98,y2)~1.1. Electron temperature in the core region only has slight change after the impurity seeding, while electron density has a ~10% increase. Furthermore, ion temperature near the axis also has a remarkable increase. These achievements provide an important demonstration for the actively-controlled radiative divertor to mitigate the heat loads with a good core confinement, which is an essential step towards steady-state operation of fusion reactor.

A major challenge facing the steady-state operation of tokamak fusion reactors is to develop a viable divertor solution with order-of-magnitude increases in power handling capability relative to present experience. A new divertor concept for this end has been developed and tested recently on the EAST superconducting tokamak through combining the effects of a closed divertor corner and ExB drifts. With BxgradB away from the divertor, the ExB drifts in the divertor move particles towards the outer divertor corner area in the scrape-off layer (SOL), which can significantly enhance the particle concentration there, facilitating divertor detachment. The effects have been demonstrated in the recent EAST experiments, where the lowest electron temperatures near the divertor target plates are obtained in the corner area with BxgradB away from the divertor. These experimental results are in good agreement with simulations using SOLPS-ITER code including drift effects, suggesting that the new divertor concept may potentially provide a promising divertor solution for long-pulse operations of future tokamak fusion reactors with much higher power fluxes.

Various edge low-frequency fluctuations with distinct characteristics exist in different detached divertor states. This study systematically assesses three edge low-frequency fluctuations (f<10 kHz), namely low-frequency quasi-coherent fluctuation (LFCF), low-frequency broadband frequency fluctuation (LFBF) and low-n X-point mode (LNXM) on EAST. The basic features of these fluctuations such as spectral width, location, mode number, propagating direction and particle transport capacities are examined. LFCF occurs when the inner strike point is energy detached or nearly energy detached with T_(et,inner)~ 8–15 eV (T_(et,inner) is the electron temperature of inner strike point), and large electron temperature gap between the inner and outer strike points with ∆T_et> 25 eV (∆T_et is the electron temperature gap between the inner and outer strike points) is essential for the occurrence of LFCF. By contrast, LFBF occurs when the inner strike point is energy detached with T_(et,inner)< 8 eV, while the outer strike point is nearly energy detached or attached. The ∆T_et of LFBF is generally lower than that of LFCF, which is < 25 eV. The LNXM is related only to the radiative divertor with impurity seeding, and considered to be excited by the geodesic acoustic mode proposed by [Sun 2021 Nucl. Fusion 61 014002, A.Diallo 2020, 28th IAEA Fusion Energy Conference] or the coupling of impurity radiation condensation instability and the drift waves proposed by [Ye 2021 Nucl. Fusion 61 116032]. In addition, the possible physical mechanisms of LFBF and LFCF are proposed, with LFBF being the purely growing rippling mode, and LFCF being the mode formed by the coupling of the rippling mode to the drift waves. Related research may be beneficial to better clarify the various low-frequency fluctuations that occur during different divertor states.