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For the problem of excessively high divertor heat flux, active impurity seeding is an effective method to radiate the plasma energy reaching the divertor and thus achieve the divertor detachment. Neon is a very effective radiation impurity on many current tokamaks, which is also a candidate species to be applied on ITER. In the EAST 2019 experimental campaign, a series of experiments were performed by seeding a mixture of neon and deuterium (Ne-D2) for detachment and core-edge-divertor integration in H-mode plasmas. The divertor partial detachment with high-confinement core plasma has been achieved by using Ne-D2 seeding in EAST with ITER-like tungsten divertor. Both the plasma stored energy and H 98,y2 > 1.1 are maintained, with the divertor electron temperature, heat flux and the surface temperature near the strike point being all significantly reduced. The differences between Ne-D2 seeding at the scrape-off layer (SOL) upstream and downstream have been experimentally investigated in detail. It is found that impurity seeding at SOL downstream is more beneficial to reducing the divertor electron temperature and peak heat flux. By comparison with experiments using divertor D2 fueling, it is further demonstrated that gas seeding in the SOL downstream will enrich more particles near the strike point, while the seeding in the SOL upstream will influence the entire outer target more evenly. Furthermore, in most of the experiments, gas seeding does not cause obvious toroidal asymmetry in the divertor plasma. However, when D2 is injected in an amount similar to that used to build the plasma, it causes the particle flux near the gas-puff to increase locally, i.e., much more than that at the toroidal location far from the gas-puff location. It is a competition between particle source and transport. When the particle source is stronger, it will naturally increase the local particles. In addition, dedicated experiments with different poloidal distances between impurity seeding and strike point on the radiation ability were carried out. Both experimental results and SOLPS simulation show that the seeding close to the strike point is more conducive to neon ionization and energy radiation.

The compatibility of efficient divertor detachment with high-performance core plasma is vital to the development of magnetically controlled fusion energy. The joint research on the EAST and DIII-D tokamaks demonstrates successful integration of divertor detachment with excellent core plasma confinement quality, a milestone towards solving the critical plasma–wall-interaction (PWI) issue and core-edge integration for ITER and future reactors. In EAST, actively controlled partial detachment with T et,div ∼ 5 eV around the strike point and H 98 > 1 in different H-mode scenarios including the high β P H-mode scenario have been achieved with ITER-like tungsten divertor, by optimizing the detachment access condition and performing detailed experiments for core-edge integration. For active long-pulse detachment feedback control, a 30 s H-mode operation with detachment-control duration being 25 s has been successfully achieved in EAST. DIII-D has achieved actively controlled fully detached divertor with low plasma electron temperature (T et,div ⩽ 5 eV across the entire divertor target) and low particle flux (degree of detachment, DoD > 3), simultaneously with very high core performance (β N ∼3, β P > 2 and H 98 ∼ 1.5) in the high βP scenario being developed for ITER and future reactors. The high-β P high confinement scenario is characterized by an internal transport barrier (ITB) at large radius and a weak edge transport barrier (ETB, or pedestal), which are synergistically self-organized. Both the high-β P scenario and impurity seeding facilitate divertor detachment. The detachment access leads to the reduction of ETB, which facilitates the development of an even stronger ITB at large radius in the high β P scenario. Thus, this strong large radius ITB enables the core confinement improvement during detachment. These significant joint DIII-D and EAST advances on the compatibility of high confinement core and detached divertor show a great potential for achieving a high-performance core plasma suitable for long-pulse operation of fusion reactors with controllable steady-state PWIs.

A new lower tungsten divertor has been developed and installed in the EAST superconducting tokamak to replace the previous graphite divertor with power handling capability increasing from −2 to ∼10 MW m−2, aiming at achieving long-pulse H-mode operations in a full metal wall environment with the steady-state divertor heat flux of ∼10 MW m−2. A new divertor concept, ‘corner slot’ (CS) divertor, has been employed. By using the ‘corner effect’, a strongly dissipative divertor with the local buildup of high neutral pressure near the corner can be achieved, so that stable detachment can be maintained across the entire outer target plate with a relatively lower impurity seeding rate, at a separatrix density compatible with advanced steady-state core scenarios. These are essential for achieving efficient current drive with low-hybrid waves, a low core impurity concentration and thus a low loop voltage for fully non-inductive long-pulse operations. Compared with the highly closed small-angle-slot divertor in DIII-D, the new divertor in EAST exhibits the following merits: (1) a much simpler geometry with integral cassette body structure, combining vertical and horizontal target plates, which are more suitable for actively water-cooled W/Cu plasma facing components, facilitating installation precision control for minimizing surface misalignment, achieving high engineering reliability and lowering the capital cost as well; (2) it has much greater flexibility in magnetic configurations, allowing for the position of the outer strike point on either vertical or horizontal target plates to accommodate a relatively wide triangularity range, δ l = 0.4–0.6, thus enabling to explore various advanced scenarios. A water-cooled copper in-vessel coil has been installed under the dome. Five supersonic molecular beam injection systems have been mounted in the divertor to achieve faster and more precise feedback control of the gas injection rate. Furthermore, this new divertor allows for double null divertor operation and slowly sweeping the outer strike point across the horizontal and vertical target plates to spread the heat flux for long-pulse operations. Preliminary experimental results demonstrate the ‘corner effect’ and are in good agreement with simulations using SOLPS-ITER code including drifts. The EAST new divertor provides a test-bed for the closed divertor concept to achieve steady-state detachment operation at high power. Next step, a more closed divertor, ‘sharp-cornered slot’ divertor, building upon the current CS divertor concept, has been proposed as a candidate for the EAST upper divertor upgrade.