In order to enhance power handling capability of the EAST (Experimental Advanced Superconducting Tokamak) lower divertor, scientists have put forward a new comprehensive solution called Corner Slot divertor, which has been upgraded in both shape and material.

The solution was a full chain program from physics design to engineering manufacturing as well as a result of international collaboration.

In this new Corner Slot divertor, vertical and horizontal target plates together formed a right angle, and its slot was changed from V-shaped to corner-shaped so that the cooling water could run faster and the heat could be taken away in a shorter time.

Corner Effect, produced by the corner shape, generated a high-pressure neutral area near the corner so that the stable detachment could be maintained across the entire outer target plate with a relatively lower impurity seeding rate.

During operation, this Corner Slot divertor offered much more candidate area for heat loading, and outer strike point could be on either vertical or horizontal target plates or slowly scanned in between, enabling advanced scenarios exploration.

As a large and extremely complex machine, fusion facility assembly always requires high precision. Compared with V-shaped, Corner Slot divertor is much simpler to be manufactured and installed as well. Then the assembly precision ca be relatively ensured, making the entire facility more reliable and economical.

Except for the modification in the slot shape, tungsten-copper structure was employed instead of the previous graphite. Tungsten is a kind of material showing a greater ability for being cooled down and reducing fuel retention.

Looking back to this spring, the new Corner Slot divertor was successfully manufactured till March and had been equipped in vacuum vessel of EAST facility in April after a series of modeling work. It was the time for the engineeringly experienced team to bring their physics idea into reality.

In the following three months, together, the new lower divertor with the EAST has witnessed that EAST achieved a plasma temperature of 120 million degrees Celsius for 101 seconds, a new record and also another milestone in EAST progress toward the test running of a fusion reactor.

A series of parameter obtained from the experiment suggested a great improvement in EAST performance. The upgraded divertor was a big factor behind the achievement. The researchers were pleased to see a dramatic enhancement in lower divertor's power handling capability from less than 2 MW/m² to around 10 MW/m², nearly five times of the previous graphite divertor.

For its physics design and its follow-up verification, the researchers used a plasma edge modelling tool, SOLPS-ITER, which is a powerful and advanced tool developed parallelly by ITER members (distributed and maintained by ITER Organization) to solve fusion-related key problems. With the help of the modelling tool, series of physics design and repeated verification could be done.

Result from EAST experiment spoke clearly of the Corner Effect. The researchers also conducted further verification using SOLPS-ITER tool, and the experimental results were in agreement with those of the simulation.

The success of new Corner Slot lower divertor has inspired the upgrade of its upper part. A more closed divertor, Sharp-cornered Slot divertor, according to the researchers, is being developed, which is considered as a preferred candidate for the EAST upper divertor.

 (a) 2D schematic of the new lower divertor on EAST. The horizontal and vertical target plates of the outer divertor constitute a right-angled corner. (b) A divertor module. The target plates are made of ITER-like monoblock PFUs. The dome and end-boxes are covered with flat-type PFUs. (Image by JIA Guozhang)