Recent Progress in the Development of Helium-Cooled Divertor for DEMO

A helium-cooled divertor concept for DEMO has been continuously developed over the past decade at the Karlsruhe Institute of Technology within the framework of the former European Fusion Power Plant Conceptual Study. Over the years, research results and progress of the divertor development with numerous earnings representations have been continually reported. This paper first gives a retrospect of the past results achieved so far and then reports on recent progress of the divertor development. In the course of developing the conceptual design with the goal of reaching a divertor heat flux performance of 10 MW/m², the He-cooled modular divertor with jet cooling (HEMJ) was selected in the early 2000s as the reference concept out of a series of conceptual design studies. For verification of the design principle, a combined high-heat-flux (HHF) test facility with helium loop was built in 2004 at the Efremov Institute for the divertor experiments under specified DEMO conditions. There, the cooling performance of the divertor finger with helium under the heat load of 10 MW/m² was confirmed already at an early stage. In parallel, the HEMJ divertor design was successively improved in terms of its robustness and quality of production in order to achieve a long service life against thermocyclic loading. A breakthrough was achieved in 2010 when an optimized HEMJ cooling finger survived more than 1000 HHF cycles at 10 MW/m² without damage. In the context of long-term planning for DEMO divertor development, research and development work on the development of larger divertor components has been started, particularly focusing on certain fabrication techniques covering, e.g., high-temperature brazing and mass production of the divertor components. Recent progress—a part of this paper—was achieved in the HHF experiment of the tungsten nine-finger module in Efremov, development of nondestructive testing methods for testing multifinger modules in collaboration with CEA, and a study on the integration of multifinger modules on the target plate.