Hawryluk and Zohm reply: Wallace Manheimer points out that remaining scientific and technological issues will need to be addressed to produce commercially competitive electricity from nuclear fusion on the completion of the ITER research program. True, ITER was not designed to produce net electricity, but power generation can be accelerated by doing more R&D in parallel with ITER’s construction. The fusion community recognizes that; it was a central topic of the research initiatives highlighted in the 2019 National Academies of Sciences, Engineering, and Medicine report on burning plasma research.1 Examples of such parallel activities that are underway include the proposed China Fusion Engineering Test Reactor and the new initiatives Jesse Treu and colleagues mention. Those efforts will profit from both the technological progress made during ITER construction and the physics understanding gained from its operation.
The “conservative design rules” Manheimer mentions are indeed recognized by the tokamak community. Research initiatives around the world are actively addressing them and seeking scientific and technological innovations to increase fusion power for a given facility size. Among them are increasing the magnetic field strength by using high-temperature superconducting magnets and increasing the power heat flux capabilities through the use of liquid-metal divertors.
Manheimer notes that fusion at an ITER-like tokamak can be used to breed nuclear fuel, such as uranium-233 from thorium-232. That may decrease the R&D effort in order to extend the parameters for fusion power plants, but it will entail other technological developments to breed 233U from Th and incorporating them into a nuclear fission economy. Some ITER partners have considered that approach, but others find it less attractive because of its link to fission.
Treu and coauthors note the strong need for innovation, as identified in the National Academies report; the community consensus recommendations for a strategy to address those opportunities; and the increased role of private companies to accelerate development. We share with them the hope that new designs for fusion facilities combined with results from ITER can accelerate the development of commercial fusion.
Finally, the triggering mechanism for the transition to the high-confinement barrier (H-mode) in the edge region is an ongoing area of research that is building on early work by Akira Hasegawa, Masahiro Wakatani, and many others. Experimentally, the criterion for entering the H-mode is characterized in terms of an ion heat flux. For a fully developed H-mode state, the shear flow necessary to suppress turbulence in the edge region is provided by the zeroth-order diamagnetic flow—that is, the pressure gradient and not the turbulence itself. Hence, sustaining H-mode confinement after the transition has occurred does not rely on additional external energy or momentum input; it can be sustained by the heat flux arising from the α-particle heating. We now realize that this important aspect regarding sustaining the H-mode was not clear enough in our article.