The speed at which fusion energy can be deployed is considered. Several economical factors are identified that impede this speed. Most importantly, the combination of an unprecedentedly high investment level needed for the proof of principle and the relatively long construction time of fusion plants precludes an effective innovation cycle. The valley of death is discussed, i.e. the period when a large investment is needed for the construction of early generations of fusion reactors, when there is no return yet. It is concluded that, within the mainstream scenario-a few DEMO reactors towards 2060 followed by generations of relatively large reactors-there is no realistic path to an appreciable contribution to the energy mix in the twenty-first century if economic constraints are applied. In other words, fusion will not contribute to the energy transition in the time frame of the Paris climate agreement. Within the frame of this analysis, the development of smaller, cheaper and most importantly, fast-to-build fusion plants could possibly represent an option to accelerate the introduction of fusion power. Whether this is possible is a technical question that is outside the scope of this paper, but this question is addressed in other contributions to the Royal Society workshop. This article is part of a discussion meeting issue 'Fusion energy using tokamaks: can development be accelerated?'.
Keywords: deployment; economics; fusion energy; nuclear fusion.