Review. The path to fusion power 1101 8. What remains to be done? The science and technology challenges for fusion power A number of technical challenges must be met in order to progress to a demon- stration reactor and commercial fusion power. The critical goals are as follows. — To sustain a high-gain burning plasma. This requires demonstrating that large amounts of fusion power (greater than 10 times the input power) can be produced in an essentially self-heated (by fusion alpha particles) plasma without provoking uncontrolled instabilities, over-heating the surrounding materials or compromising the purity of the fusion fuel. The plasma current must be driven so that steady state is also demonstrated. ITER (see below) is designed to attain this goal. — To develop a reactor-compatible plasma exhaust system. This involves (i) optimizing a divertor magnetic ﬁeld design that steers the particles away from the plasma and spreads the heat over the largest possible area; and (ii) developing appropriately tough materials for the reactor walls and divertor. — To fabricate and test long-lasting, low-activation structural materials. This requires continued research and testing with reactor-level neutron ﬂuences. The International Fusion Materials Irradiation Facility (IFMIF; see below) will provide the critical tests. — To design and prototype fusion blankets. Blanket concepts have been developed but, because they have never been tested with 14 MeV neutron ﬂuxes, which is a huge challenge. The ﬁrst blanket tests will be performed in the later stages of ITER. — To develop reactor-relevant tritium-handling systems. Tritium has been handled on JET in small quatities but there is no actual operational experience of extracting tritium from a blanket. — To integrate components into a reliable and maintainable reactor design. Progress on the previous goals will inform the evolution from present conceptual designs to the engineering designs of the ﬁrst reactors. Making a reliable reactor is possibly the greatest challenge. Meeting the above goals would deliver a fusion reactor with adequate performance. However, performance would be enhanced and the cost of fusion- generated electricity reduced if a further series of goals were met. These include the following. — The development of a reactor-compatible, high-conﬁnement, high β magnetic conﬁguration. This requires further suppression of turbulence and the extension of the stable operating regimes. It would allow a reactor at lower ﬁeld strength and, therefore, cheaper and perhaps simpler coils. Spherical tokamaks may point the way. — The fabrication of high-temperature structural and plasma-facing materials. This would allow higher operating temperatures of the walls and blanket structure, which would increase thermodynamic efﬁciency. Recognizing the challenges outlined above, Culham developed in 2004 the ‘Fast Track to Fusion’—a strategic plan to develop fusion power (see below and UKAEA FUS 521 2005). This plan outlines the facilities that are needed and Phil. Trans. R. Soc. A (2010)