1102 C. Llewellyn Smith and S. Cowley the expected timescales. We argue that this almost certainly must include a device to test the key engineering components—the CTF. Although this facility was proposed as a possible option in UKAEA FUS 521 (2005) we now consider it essential. 9. What next? (a) The ﬁnal science stages—the International Tokamak Experimental Reactor and the International Fusion Materials Irradiation Facility It is clear that much of what is required is technology rather than scientiﬁc development. Two intermediate facilities, ITER and IFMIF, are necessary to complete the science for the ﬁrst generation of reactors. In the Fast Track they are to be constructed in 10 years, facilitating the start of construction of a demonstration reactor, DEMO, in 20 years. We provide only a short summary of ITER and IFMIF here—further details can be obtained from Ikeda (2007) and Moeslang et al. (2006). (i) The International Tokamak Experimental Reactor ITER, which is shown in ﬁgure 4, will be approximately twice the size of JET in linear dimensions, and operate with a higher magnetic ﬁeld and current ﬂowing through the plasma. The aim of ITER is to demonstrate integrated physics and engineering on the scale of a power station. The design goal is to produce at least 500 MW of fusion power, with an input of approximately 50 MW. The engineering systems of ITER have all been tested in smaller devices but not at this scale or all together. The mission of ITER is complex and detailed but it can be summarized in three goals. — Sustain a high-gain (Q > 10) burning plasma. Recent studies indicate that the gain on ITER may exceed the Q = 10 target and approach ignition (Budny 2009). — Access high-performance steady-state regimes of tokamak operation. The aim is to extend high β, high-conﬁnement (low turbulence), steady-state regimes found in smaller tokamaks. — Test the ﬁrst blanket modules at moderate neutron ﬂux. ITER can deliver approximately 30 per cent of the expected neutron ﬂux in a reactor—thus, low-power blanket tests can be performed. However, ITER will operate for at most a few hours per day; therefore, the total neutron ﬂuence is not sufﬁcient for a test of the blanket materials. ITER is being funded and built by a consortium of the European Union, Japan, Russia, USA, China, South Korea and India. The design has recently been reviewed and updated, having been frozen since negotiation of the Agreement began in 2001, and the construction cost (originally estimated at ¤5 billion in 2008 prices) is currently under review. Prototypes of key ITER components have been fabricated by industry and tested. The site, at Cadarache in France, has been cleared and construction of components is beginning. The initial activity was slower than expected but the project is now gaining momentum. Phil. Trans. R. Soc. A (2010)