AbstractRecent experiments on JET with increased additional power have resulted in plasma parameters close to those of a thermonuclear reactor. Electron and ion temperature significantly in excess of 10 keV have been simultaneously achieved at a plasma density of 2 × 9m−3; transiently, during an H-mode the fusion product (n̂iT̂iτE) has reached 3 × 1020 m−3 keV s at temperatures exceeding 5 keV; and plasma current up to 7 MA (for 2 s) have been achieved. However, degradation of confinement with increased power is observed in all regimes.The major heat and particle transport phenomena observed in tokamaks can be interpreted as resulting from a magnetic turbulence, in some respects analogous to the turbulence existing in fluids when the fluid velocity exceeds a certain threshold value. This interpretation has led to local transport and global scaling laws giving satisfactory agreement with experimental results. Accordingly, the fusion product would scale as I2BtR12 for a fixed Troyon factor. Reaching ignition would require a plasma current close to 30 MA at a moderate field value of 4.5 T.To fully tackle the problems of a controlled burning plasma for at least days in semi-continuous operation, the plasma of the next step tokamak should be similar in size and performance to an energy producing reactor. The scientific and technical aims of such a machine should be to study a burning plasma, to test wall technology, to provide a test-bed for breeding blankets and above all to demonstrate the potential and viability of fusion as an energy sourceThe main characteristics of the design of a thermonuclear furnace dedicated to these objectives are presented. Basically the plasma parameters are scaled up from JET by a linear factor of 2.5. Magnets, either superconducting or of copper, should be able to operate continuously. The present design uses watercooled copper magnets to benefit from proven technology and consists of 20 identical sectors. Each incorporates a toroidal field coil, mechanical structure and a part of the vacuum vessel wall as one integrated unit. A single-null divertor configuration ensures helium exhaust and possibly benefits from an H-mode to reach the ignition domain. The X-point position relative to the dump plates would be swept to limit the wall loading to 2 MW/m2. By changing the operating density, the thermonuclear power could be varied from 0.5 to 4 GW(th), according to requirements on power loading and tritium consumption.