JET results and the prospects for fusion

Recent experiments on JET with increased additional power have resulted in plasma parameters close to those of a thrermonuclear reactor. Electron and ion temperature significantly in excess of 10 keV have been simultaneously achieved at a plasma density of 2x1019 m-3; transiently, during an H-mode the fusion product (niTiτE has reached 3x1020 m-3 keV s at temperatures exceeding 5 keV; and plasma current up to 7 MA (fo 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 I2BtR1/2 for a fixed Troyon factor. Reaching ignition would require a plasma curent close to 30 MA at a moderate field value of 4.5 T. To fully tackle the problems of a controlled burning plasmafor 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 source. The main characteristics of the design of a thermonuclear furnace dedicated to these objectives are presented. 13 refs.; 25 figs.; 8 tabs