Tokamak Development and Future Reactors

To satisfy the conditions of thermonuclear fusion, using deuterium-tritium fuel, the plasma temperature T must be in the range 1~3108 K, the energy confinement time tE must be about 1~3 s and the density n must be around 1~31020 particles/m3. To startup a reactor some means of auxiliary heating must be used to attain the minimum initial temperature of about 108 K. After the ignition of the fuel mixture the plasma will be heated by the alpha-particles released in the reaction and the source of auxiliary heating may be turned off. The rate of fusion reactions increases with the square of the plasma density. However, the density cannot increase above certain limits without spoiling the plasma stability. On the other hand, the energy confinement time increases with the density, with the degree of plasma stability, and with the plasma volume. Balancing these requirements, it is possible to determine the minimum size for a reactor, which depends on the magnetic configuration adopted.

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The International Thermonuclear Experimental Reactor (ITER) is a conventional tokamak reactor (major radius R ~ 6.2 m and aspect ratio A ~ 3.1) presently being pursued by Russia, Europe(EFDA), Japan (Rokkasho), Canada (Iter Canada), China, United States and the Republic of Korea. The toroidal magnetic field of 5.3 T will be produced by superconducting magnets and the nominal plasma current is 15 MA. This tokamak reactor will produce 410 MW of fusion power.

The figure was obtained from the ITER site.


The graph illustrates the possible routes in the development of reactors based on the tokamak configuration. After design optimization and assuming operation of the reactor in advanced plasma regimes, the conventional tokamak configuration still leads to relatively large and expensive devices. The spherical tokamak offers the prospect of small size, low cost reactor development.

The compact design, low magnetic field, overall good plasma stability and confinement properties of the spherical torus may lead to the development of tokamak reactors at a low cost with improved performance.

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