The Tore Supra Tokamak at the Association EURATOM-CEA (Cadarache, France) has pioneered the use of superconducting coils to generate its strong toroidal magnetic field. This is an important technology for future machines, since it opens the way to very long plasma pulses, and perhaps even steady state operation. This unique feature of Tore Supra means that it is the right machine for making long pulse experiments. In fact Tore Supra holds the world record of 2 minutes for a hot Tokamak plasma.

The limitation on the plasma performance in these very long pulses is due to heating of the inner surfaces of the machine which define the plasma edge (the “limiter”), and which must absorb most of the injected power. In order to achieve higher performance in long pulses, the physicists and engineers at Cadarache are now  working on a major upgrade of the machine called CIEL (“Composants Internes et Limiteur”).

The most important new component of CIEL is a “pump limiter” made up of 576 individual fingers. Each finger consists of plasma facing tiles made of a composite of graphite and carbon fibre bonded to a copper substrate which contains a cooling water channel.

This high tech structure has ideal properties for a fusion machine : a refractory, low atomic weight material in contact with the plasma, and a high thermal conductivity substrate which can be water cooled. The bonding of the two uses state of the art techniques developed in collaboration with industry. Prototypes have sustained heat fluxes up to 15 MW/m2.

The whole set of fingers is designed to be able to support a 15 MW load in steady state. Apertures located under the fingers allow exhaust gas from the plasma to be pumped away. Other elements of the upgrade include improved protection for the remainder of the inner surface of the machine, infrared cameras to monitor the surface temperature of the limiter, and enhanced control systems to allow the long pulses to be operated safely. Full scale plasma operation of the upgraded machine will start in 2002.