Long pulses: a speciality of Tore Supra

6) Long pulses: a speciality of Tore Supra (p 1 - 2 )

Tore Supra opens the path to the continuous regime of the reactor, with its discharge record of 2 min. How is this performance achieved?

What recipe is needed to achieve a long pulse? A number of parameters come into play. First of all, the magnetic configuration : a system is needed to allow for continuous confinement of particles (supra-conducting magnet for the toroidal magnetic field, non inductive current generation using hybrid frequency heating for the poloidal magnetic field). Then, power and matter injection: heating systems and particles plasma supply working over long periods are needed. And then, of course, as a corollary, the power and matter extraction : plasma facing components cooled by flow water are needed, capable of withstanding enormous heat continuously without excessive temperature rise, while evacuating particles. Then, the plasma parameters must be well chosen (magnetic field, plasma current, density and so on), so that the magnetic equilibrium is stable, the coupling of heating to plasma is satisfactory, the efficiency of current generation optimal and the plasma facing components do not overheat. All this must be monitored in real time thanks to efficient diagnostics in order to be able to intervene on control systems if necessary (magnets, heating, gas injection and so on).

The integration of cutting edge technology and the latest breakthroughs in plasma physics has enabled Tore Supra to achieve the world record of energy injected into a tokamak with 280 MJ in a 2 minutes discharge.

To go further and thus enrich the database, which is used to scale the next step ITER, Tore Supra is now aiming at a performance of 25 MW injected for 1000 seconds (or 25 GJ). At this stage, it will only be one order of magnitude down from ITER in terms of energy (intended for 200 GJ) and will achieve the same pulse duration (1000 s). Moreover, even if the coupled power in Tore Supra is lower, the power density (i.e. the power divided by the surface collecting it) attained on plasma facing components is comparable, the machine being smaller. This is therefore the opportunity for real life testing of next step technologies.

To be up to these ambitious goals, Tore Supra has undergone a facelift:

first of all, its plasma facing components have been renovated, so as to be capable of extracting heat and particles continuously up to running at 20 MW (CIEL project, standing for " Internal Components and Limiter"). The new components have been installed on the machine, which started again in August 2001 in this configuration (cf re-starting Tore Supra).
then, its heating and matter injecting systems are in the course of upgrading, to be capable of injecting the desired power into the machine and continuously supply the plasma with particles (CIMES project for "Components for Injection of Matter and Stable Energy ").

We see here the last preparatory measures to start up Tore Supra again in the CIEL configuration. The operator is working in white overalls to minimise the introduction of impurities into the machine. We see at the bottom of the picture the main plasma facing component, intended to evacuate most of the power and extract the particles, the CIEL start-up limiter (LDC) pending the arrival of the definitive limiter (LPT) for 2002. In the foreground on the left are the carbon safety rings, placed on the stainless steel inner wall. Finally, on the right we can see openings in the wall, enabling access to the plasma for diagnostics or the introduction of the heating antennae.

In 2001, Tore Supra starts again in a totally new configuration, with a CIEL start-up limiter (LDC) enabling tests of the plasma facing component technology. In 2002, the whole CIEL project will be up and running, with the pumped toroidal limiter (LPT): the machine will then be ready to extract 25 MW continuously, all ready to break the GJ record. In parallel, progress in heating systems is still being made, with the first stage CIMES1, which, with the enhancement of the hybrid system, will help along the path to even longer pulses.