Plasma to wall interaction : The materials

5) Plasma-wall interaction and particles and heat extraction (p 1 - 2 - 3 - 4 - 5 - 6 - 7 )

The plasma attacks the wall by submitting it to intense heat and particles fluxes. The wall has its revenge on the plasma by emitting impurities which pollute it. What material can be found to reconcile the two adversaries?

b) The materials

It is by obtaining more and more performing plasmas that we have become aware of the importance of plasma-wall interactions. Indeed, in the first experiments, the duration of the pulse was too short to be able to observe significant heat or damage to plasma facing components. It was with the increase in power coupled to the plasma that we noticed that the wall, under bombardment from particles, emitted impurities (by erosion), like water gradually eroding the rock over which it flows. These impurities went on to pollute the central plasma and decrease the machine performance by radiating the energy coupled to the plasma, which is then lost instead of heating the discharge (see energy balance in a tokamak).

One of the first ideas was to change the wall material, and we then went from the first metal machines to components made of so-called light materials, like carbon or beryllium. Indeed, in addition to thermal properties, these materials have the advantage of radiating less strongly than metals when they are pulverised in the plasma. Thus, the Tore Supra tokamak inner vessel is largely covered with components made of carbon, a material also used for thermal shields in the space industry. In addition, original technologies of combining copper (cooling material) and carbon (plasma facing material) have been developed for the specific needs of Tore Supra, the only tokamak to operate with long pulses necessitating, as in the future reactor, the use of cooled components (i.e. criss-crossed by pressurised water circuits). These technologies have been implemented for the CIEL project, destined to improve the heat extraction capacity of Tore Supra.

NB : carbon also has other physical and chemical properties in terms of hydrogen retention and erosion in particular, which have major repercussions on plasma-wall interaction. The problems linked to retention of radioactive tritium in a reactor type machine mean that we are continuing to analyse alternative materials such as tungsten.

We see here, in the Tore Supra vacuum chamber, a welder working on the first innerl wall, made up of hundreds of carbon tiles. We can also see the cellular structure of the inner vessel, which has remained metallic, since the plasma is not in direct contact with it.

The JET tokamak has, in addition to high flux carbon elements, a wall covered by beryllium. A protective suit must be worn inside the chamber on account of the toxic dust generated by beryllium.