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Tungsten and fusion plasma: Cohabitation without contamination!

​Physicists from the IRFM explain for the first time why, contrary to what is observed in most tokamaks, tungsten atoms from the inner walls do not accumulate in the WEST fusion plasma.  ITER should also be free of this contamination, which degrades plasma performance.

Published on 1 March 2021

The accumulation of tungsten in the plasma core causes significant radiative losses, often observed in the European tokamak JET (Great Britain) and ASDEX-Upgrade (Germany), but extremely rarely in WEST.

Tungsten has been chosen to line the vacuum chamber of tokamaks because it can withstand the millions of degrees that fusion plasma can reach and because it does not absorb tritium (one of the two reactants).  However, tungsten atoms are torn from the walls and transported either by successive collisions or by turbulent convection.  A high rotation of the plasma can strongly increase the collisional transport of these "heavy" atoms.

In JET and ASDEX-Upgrade - as in many other machines - the plasma is heated by the injection of beams of neutral particles which drive it in rotation. The centrifugal force then favors the accumulation of impurities in the outer part of the plasma, from where they "naturally" move to the central part, and radiate intensely, leading to a degradation of the energy confinement.

In WEST, the plasma is heated by radio frequency waves and its rotation remains weak.  In ITER, the situation will be similar to that of WEST because the contribution of the beams of neutral particles at high energy (1 MeV) to the plasma acceleration will be modest.

Based on calculations performed with turbulence and collisional transport codes, the IRFM scientists demonstrate that under the experimental conditions of WEST, the collisional transport of tungsten atoms remains weak, and the turbulent transport dominates.

This result, important for the operation of ITER and for the design of future fusion reactors, will facilitate the study of the rare cases of accumulation still unexplained and the remedies to be considered.

The description of these processes will finally be integrated into the fast simulator of plasma evolution to be realized in 2021 in the framework of an European project.

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