Runaway electrons (REs) generated during disruption are identified as a major issue for ITER and reactor-size tokamaks. Such electrons are produced when a large toroidal electric field is generated in the plasma. This field continuously accelerates low-collisional electrons up to relativistic energy. Such a large electric field occurs both in the plasma core at thermal quench of the disruption when the current profile flattens due to high magnetohydrodynamic activity, and during the current quench (CQ) of a disruption. These REs may initiate secondary RE generation during CQ due to the avalanching process, leading to a multiplication of these relativistic electrons. The impact of REs on the first wall is well localized due to their very small pitch angle. The energy deposition may be huge, and plasma-facing component damages are often reported.

Mitigation techniques are thus mandatory to suppress RE formation or/and reduce their heat loads. Two ways are explored on Tore Supra: (a) suppressing the RE beam formation and avalanche amplification by multiple gas jet injections at CQ and (b) controlling the RE beam when it is formed and increasing the collisionality to slow down the relativistic electrons.