IMPACT OF RUNAWAY ELECTRONS

Runaway electrons can cause severe damage to plasma facing components of large tokamaks. The designs proposed for the first wall and divertor of the next large fusion experiment, ITER-CDA (International Thermonuclear Experimental Reactor - Conceptual Design Activities) are investigated. Energies of up to 300 MeV per electron and surface energy depositions of 30 MJ/m2 are assumed. The GEANT code originating from high energy physics was used to model the energy deposition [J/cm3] quantitatively as a function of the penetration depth and material. The magnetic field was included in the analysis. The energy deposition in the bulk material for a given surface energy load is roughly independent of the incident angle and energy (above 100 MeV) since the main physical process of the energy loss is the formation of an electromagnetic shower, i.e. rapid dissipation of the initial energy into many electrons, positrons and photons. Typical divertor designs protect the cooling tubes with a 1 cm thick graphite layer. Melting of such molybdenum (copper) cooling tubes occurs at a heat load of 50 (25) MJ/m2. Every additional cm of graphite roughly doubles the runaway protection. For low cooling water temperatures (T(H2O) almost-equal-to 100-degrees-C), water pressurization due to runaway electron impact is not a serious problem if the cooling pipes do not melt. If the first material facing the plasma is metallic, melting must be expected for heat loads of around 15 MJ/m2.