Feasibility study of fast swept divertor strike point suppressing transient heat fluxes in big tokamaks

Abstract

In order to avoid metal surface melting of divertor targets of big tokamak fusion reactors by localized ELM heat loads, sudden detachment loss or VDEs, we study a new technique of spreading the heat flux by harmonic divertor strike point sweeping using a dedicated divertor coil. We ran 2D dynamic heat conduction simulation using real infra-red data of large ELMs heat fluxes on JET divertor target, rescaled for EU DEMO reactor (B0=6T, Ip=21MA, R0=9m). Aiming for the surface temperature suppression factor of 4, this requires sweeping with amplitude*frequency=7cm*2kHz. Building the divertor coil out of 27 tons of copper, this requires 0.8MW cooling. Triggered by analog divertor heat flux signal, dedicated dynamic Fiesta simulation scenario requires 54 capacitive energy storages at 1500V (6mF) discharged into 54 divertor coils, each with AC current of 130kA for each ELM event for ∼4ms (with waiting time ∼40ms). The Ix Btor forces would yield less than 0.1mm rotational vibrations of the coil at the ELM frequency (∼20Hz). The DEMO divertor surface temperature suppression reaches factor of 9 with 18kV, 20cm, 7.5kHz, 400 kA. We also calculate system requirements (0.6mF, 5kV & 60kA→suppression factor=2) for ASDEX Upgrade using the upper divertor coil. Since we found no show-stoppers, this technique seems attractive for big tokamaks.