Topology analysis of electromagnetic fields during plasma start-up in tokamaks

At plasma start-up phase, external electromagnetic fields are applied in a tokamak to ionize pre-filled neutral gas molecules, resulting in an electron avalanche. Based on the traditional Townsend avalanche theory, various methods have been developed to evaluate the quality of external electromagnetic fields. However, it was recently revealed (Yoo 2018 Nat. Commun. 9 3523) that the avalanche process is completely different from the Townsend avalanche because a strong electric field generated by the plasma space-charge reduces the avalanche growth rate and causes the dominant turbulent E×B transport. Considering the important self-generated electric field effects, we propose a novel topology analysis method (Yoo 2022 PPCF 64 054008) that can very easily predict the overall plasma evolution in complex electromagnetic structures. The topology analysis method is verified with particle simulation results for various electromagnetic configurations. The main plasma locations observed in simulations are successfully predicted by the topology analysis. The topology analysis method will be of great help in designing reliable start-up scenarios in ITER and future fusion reactors.