Kinetic modelling of start-up runaway electrons in KSTAR and ITER

Understanding the formation of start-up runaway electrons (REs) is essential to ensure successful plasma initiation in ITER. The design of ITER start-up scenarios requires not only predictive simulations but also a validation of assumptions. The objective of this study is to strengthen the physical background required for predictive simulations aimed at ITER plasma start-up design, by validating the model assumptions. Through kinetic simulations, this study examines the validity of steady-state models for Dreicer generation under slowly-varying time scales relevant to plasma start-up and investigates the finite energy effect, commonly neglected, on the runaway avalanche growth rate. The research findings provide insights into situations where kinetic simulations are necessary. To secure a margin-of-control scheme without kinetic simulation, we suggest a strategy of scanning the Coulomb logarithm in fluid simulations as an alternative to predict runaway current takeover and avoid RE dominant scenarios. Ultimately, this paper seeks to offer a robust physical background, practically supporting the successful design of ITER start-up scenarios.