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

Abstract

Understanding 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 validation of assumptions. There are two generation mechanisms of REs: the Dreicer generation and runaway avalanche. Steady-state models for the Dreicer generation are likely to be valid due to the slower evolution of the plasma parameters during the plasma start-up. This is confirmed through the kinetic simulation, which demonstrate that the Dreicer generation can be extracted from the distribution function in dynamic scenarios. However, the runaway avalanche growth rate can differ from its steady state. There are two stages of evolution in the runaway avalanche growth rate before it reachs the steady state. In stage I, REs incapable of participating in the RE amplification reduce the runaway avalanche growth rate. In stage II, REs with finite energy can increase the runaway avalanche growth rate, but the effect is not significant. An error in the RE density due to the runaway avalanche is usually insignificant in most start-up RE modelling. The kinetic modelling of the KSTAR standard ohmic scenario confirms that the RE density can be modelled using the analytic formulas if it is the runaway-free discharge. However, the runaway-dominant discharge may demand the kinetic modelling. The scan of the Coulomb logarithm for the nonstandard ITER plasma start-up suggests that the timing of runaway current takeover predicted by the fluid simulation may change depending on the choice of the Coulomb logarithm.