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Runaway electron modelling efforts are motivated by the risk these energeticparticles pose to large fusion devices. The sophisticated kinetic models cancapture most features of the runaway electron generation but have highcomputational costs which can be avoided by using computationally cheaperreduced kinetic codes. In this paper, we compare the reduced kinetic andkinetic models to determine when the former solvers, based on analyticalcalculations assuming quasi-stationarity, can be used. The Dreicer generationrate is calculated by two different solvers in parallel in a workflow developedin the European Integrated Modelling framework, and this is complemented bycalculations of a third code that is not yet integrated into the framework.Runaway Fluid, a reduced kinetic code, NORSE, a kinetic code using non-linearcollision operator, and DREAM, a linearized Fokker-Planck solver, are used toinvestigate the effect of a dynamic change in the electric field for differentplasma scenarios spanning across the whole tokamak-relevant range. We find thaton time scales shorter than or comparable to the electron collision time at thecritical velocity for runaway electron generation kinetic effects not capturedby reduced kinetic models play an important role. This characteristic timescale is easy to calculate and can reliably be used to determine whether thereis a need for kinetic modelling, or cheaper reduced kinetic codes are expectedto deliver sufficiently accurate results. This criterion can be automated, andthus it can be of great benefit for the comprehensive self-consistent modellingframeworks that are attempting to simulate complex events such as tokamakstart-up or disruptions.

Validity of models for Dreicer generation of runaway electrons in dynamic scenarios