When modeling the generation of runaway electrons in tokamak disruptions, it is essential to account for the evolution of the electric field in a self-consistent way. This is achieved by the ARENA code, which is described in the present paper. In this code, the relativistic electron kinetic equation is solved by the Monte Carlo method, supplemented with a weighting scheme to enhance the accuracy of the simulated fast-electron dynamics. Finite elements are employed to solve Maxwell's equations governing the electric field, and this solution is coupled to the Monte Carlo solution of the kinetic equation in a semi-implicit way in order to maintain numerical stability. This numerical scheme thus makes it possible, for the first time, to simulate runaway avalanche kinetics in a disruption self-consistently, accounting both for the acceleration of runaway electrons by the electric field and for the change in the electric field induced by the runaway current. The first results of such a simulation of a JET-like disruption are presented.
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