A 3D electromagnetic model of the iron core in JET

Abstract

The Magnet and Power Supplies system in JET includes a ferromagnetic core able to increase the trans-former effect by improving the magnetic coupling with the plasma. The iron configuration is based onan inner cylindrical core and eight returning limbs; the ferromagnetic circuit is designed in such a waythat the inner column saturates during standard operations [1]. The modelling of the magnetic circuitis a critical issue because of its impact on several applications, including equilibrium and reconstruc-tion analysis required for control applications. The most used model in present applications is based onEquivalent Currents (ECs) placed on the iron boundary together with additional specific constraints, in a2D axisymmetric frame. The (circular) ECs are chosen, by using the available magnetic measurements, tobest represent the magnetic polarization effect [1]. Due to the axisymmetric assumption such approachis not well suited to deal with significant 3D effects, e.g. arising in operations with Error Field CorrectionCoils (EFCC). In this paper a new methodology is proposed, based on a set of 3D-shaped ECs and ableto better model the actual 3D magnetization giving rise to a linear system to be solved. According toa well assessed approach [2], the 3D shape of ECs is represented by a set of elementary sources. Themethodology has been successfully validated in a number of JET dry-run experiments where 3D effectsare generated by EFCC currents. The new procedure has been designed to be easily coupled with equi-librium or reconstruction codes such as EFIT/V3FIT. The proposed model resulted to be very effective inrepresenting 3D iron magnetization, especially if compared with typical 2D models.