Impact of flux expansion on tungsten influx and radiation in JET-ILW

Recently, the power exhaust handling has been recognized as one of the main issues in magnetic fusion devices. The problem has become even more relevant after realizing that Plasma Facing Components (PFC) of future devices must be built of metallic materials such as tungsten. After the installation of the ITER-like Wall (ILW) at JET, new strategies such as gas puffing, impurity seeding and control of magnetic poloidal flux expansion were developed and are under investigation to avoid high heat loads in the divertor region and to reduce the tungsten influx from the strike point target plates. Among these, changing the magnetic flux expansion in the tokamak divertor region is a simple way to modify plasma-divertor interactions in terms of different effects like heat and particle fluxes at the targets, impurity influxes and radiation distribution. In the extreme case the flux flaring towards the targets (X-divertor) has been proposed as an alternative divertor configuration able to improve power exhaust handling. Even without reaching XD configuration, flux expansion can significantly affect plasma-divertor behaviour. Therefore, in this paper we study the effects of reducing flux expansion of the inner leg on tungsten influx at JET. Plasma pulses were performed in the baseline scenario with strikes points at the horizontal targets but close to the vertical ones. The aim of the flux expansion reduction (also, flux compression) was the mitigation of the interaction with the high field side of the divertor and a better screening toward the core of tungsten influx coming from the divertor. The flux compression was achieved by reproducing reference equilibria previously executed during relevant pulses at JET-ILW with the CREATE-NL code [1]. Then, an optimization of the currents in the divertor coils was performed [2] to keep the plasma current and the plasma shape constant out of the divertor region where the flux compression was achieved. Taking also advantage from reversing the polarity of the divertor coil closest to the inner strike point (ID1), an experimental poloidal flux compression over 40% was achieved at the inner target and a poloidal flux compression of 8% on the inner X-point plane (with respect to equilibria performed with ID1 = 0). However, differently from the expectation tungsten influx increased far from strike points by about 10% and remains constant at strike points. The final effect of the smaller flux expansion was an increase of core radiation due to a corresponding growth of the tungsten content in the core and its influx far from strike points. The paper presents also the results of the simulations performed with edge codes SOLEDGE2D [3] and EMC3 [4] to understand the reason of previous results.