Numerical investigation of the impact of conductivity modelling in fluid simulations of RF drivers for giant negative ion sources

Current designs for ITER giant NBI sources involve the use of multiple large radio-frequency (RF) drivers to heat the plasma from which negative ions are extracted. A clear example is the ITER Neutral Beam Injector (NBI) prototype, MITICA (Megavolt ITer Injector & Concept Advancement), currently under construction at Consorzio RFX in Padua, and its separate mock-up, SPIDER (Source for Production of Ion of Deuterium Extracted from RF plasma), currently in operation; both accommodate eight separate RF drivers operating at 1MHz. Maximizing the power coupled to the plasma for such systems is mandatory in order to sustain operation and ensure a high overall efficiency of the NBI heating system. A powerful numerical method to address this challenge is a fluid-model approach to the driver plasma, which can offer at least qualitative predictions of the operational trends of the machine. A critical factor when applying this approach to study the plasma response to the induction fields generated by the RF coil is the modelling of its electrical conductivity, which determines the amount and distribution of heat delivered. In this paper, we present a thorough investigation of this aspect using the FSFS2D code (Fluid Solver for SPIDER 2D), currently under development at Consorzio RFX. In addition, the influence of the static magnetic field generated by magnets embedded in the driver backplate is analysed, given its major expected role.