Global analysis of tungsten migration in WEST discharges using numerical modelling

Abstract

Plasma discharges in the tungsten (W) Environment Steady-state Tokamak (WEST) are strongly impacted by W contamination. In WEST experiments, due to W contamination, the power radiated in the plasma ( P _Rad ) is on average, around 50% of the total power injected into the plasma ( P _TOT ). Furthermore, this radiated power fraction ( f _Rad ) is almost insensitive to plasma conditions. The causes behind this experimental trend are not fully understood. In this contribution, a 3D numerical model is used to analyze the W migration in the WEST boundary plasma in different plasma scenarios. The WEST experimental database is sampled to obtain a scan of simulation input parameters. These parameters mimic the WEST plasma conditions over a chosen experimental campaign. The simulation results are compared to WEST diagnostics measurements (reflectometry, Langmuir probes, and visible spectroscopy) to verify that the simulated plasma conditions are representative of the WEST database. The W contamination trend is analyzed: the W density ( n _W ) strongly decreases when the radial distance between the separatrix and WEST antennas (Radial Outer Gap, ROG) increases. On the other hand, at a given ROG, n _W increases proportionally with the power entering the scrape-off layer ( P _SOL ). P _Rad is estimated with a simple 0D model. For a fixed ROG, f _Rad is not sensitive to plasma conditions. These trends are qualitatively and, at times, quantitatively comparable to what is observed in WEST experiments: the simulated trends are related to the poorly screened W influx caused by the erosion of the main chamber Plasma-Facing Components. Thus, this numerical analysis suggests a possible interpretation of WEST experimental trends.

Keywords

• tokamak plasma
• numerical modelling
• impurity transport
• WEST