Heating of SPIDER drivers during nominal operations: Calorimetry and numerical estimation

Abstract

SPIDER, the ion source prototype for ITER Neutral Beam Injectors, is under development at the Neutral Beam Test Facility (NBTF) in Padua, Italy. A first experimental campaign took place between 2018 and 2021. This paper focuses on the heating of SPIDER drivers during operations, primarily caused by RF power dissipation through eddy currents in passive structure elements. Specifically, it examines short pulses with low RF power (about 20 to 50 kW per driver) of the 2018–2021 campaign. It aims at validating the results of a 3D electromagnetic model that simulates RF coupling with plasma, by providing a realistic evaluation of driver temperature and incident power. A calorimetric study allows to estimate the share of RF power absorbed by the driver passive elements. In line with existing literature, results show that around 50% of the RF power is typically lost, with 90% of this loss occurring in the Faraday Shield Lateral Wall (FSLW). The second part of this study describes a self-consistent set of simulations to assess driver heating. Aforementioned electromagnetic code calculates the spatial distribution of power on the FSLW, and ANSYS Fluent models of the Faraday Shield cooling circuits are coupled with a thermal steady-state model that accounts for radiation, plasma heating and backstreaming ions (BSI). Three load cases are considered: low RF power without BSI (typical of 2018–2021 campaign), full target power, and a “worst-case scenario” with maximum RF power on the Faraday Shield. The temperature at which the mechanical properties of electrodeposited copper degrade is never exceeded. The electromagnetic code assumes constant electrical conductivity on the FSLW — that of copper at 50 °C. Thermal simulations support this hypothesis at low power but predict important variations of temperature — and therefore conductivity — at target conditions, locally modifying the RF coupling.