Design and Analysis of NSTX-U Inner Poloidal Field Coils

NSTX-U is an innovative magnetic fusion device constructed at the Princeton Plasma Physics Laboratory (PPPL) and is an upgrade of the original NSTX device that operated successfully for more than ten years. During the initial operation of NSTX-U in 2016, one of the inner poloidal field (PF) coils experienced a turn fault that necessitated a complete shutdown. A precise cause could not be identified but several contributing factors became apparent. It has since been decided to replace all three inner PF upper and lower coil pairs. Prototype and replacement inner PF coils with an improved design and construction have developed that factor in findings from the root cause analysis of the failed coil. Detailed electromagnetic (EM) and transient thermal analyses were performed for both the energized state and the cool down cycle. This article summarizes the effort taken to replace inner PF coils including improvements in design, results of analysis, experience with prototype coils, and progress with the fabrication of replacement coils.