Design and optimization of the Columbia Stellarator eXperiment

The Columbia Stellarator eXperiment (CSX), currently in the design phase at Columbia University, aims to explore quasi-axisymmetric (QA) plasmas at low aspect ratio and to validate recent advances in stellarator optimization, theory, and technology. A central physics goal is to test key predictions of QA theory, including reduced neoclassical flow damping. CSX employs a compact coil architecture comprising two circular planar poloidal field (PF) coils, repurposed from the Columbia Non-neutral Torus (CNT), and two interlinked (IL) coils fabricated in-house using non-insulated high-temperature superconducting (NI-HTS) tape. Additional shaping coils may be incorporated to increase flexibility. A major design challenge is to realize plasma configurations that meet physics objectives within the tight engineering constraints imposed by a minimal coil set. These constraints render the traditional two-stage optimization approach, in which plasma and coils are optimized sequentially, ineffective. Instead, CSX leverages a novel single-stage optimization framework that co-optimizes plasma and coil geometry simultaneously. This approach, though computationally intensive, enables the discovery of viable QA configurations that balance physics performance with manufacturability. We present optimized configurations for CSX that demonstrate this balance, refined further through multifilament modeling to account for coil thickness, and by assessing sensitivity to manufacturing and alignment errors. Physics performance is evaluated using the neoclassical code SFINCS, confirming the viability of the selected configurations. These advances highlight the effectiveness of integrated optimization techniques in addressing complex stellarator design challenges and position CSX as a testbed for QA stellarators and NI-HTS coil technology.