Integrating Recent Innovations in Single-Stage Stellarator Optimization for the Columbia Stellarator eXperiment

Date
Jun 13, 2024, 9:45 am10:45 am

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Abstract:

The Columbia Stellarator eXperiment (CSX), currently in the design phase at Columbia University, is focused on investigating quasi-axisymmetric plasma with a small aspect ratio, and on validating recent developments in stellarator technology, theory, and optimization techniques. It is designed to test some of the theoretical predictions of quasi-axisymmetric plasmas, in particular plasma flow damping, MHD stability properties, and the study of trapped particle confinement. The magnetic field is generated by a set of two circular and planar poloidal field coils (PF coils) alongside two shaped interlinked coils (IL coils), with the potential consideration of additional coils to enhance shaping or experimental flexibility. The PF coils and vacuum vessel are repurposed from the former Columbia Non-Neutral Torus (CNT) experiment [1]. The two IL coils will be wound "in-house" at Columbia University, using non-insulated High-Temperature Superconducting (HTS) tapes. These coils undergo shape and strain optimization to produce the desired plasma configuration while adhering to numerous engineering constraints. Discovering a plasma shape that aligns with the physics objectives and can be produced by such a restricted number of coils poses a significant challenge. Indeed, the constrained coil set’s limited capacity to produce varied plasma shapes hinders the application of the traditional two-stage stellarator optimization approach. Instead, novel single-stage optimization techniques are employed, where plasma and coils are optimized concurrently. Despite an increased problem complexity due to the larger number of degrees of freedom, these methods find optimized plasma shapes that can be generated by coils that satisfy engineering constraints. We discuss two single-stage optimization methodologies[2, 3, 4]. We explore their application to the CSX experiment’s design, aiming to identify configurations that fulfill engineering constraints and generate a plasma within a desired regime for the experiment’s physics objectives.

[1] Pedersen, T. S. et. al. (2006). Construction and Initial Operation of the Columbia Nonneutral Torus. Fusion Science and Technology 50 (3), 372-381

[2] Jorge, R. et. al. (2023). Single-stage stellarator optimization: combining coils with fixed boundary equi libria. Plasma Physics and Controlled Fusion 67 (7), 074003

[3] Giuliani, A. et. al. (2022). Direct computation of magnetic surfaces in Boozer coordinates and coil opti mization for quasisymmetry. Journal of Plasma Physics 88 (4), 905880401

[4] Giuliani, A. et. al. (2023). Direct stellarator coil design using global optimization: application to a compre hensive exploration of quasi-axisymmetric devices. arXiv:2310.19097