Initial Coil Configurations for a Novel, Flexible, Hybrid Tokamak-Stellarator Concept

We outline early design results for the Columbia Reconfigurable EXperiment (C-REX), a hybrid tokamak-stellarator experimental concept with planar toroidal field coils (TFs) and magnetic dipole coils. C-REX will operate with magnetic fields on the order of 0.5-1T and can be used for multiple experimental studies, primarily focusing on 3D physics unique to this configuration. These include the effects of quasi-axisymmetry (QA) vs. axisymmetry, divertor and island physics, and varying the amount of internal vs. external transform using the central solenoid. Magnetic shaping is expected to be provided by a combination of shifts/tilts of the TFs and varying the current in the dipoles, allowing for both axisymmetric and non-axisymmetric equilibria. Our initial efforts focus on demonstrating the ability for the same coil configuration with realistic engineering constraints to create both axisymmetric and QA plasmas. We discuss the necessary magnitudes of TF shifts/tilts, dipole quantity and size, and equilibrium properties best suited for this experiment. As magnetic dipoles are a recently proposed alternative to non-planar modular coils for stellarators, we also aim to answer open questions in the field such as the effects of dipole current regularization and multiple dipole sizes/shapes.