Quantitative Linear MHD Stability Analysis of the nT-Tao Pulsed Stellarator Concept via STRUPHY

The nT-Tao compact-core pulsed stellarator aims to reach fusion conditions in a magnetically confined D-T plasma, targeting ~10 keV temperature, ~10²² m⁻³ density, and confinement times of ~0.01 seconds. These properties dictate hundreds of MW of power required to heat the plasma, achieved by tight coils driven by pulsed power generators. The underlying confinement is based on optimized quasi-symmetric stellarator MHD equilibria designed with DESC; however, the pulsed mode operation and the high input power may initiate destabilizing MHD modes. Nevertheless, instabilities are permitted to grow if their growth rate is much smaller than a single pulse duration.

In this work we developed a method to assess the quantitative MHD stability of our stellarator designs using the Struphy code developed in IPP. We extract physical growth rates of unstable modes and study their dependence on perturbation mode structure, β, aspect ratio, and plasma radius. We identify interchange and ballooning instabilities and connect their growth rates to rational flux surface, iota, and the Mercier criterion. The method is demonstrated on various benchmark stellarator designs. This study forms the first step in a model hierarchy for design integration, from linear to nonlinear MHD, hybrid MHD-kinetic models, and gyrokinetics. With a large range of experiments, including on our current prototype the C-2A, we aim to detect the signature of specific instabilities, and thus to validate the predictions and benchmark the simulations.