Conceptual Design of Formosa Integrated Research Spherical Tokamak

The preliminary plasma parameters for the new FIRST (Formosa Integrated Research Spherical Tokamak) device have been determined as follows: major radius 𝑅₀ = 0.45 m, minor radius 𝑎 = 0.32 m (aspect ratio 𝐴 = 1.4), elongation 𝜅 = 2.4, triangularity 𝛿 = 0.5 and magnetic field 𝐵 = 0.8 T. The safety factor (q) profile is monotonic, aiming to achieve a high beta of 𝛽 = 38% and the normalized beta of 𝛽_𝑁 = 7.7. Considered are both fixed-boundary and free-boundary equilibria for FIRST. The fixed boundary equilibrium is computed using the CHEASE code [1] and its stability properties are analyzed with the DCON code [2]. The design parameters are selected based on the β_𝑁 scans while varying the plasma pressure profile, elongation and triangularity. Under these configurations, low-n ideal external kinks, internal modes, and ballooning modes are all found to be stable. To construct the device, we also need to know how to replicate the fixed boundary equilibrium from the externally imposed toroidal magnetic field (TF) coils and poloidal magnetic field (PF) coils. The EFIT code [3] is used to determine the configuration and current of PF required to reproduce the fixed boundary equilibrium.