Clarifying the Role of Aspect Ratio in 0D Tokamak Design Studies

Reducing tokamak aspect ratio leads to improved stability limits (beta, elongation) and potentially larger bootstrap fraction, offering the promise of achieving 100% non-inductive reactor designs in a relatively compact configuration. However, these potential benefits will be constrained by the reduction in achievable on-axis magnetic field, the need for inner wall shielding and a central solenoid, and potentially larger mechanical stresses. As such, this study uses 0D design methods to clarify and illustrate the interplay between these constraints when optimizing aspect ratio for various tokamak design targets, such as a pilot plant, DEMO, or FNSF. We further consider the sensitivity of the optimization to variation in assumed technologies and design choices (HTS vs. LTS, NBI heating vs. RF heating, non-inductive vs. inductive current drive, etc.) and confinement and stability assumptions. Simple cost metrics are also considered to aid illustration of potential optimal designs.