Constraints on Tokamak Power Plants

Issues of tokamak power plants, such as the maximum possible length of a pulse of fusion power, the required time between fusion pulses, and the evolution of the current profile towards disruptive states, are tightly constrained by a simple evolution equation for the magnetic field. This equation is determined by Faraday's Law and the mathematical relation between the magnetic and the electric fields. The constraints present challenges to the fast paths to tokamak fusion power that are being pursued by Commonwealth Fusion Systems and Tokamak Energy. Although their proposals are based on detailed simulations, the validity of these simulations is tested by consistency with Faraday's Law, not the other way around. Stellarators demonstrate that these constraints can be avoided. Feasible simulations from startup to shutdown for prototype plasmas could greatly clarify these constraints and the feasibility of their avoidance in tokamaks. The paper https://arxiv.org/pdf/2507.05456 gives details and has two purposes: (1) Illustrate issues on which research must be focused for the credibility tokamak power plants. (2) Encourage thought on the allocation of resources among the various fusion concepts to minimize the time and the cost to the achievement of practical fusion power.