Plasma power balance models for self-sustained tokamak reactors

The plasma physics and engineering constraints determining the minimum size of a tokamak reactor are analysed. Three different models of increasing complexity are considered: (1) a simplified plasma-engineering model for a preliminary choice of the parameters, (2) a self-consistent model based on a space-averaged (zero-dimensional) plasma power balance, and (3) a self-consistent model including the profile effects on the power balance as provided by the Duechs code. As a reference example, the parameters of a minimum-size experimental reactor (FINTOR 1) are deduced, assuming that plasma confinement not worse than 10 times below neoclassical is possible and adopting conservative technological feasibility conditions. The main parameters resulting are a minor plasma radius a=2.25m, an aspect ratio A=4 and a magnetic field on axis of Bsub(To)=3.5T. A sensitivity analysis of the effects of changes in the main input parameters is performed. As an example of a different assumption on plasma transport, the case of losses induced by the trapped-ion instability is touched upon. The method exposed can readily be used to deduce the parameters of reactors of given (not minimum) power or wall loading. (author)