Performance analysis of compact tokamak reactors

A new approach to fusion power has been recently considered - to demonstrate early power production in a compact reactor with low first wall load. The reduced load allows using presently available first wall technologies, until new materials are developed and tested by ITER and future component-testing facilities. However, the use of the small fusion power output of the pilot plant has to be maximized either by energy multiplication methods (fuel breeding) or in applications such as hydrogen production at high temperature, high-level waste transmutation, and testing of fusion nuclear technology components. Low aspect ratio tokamaks with increased toroidal field seem to be the ideal candidates for these applications, either by using replaceable central copper rods or high temperature superconductor technology. In this paper the performance of compact tokamak reactors is analyzed, considering the fusion power, power gain and average wall load. Stability issues related to the toroidal β limit, safety factor and density limit are taken into account. The analysis is based on the solution of the global power balance equation with the convection and conduction losses modeled by empirical scaling laws (ITER scaling law in particular). The plasma model includes geometrical aspects, profiles and impurities effects, neoclassical effects, and stability constraints. A convenient normalization of the plasma temperature and density, and of the auxiliary power, is introduced, which leads to the definition of a simple figure-of-merit parameter. This figure of merit sets the operating conditions of the tokamak reactor according with the required performance. In this way, it is possible both to search for a set of machine parameters that satisfies the performance goal and to classify different tokamaks by their figure-of-merit value. (author)