Problems related to ignition in the next step tokamak

The working point (i.e the central density and temperature) is assumed and the possibility of ignition of a given device is discussed: because of the analytical form of the confinement scaling, it appears that the conclusion depends only weakly on the assumption about the working point but strongly on the volume of the device and on the plasma quality symbolized by three factors (purity, profiles, confinement). An analytical criterion is presented which allows an easy estimate of the balance between technical and physical constraints. In an ignited plasma the fuel is mainly heated by collisions with electrons. This fact is not taken into account in an one-fluid model. A two-fluid model is presented which allows to estimate the impact of the energy equipartition constraint on a tokamak scaling. These models are used to evaluate the ignition capability of ITER. Suggestions are made in conclusion to increase the safety margin for ignition