Preliminary investigation of burn control in experimental tokamak reactors

The paper content is addressed to the fusion machines of the generation NET, INTOR, FED, in which an exhaust system is envisaged, capable of keeping impurities at a constant prescribed level. The problem of the choice of the reactor working temperature is conveniently separated from the problem of the stabilization (if necessary) of the temperature. For both problems, solutions are proposed which rely on established physics and proven technology. For the first problem it is proposed to rely on the enhancement of radiation losses due to impurities. It is to be observed that the resulting impurity-rich scenario relieves some of the physical and technological difficulties of the exhaust system. For the second problem the combination of two methods is proposed, namely the modulation of a fraction of the additional heating and the modulation of the equilibrium vertical field. It is important to note that the required apparata are already present in the tokamaks for other purposes. With reference to the modulation of additional heating, the control aspects are studied in terms of the theory of systems, showing that the conditions are verified for ''complete observability'' and ''complete controllability'', so that a flexible and efficient control is achievable. Some diagnostics, useful as control inputs, are considered, and their basic physical limitations are investigated. (author)