PLASMA MAGNETIC AND KINETIC CONTROL IN A TOKAMAK

The search of new resources has become crucial in this time of increasing energy demand. Research in nuclear fusion field tries to give an answer to this request. Its main aim is to bring the Sun on Earth", that is to find a viable way to produce energy by the same fusion reaction that occurs in the stars. If one day this goal is achieved a clean, safe and theoretically inexhaustible source of energy will be made available to all mankind. The role of automatic control in today experimental thermonuclear fusion devices is becoming more and more significant. New control systems should be designed to achieve the requirements for an economically attractive steady-state fusion power plant. The control systems which interact with the fully ionized gas (plasma)in the reactor vacuum chamber can be classified into two main categories: - magnetic control systems - kinetic control systems The magnetic control systems deal with macroscopic properties of the plasma such as its vertical velocity, its shape and the current flowing through it. All these features can be related to the magnetic field generated by several surrounding coils. The kinetic control systems, instead, control the transport characteristics of the plasma, that is the internal profiles of current density, temperature and density. To achieve this goal the kinetic control systems make use of the heating and fuelling actuators. Thanks to the availability of reliable models, the magnetic control problem has been successfully solved and different approaches have been proposed to control the plasma position, shape and current in all the existing thermonuclear fusion facilities. As far as the control of the internal kinetic profiles is concerned, only preliminary results have been achieved so far, and only some first attempts of feedback control have been performed. The main problems preventing a good performances achievement with a kinetic control system are: - the lack of simple models able to catch the main transport phenomena which occur during a plasma discharge - the reliability of the available measurements, which is not always satisfactory, especially for those near the plasma center. For these reasons further improvements in modelling and diagnostics are crucial and should be made before a controller with acceptable performances could be designed. In this thesis we deal with both the magnetic and kinetic control problems. In particular the thesis has been divided into two parts: Part I is mainly related with the design, the implementation and the experimental results of the new shape controller (eXtreme Shape Controller, XSC) that has been deployed at the Joint European Torus tokamak. Part II deals with the identification of a dynamic model suitable for the design of a plasma kinetic profiles controller. Special attention is given in Part I to the implementation details, since the author of this thesis has temporarily joined the JET plasma position and current control team during the shape controller development. It is worth to notice that, apart from the review of plasma transport physics, only preliminary results are given in Part II as far as the modelling is concerned. In fact the modelling aimed to the design of a controller for the plasma internal profiles is among the subjects that are still under investigation by both the plasma physics and control communities.