Axisymmetric magnetic control in ITER

In magnetically confined fusion plasmas, feedback control of plasma parameters is assuming an increasingly important role. The complexity of the phenomena that occur in the plasmas and the limited number of actuators available require the implementation of sophisticated control systems to achieve adequate quality of plasma confinement. The range of requirements is wide and the relevant timescales can range from a few to hundreds of milliseconds. In addition, the high degree of coupling between control parameters increases the level of complexity that the control systems have to address. In this respect, the ITER device, which is under construction and will be the world's largest magnetic fusion experiment, doesn't differ conceptually from a range of smaller scale experiments currently in operation across the world. Nevertheless, limited operational space, together with more demanding requirements in machine protection, implies more restrictive constraints for the control systems. One area of particular importance is that of axisymmetric magnetic control of the plasma. ITER will produce D-shaped toroidal plasma with a "tokamak" configuration in which the hot plasma will be confined and controlled using magnetic fields generated by a set of superconducting coils that surround the vacuum chamber. Although magnetic control is well understood and well developed, being the basic control required for operation of a tokamak, in the case of ITER, new constraints need to be taken into account in the control strategies: long response times due to the thick metal shell of the vacuum vessel, achieving an acceptable level of magnetic noise, that can compromise the quality of control, and establishing an acceptable range of controllability are some of the key issues that need to be addressed. Axisymmetric magnetic control encompasses primarily control of the plasma shape and position. Two controllers are usually envisioned for this purpose: the controller responsible for Plasma Current, Position and Shape and the controller stabilizing plasma vertical displacements which are unstable for plasmas that have an elongated poloidal cross section (Vertical Stabilization, VS). The dynamics of the two control systems differ by almost an order of magnitude in timescale, but the two share some of the magnetic actuators. Additional constraints on the control capability are associated with the quality of the diagnostic data available for the control. Sensitivity of the measurements to the 3D non-axisymmetric components of the magnetic fields, the noise level in the feedback loops and real-time constraints in the parameter estimation would increase complexity in the plasma axisymmetric magnetic control. This paper will illustrate the present status of development of the magnetic control system for ITER and identify the major outstanding issues. Some possible solutions will be presented together with open questions under investigation.