Since the Outline Design Report, a characteristic feature of the ITER EDA Design is the layer wound Central Solenoid (CS), with four conductors in hand, under compression from the Toroidal Field (TF) coils. The main advantage of this solution is a structurally integrated and compact design. The disadvantages include certain limitations to plasma shaping and control and the requirement on the CS power supply to interrupt currents of about 170 kA at plasma initiation and for quench protection. To improve the design, alternative magnetic configurations have been the subject of intensive studies during the last two years.

This paper will present the two main alternatives and will compare their plasma shaping and control capabilities with those of the configurations presented in the Detailed Design Report and chosen for the Final Design Report.

Firstly a segmented CS has been studied: it was made up of five modules, pancacke wound and supplied independently with current of about 45 kA. This design would have improved the plasma shaping and control capability (as the paper will show) and solved the power supply problem but was ruled out because superconducting joints working at very high field (~ 13 T) were considered not feasible.

Secondly a so called "hybrid" CS has been analysed: it was made up of three independently powered modules, the central module was layer wound with three conductors in hand reducing the requirements on its power supply (~ 135 kA); the upper and lower module were pancacke wound. The plasma shaping and control capability improved, the triangularity increased and the separatrix deviation decreased: the paper will report the comparison among the alternatives. But the space required for joints among the modules introduced a gap in the CS design, therefore the flux available to induce the plasma current was somewhat reduced (~ 14 Wb) and, perhaps more important, the stray field throuth these gaps, useful for plasma shaping, caused increased mechanical stresses, beyond the acceptable level. Therefore also this alternative has not been retained.

These studies have confirmed that the original concept of the magnetic configuration is the best suited for the ITER design. Moreover the two Poloidal Field (PF) coils of large cross section and small diameter, which used Nb₃Sn as a superconducting material, have been split into two smaller coils. Therefore the PF system for the Final Design Report will have nine PF coils, all using NbTi superconductors.