TOROIDAL FIELD RIPPLE STUDIES FOR THE FAST PROJECT AND ITER DESIGN

A new facility for fusion, the Fusion Advanced Studies Torus ( FAST), has been proposed to prepare ITER scenarios and to investigate non linear dynamics of energetic particles, relevant for the understanding of burning plasmas behavior, using fast ions accelerated by heating and current drive systems. This new facility is considered an important tool also for the successful development of the demonstration/prototype reactor (DEMO), because the DEMO scenarios can take valuable advantage by a preparatory activity on devices smaller than ITER with sufficient flexibility and capable plasma conditions, before to testing them on ITER itself. In the regimes proposed for FAST the magnetic Toroidal Field (TF) ripple could lead to significant losses of high-energy particles, as also demonstrated in JET and JT60U experiments(1&2), so a careful analysis is necessary to achieve a low value of the TF ripple as far as compatible with the general load assembly design issues. Two different approaches to reduce TF ripple had been considered: Ferromagnetic Insets and Active Coils. For both solutions, different geometric parameters were investigated and the relative benefits and drawbacks evaluated. The analysis was carried out by 2D and 3D electromagnetic F.E.M codes, dealing with different design solutions, chosen between those compatible with the relevant geometric dimensions of the plasma (i.e. the vacuum vessel), the access to the plasma and the divertor needs (i.e. the vacuum vessel ports dimensions) and other design constrains. A magnet consisting of 18 coils, each made of 14 copper plates suitably worked out in order to realize 3 turns in radial direction has been proposed. To limit within acceptable value the TF magnet ripple, the ferromagnetic insets solution has been chosen for FAST. The ripple on the plasma separatrix (near the equatorial port), has been so reduced from 3% to 0.3%. Due to the good results obtained also with Active Coils a study for applying the Active Coils concept also in ITER design was made, confirming even in this case the possibility to reduce considerably the TF ripple.