Description
The Toroidal Field (TF) is the strongest magnetic field component in tokamak devices. It is produced by a dedicated set of windings (the so called TF coils), which ideally should generate a purely toroidal magnetic field decaying as 1/r, r being the distance from the vertical axis. Due to the deviations from the ideal geometry of a perfect toroidal solenoid (segmentation, non-cylindrical geometry, misalignments etc.), both a toroidal non-homogeneity and a spurious poloidal component arise, giving rise to the so-called TF ripple. It is well known that even a small TF ripple may have a dramatic effect on plasma rotation [1] and on fast ion losses [2], potentially impacting plasma fusion performance. This is the reason why specific efforts are dedicated to the minimization of the TF ripple, both at design level [3] and with the introduction of ad-hoc solutions, like ferromagnetic inserts [4].
This paper focuses on the implications of the presence of TF ripple on the electromagnetic loads arising as a consequence of a disruption. During such events, significant amounts of current flow in the conducting structures surrounding the plasma, both induced by plasma current and position variations (eddy currents) and directly injected from the plasma to the structures and vice-versa (halo currents). Such currents interact with the magnetic field, giving rise to forces and moments on various parts of the device. In the paper we will show that the TF ripple is responsible for an unexpected vertical moment (i.e. describing a rotation around the vertical axis) on the conducting structures. Both a theoretical explanation and a numerical quantification using detailed 3D computational models will be provided. It will be shown that the vertical moment can be as high as several tens of MN*m in the case of ITER, when realistic values are considered for the TF ripple and for disruption parameters.
[1] P. De Vries et al., Nucl. Fusion 48 (2008) 035007
[2] G. Spizzo et al., Nucl. Fusion 61 (2021) 116016
[3] L. Kripner et al., Fus. Eng. Des. 187 (2023) 113378
[4] A. Portone et al., Fus. Eng. Des. 83 (2008) 1619-1624