Feedforward error field correction by 3-D fields in ITER*

Error fields (EFs) are intrinsic small 3D fields that inevitably arise in the manufacturing of components and misalignment in the assembly process. Low-n EFs can drive large magnetic islands at rational surfaces in the core and induce mode-locking. Therefore, it is critical to correct these EFs to avoid disruptions in ITER, which is equipped with ex-vessel 36 error fields correction coils (EFCC) for n=1 EFC.

For discharges with fast variations of plasma conditions, such as plasma beta, good EFC requires rapid modification to the applied correction (both amplitude and toroidal phase). However, being outside of vacuum vessel, penetration of fields produced by EFCC in ITER is slower than these transients and unable to respond quickly enough. Therefore, in this case, in-vessel ELM control coils can be utilized to correct EFs. Another reason to use ELM control coils for EFC in ITER is that experiments have shown n=2 EFs contributing to mode-locking; EFCC in ITER cannot perform n=2 EFC and only ELM control coils can contribute to this mission.

This talk reports feedforward EFC using 3-D fields in ITER while maintaining ELM control. In addition to ITER’s intrinsic n = 1 EFs, the EFs due to misalignment of TF, PF and CS coils, are evaluated for various phases of key ITER scenarios. Then plasma response to these EFs is evaluated, and the time variation of the required correction coil current will be demonstrated.