Non axi-symmetric magnetic fields can be responsible for the onset of large locked modes in tokamak discharges, which in turn often trigger disruptions.

The internal saddle coils on JET have been used to study the generation and correction of error fields. However, these internal coils had technical problems and therefore a new set of external correction coils were proposed as an EFDA enhancement project. The installation of the coils start- ed during the summer 2001 shutdown and two coils have been completed successfully. The remaining two coils are due to be wound in the summer 2002 intervention.

The new set of 4 external, non-symmetric coils can be used to replace the DC (direct current) and low frequency AC (alternate cur- rent) function of the existing internal saddle coils for the generation and correction of static error fields, and the seeding of neo-classical tearing modes. The internal saddle coils are also currently used for TAE studies, which involves their excitation at high frequency – this func- tion cannot be performed by the external error field correction coils due to the high inductance and the field penetration time through the JET mechanical structure.

The correction coils will be used both to reduce the ‘natural’ error field (which arises from machine asymmetries), to ameliorate its effect, and to control the level of the error field so that the threshold level can be studied as a function of plasma parameters. It is important to understand this process since the maximum tolerable error field decreases with the machine size and the permitted value on ITER is close to the value that is likely to be generated by anticipated machine asymmetries.

Features of the Error Field Correction Coils Project

Area enclosed
by the loop 6 m2
Toroidal angle
(each) 70°
of turns 16
Cable area 150mm2

Locked modes are Magnethydrodynamic modes that cease rotating (though they can still grow).

Neo-classical tearing mode: The magnetic island produced by a tearing mode perturbs the bootstrap current which further amplifies the island and degrades confinement or leads to a disruption.

TAE means Toroidal Alfvén Eigenmode, which is a fast particle driven instability.