In order to cancel the field errors at the insulating gap of the RFX stabilizing shell, which cannot be suppressed by the existing axisymmetric control system, a set of 11 coils has been designed and built. They will be placed on the external surface of the shell, across the gap.

In principle the control system, which drives the coil currents, should vanish the radial magnetic field on the inner surface of the shell, by using the feedback signals provided by a set of probes mounted on the same surface. As a matter of fact, only two large saddle coils are present on the inner surface of the shell and the installation of more probes is not possible. For this reason the feedback signals will be provided by measurement coils placed between each active coil and the outer surface of the shell. This allows to vanish the radial field only on the outer surface of the shell.

The first aim of the work here presented is the determination of a dynamic model of the system composed by the active coils, the conductive shell with the gap, the probes and the saddle coils.

The definition of the principle scheme of the dynamic system to be controlled had to take into account the presence of three different inputs, i.e. the field produced by the plasma current, by the field shaping coils and by each of the active local control coils. The output (quantity to be controlled ) is the total flux linked with the probes.

The transfer functions of the system have been evaluated by means of harmonic analyses on several 3-D eddy current finite element models of the shell with the gap and of the active coils.

On the basis of the above described model a digital control system was designed and implemented as described in a companion paper.

After the assembly of two prototype coils at the top and bottom position of the gap respectively, specific pulses without plasma were executed to validate experimentally the transfer functions between the magnetic field produced by the field shaping coils and the flux measured on either the outer or the inner surface of the shell. Moreover, the effectiveness of the feedback system in cancelling the field on the outer surface of the shell was then verified during standard plasma pulses. The results confirmed the adequate accuracy in the evaluation of the transfer functions and suggest that also a reliable estimate of the field along the inner surface can be obtained on the basis of the same model. This would allow to introduce a correcting term in the feedback signal in order to obtain a virtual feed-back control of the radial field at the inner surface of the shell and to achieve the cancellation of the error field at the plasma edge.