The quality of the magnetic configuration is crucial in the fusion experiments as even little local perturbations can induce locked modes which greatly degrade the confinement. This aspect is very relevant to the RFP experiments where radial components of magnetic field arise at both the toroidal and poloidal gaps of the conducting shell, which is always present in these machines in order to suppress the magnetohydrodynamic (MHD) instabilities.

A system capable to compensate for radial magnetic fields in the order of some tens of mT, arising in an area of some square decimeters, has been designed and implemented. It consists of two coaxially installed coils, a switching power supply unit, and a digital control system. The power coil fed by the switching power supply generates the magnetic flux for the compensation, while the measure coil meters the global magnetic flux which is used by the digital control system to produce the voltage reference for the switching power supply.

The switching power supply consists of a power rectifier connected to a power converter. The last is based on IGBT arranged in an H-bridge topology the selected modulation technique is the PWM with unipolar voltage switching, which, compared to the bipolar voltage switching scheme, has the advantages of reducing the ripple in the output current and of doubling the switching frequency of the output harmonics.

The converter reference is produced by a digital regulator which is designed so as to guarantee the compensation of a step disturbance in times in the order of some hundreds of microseconds it also provides for the conditioning of the feedback signal, for the correction of the offset superimposed and for the limitation of the converter maximum current. The system requirements are initially presented in the paper these include the amplitude of the radial magnetic field to be compensated, the cross-section of the interested area, the time scale of the disturbance compensation.

The design criteria of the system are then discussed in the paper, with particular reference to the voltage and current ratings of both the rectifier and the converter. In addition the choice of the converter switching frequency and of the system sampling frequency are discussed.

The paper will finally describe the implementation of the system, its set-up and the results obtained during the operation of the RFX experiment. The implemented system is composed of a 3 square decimeters power coil, which is fed by an IGBT converter rated 500V @ 70A and operating with a PWM switching frequency of 20kHz. The magnetic flux crossing this area, derived by integrating the voltage signal measured by the measure coil, is then fed into the digital regulator, which is implemented by using a DSP of the TI TMS320C4x family with sampling frequency at 25kHz.

The system performance was tested during the RFX operation and the results are reported in the paper. The system is able to nearly balance the measured flux in about one hundred of microseconds.