Advanced feedback control of magnetohydrodynamic instabilities: comparison of compensation techniques for radial sensors

A survey of the feedback algorithms exploiting radial sensors applied in RFX-mod is presented. A Fourier variant of the 'intelligent shell', whose efficiency is limited by the aliasing coming from the active coils' sideband harmonics, and 'clean mode control', a method to single out and cancel this pollution, are compared for both tearing mode and resistive wall mode control. Simulations for 'explicit mode control', a different technique still to be tested in RFX-mod, are also discussed: this method removes the sidebands' effect by subtracting the entire vacuum coils' field. The conclusion is that 'clean mode control' is a better way to implement radial sensors. Since the latter have the technical advantage of allowing an arrangement outside the plasma-facing structures, the efficient control scheme developed at RFX-mod can be considered reactor-relevant.