Reversed Field Pinch (RFP) configuration and dynamo process
The reversed field pinch (RFP) configuration differs from other schemes of toroidal plasma magnetic confinement since it relies on currents flowing in the plasma for the generation of both poloidal and toroidal components of the magnetic field.
In this way, a highly sheared magnetic configuration can be obtained which is stable with respect to magnetohydrodynamic (MHD) modes at low values of the safety factor and high values of beta.
MHD mode spectrum measured on RFX, showing the amplitudes of modes with m=1 and various n.
However, while a toroidal current can be efficiently provided by an ohmic transformer, a practical scheme to drive the poloidal current with a similar efficiency is presently lacking.
Experimentally the configuration is realised thanks to a spontaneous dynamo mechanism, driven by global resistive tearing modes resonant on closely spaced magnetic surfaces in the core of the discharge, inside the reversal surface.
These MHD instabilities produce coherent fluctuations in plasma velocities and magnetic fields which induce an electromotive poloidal force which generates and sustains the current distribution against dissipation ("dynamo effect").