Quasi Single Helicity states
Recent experiments performed in RFX have demonstrated that the plasma can access a Quasi Single Helicity (QSH) state, where the m=1 resistive tearing mode toroidal spectrum, which usually includes a broad range of toroidal numbers n, shrinks around a dominant m=1, n=n0 mode.
In the figure the m=1 spectra of the standard Multiple Helicity (MH) state and of the QSH state are compared together with the corresponding tomographic SXR images, which can be considered representative of the magnetic surfaces.
The SXR emissivity is poloidally symmetric in the MH state, whereas a hot m=1 island is evident in the QSH case in which helically symmetric closed magnetic surfaces are generated.
The location of the island systematically coincides with the resonance radius and poloidal phase angle of the dominant m=1 mode, which are reconstructed from magnetic measurements.
In this situation a coherent helical structure appears in the plasma core.
QSH state are obtained in RFX either spontaneously or as a result of Pulsed Poloidal Current Drive (PPCD).