MHD limit cycles on FTU.
Casolari A., Pucella G., Buratti P., Giovannozzi E., Grasso D., per il Team FTU
The study of macroscopic instabilities on FTU led to the discovery of the so-called "limit cycles", consisting in a quasi-periodic modulation of the amplitude and the rotation frequency of magnetic islands, arising in nearly disruptive conditions. The phase difference between amplitude and frequency modulations causes the phase portrait to resemble a closed cycle. The presence of a significant second harmonic of the island's frequency, suggests that the island suffers a deformation during the cycle. This deformation may result from the differential rotation of the plasma inside the separatrix, due to the presence of a shear in the diamagnetic velocity. Numerical simulations, performed with a finite difference code, support this hypothesis.