Quasi-single helicity spectra in the Madison Symmetric Torus

Evidence of a self-organized collapse towards a narrow spectrum of magnetic instabilities in the Madison Symmetric Torus (R. N. Dexter, D. W. Kerst, T. W. Lovell, S. C. Prager, and J. C. Sprott, Fusion Technol. 19, p.131, 1991) reversed field pinch device is presented. In this collapsed state, dubbed quasi-single helicity (QSH), the spectrum of magnetic modes condenses spontaneously to one dominant mode more completely than ever before observed. For the first time it is observed that the spectra are characterized not only by the increase of one m=1 mode, but also by the simultaneous decrease of the other m=1 modes. Identification of control parameters for the onset of the QSH spectra have also been performed and evidences of a hot core helical structure have been gathered. A higher probability to obtain QSH spectra is found at higher plasma current and at medium-low values of the electron density. The highest values of the SXR emission occur with QSH spectra, a result that is consistent with a hotter plasma core. Moreover, SXR measurements are consistent with the presence of a hotter m=1 helical structure in the plasma core, produced by the dominant MHD mode and associated with the formation of closed helical flux surfaces.