Investigation of the resonant helicoidal fields on the activation/inhibition of the MHD oscillations in the TBR-1

The effect of resonant helical fields (CHR) on the plasma confined by the TBR-1 tokamak was investigated. The external perturbations were created by a set of windings located around the vacuum vessel. The field helicity corresponding to the components m/n = 2/1, m/n = 3/1 and m/n = 4/1 could be chosen, from two existing panels beside the tokamak. The main diagnostic used was a set of 10 Mirnov magnetic coils. For this work, a computer program, called MHD-FOUR, was created to Fourier-analyse the detected MHD oscillations. Therefore, the time evolution of every component ranging from m = to m = 4 could be separately obtained. The Spectral Analysis done on some parts of the discharges showed that two or more modes were coupled, and this coupling could justify the disruptive instability that occurred afterwards, while rather intense helical currents (Ihel ≅ 400 - 500 A) were applied. The major disruptions were frequently observed to be preceded by a dominant m = 2 mode while, for the minor disruptions, besides the m = 2, also the m 3 component was found to be equally dominant. The modes growth rates were calculated using experimental data taken just before the occurrence of a disruptive instability. The results confirmed the resistive nature of the perturbations. The rational magnetic surfaces positions and the half-width of the corresponding islands were also estimated. The Chirikov criterion showed that the minor disruptions were preceded by an ergodization of the external part of the plasma column while before the major disruption the ergodization also comprised the internal region. This total ergodization of the plasma column was, most probably, the main cause for the destruction of the confinement. (author)