Studies performed in RTP (Rijnhuizen Tokamak Project) of the most violent and dangerous instability in tokamak plasmas, the major disruption, are presented. A particular class of disruptions is analyzed, namely the density limit disruption, which occur in high density plasmas. The radiative tearing modes that precede these disruptions are analyzed in chapter 4, where it is shown that the extended Rutherford model accounts very well for the mode growth rate and that the effective electron heat diffusivity inside the island is almost 1 order of magnitude smaller than the global electron heat diffusivity of the plasma. The temperature and density profiles inside the island are irregular and for large saturated islands it is clear that the electron density increases inside the island. In chapter 5 is discussed the evolution of the electron temperature and density during the energy quench of a major disruption as measured with a high resolution Thomson scattering for the first time. A series of peculiar phenomena was observed. It was observed that in the beginning of the well known m/n=1/1 erosion of the core temperature, the electron temperature profile is eroded at the m=2 O point A remarkable intense peak in the electron temperature was observed immediately after the almost complete flattening of the electron temperature across the plasma radius. This peak is radially localized at the position of the m=2 island but is very short lived and is poloidally asymmetric. The evolution of the density profile during the erosion of the electron temperature in the core shows a decrease in the core and a pronounced increase in the m=2 island with the density perturbation traveling outwards, opposite to the density gradient. In chapter 6 the result of a series of experiments done with the purpose to avoid or ameliorate disruptions, are presented. Avoidance of disruptions was achieved by stabilizing the radiative m=2 mode with ECRH. Both continuous and modulated power deposition was studied. The most important result was the finding that stabilization of this radiative m=2 mode with modulated ECRH in phase with the O point was not more efficient than continuous ECRH, contrary to what was expected from theory. Moreover, detailed scans of the EC power deposition and of the power intensity were in agreement with the radiative nature of the driving mechanism of this m=2 mode. Amelioration of disruptions was achieved, in a pioneering experiment, by applying ECRH at the end of the energy quench. In this way the current decay that follows a major density limit disruption could be reversed.
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