Abstract
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
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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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