Behavior of density fluctuations and electron temperature profiles in JET density limit disruptions

Abstract

The sudden degradation of the energy confinement characterizing major density limit disruptions, has been observed to initiate close to the q=2 surface, in a small and medium size tokamak. A flattening of the electron temperature profile spreads inwards from the outboard O-point of the 2/1 mode. This initiates the destruction of the energy confinement. This paper reports on observations of this sequence of events that has also been observed in JET major density limit disruptions, provoked with a Ne gas puff in ohmic D plasmas. The observation of the same phenomena in three distinct size tokamak plasmas indicates that it may be an intrinsic characteristic of this type of disruptions. Density fluctuations were also measured and it was observed an abrupt increase in the frequency of fluctuations coinciding with the onset of the T_e erosion. The evolution of density fluctuations was measured from the low field side with a recently upgraded fixed frequency, X mode, reflectometer at 76 and 78GHz with 2 micro seconds time resolution. The evolution of the temperature profile was measured with the also recently upgraded heterodyne radiometer with 96 channels, separated by 1 to 2 cm, and 100 micro seconds time resolution. This behavior of the density fluctuations seems to be consistent with the abrupt increase of the density observed in ASDEX Upgrade major disruptions. It will be discussed if these observations indicate that convection is involved in the abrupt destruction of the energy confinement, characteristic of major density limit disruptions.