Density limits and evolution of disruptions in Ohmic TEXTOR plasmas

Density limits and the evolution of disruptions were studied on TEXTOR for different wall materials facing the plasma surface. A systematic extension of the density limit was found over a large current range when the metallic wall was first exchanged by a carbon coated wall and later by a boron-carbon coated wall. The maximum attainable density nearly doubled when metallic impurities were replaced by low Z impurities such as boron and carbon. With a neutral beam additional heating power of PNI = 3.2 MW, average densities exceeding n-bare = 1020 m-3 were obtained. The maximum density observed scales like the square root of the auxiliary heating power. The sequence of processes leading to a density limit disruption was experimentally analysed in detail for ohmically heated plasmas. During the density increase in the gas feeding phase, boundary cooling by radiation and steepening of the electron temperature gradient were observed. The plasma detaches from the limiters and the magnetohydrodynamic activity is launched in the edge plasma. An m = 2 mode in the outer plasma region couples with an m = 1 mode deeper in the plasma. During a first mode locking of the m = 2 mode, a strong heat pulse is emitted and a sharp increase of the impurity line C V and O VI is found. The final energy quench is believed to be triggered by the overlapping of two coupled modes. Enhanced density fluctuations are observed by collective millimetre wave scattering in this phase of the discharge. The energy quench in TEXTOR lasts approximately 1 ms or less. Then the electron temperature profile evolution shows a structure which could be interpreted as a filamentation of the plasma current. The decay of the plasma current after the energy quench is governed by the resistance of the cold plasma. (author). 26 refs, 16 figs