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 ne=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

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