Operational limits under different wall conditions on TEXTOR-94

Generally the operational range of tokamaks is limited by the β and the density limit. Experimentally it was found, that the appearance of MARFEs [B. Lipschultz, J. Nucl. Mater. 145-147 (1987) 15] inhibited an increase of the density significantly above the so-called Greenwald density (nGW) [M. Greenwald et al., Nucl. Fus. 28 (1988) 2199]. In TEXTOR-94 the onset of MARFEs has been investigated under different wall conditions. With fresh siliconization and fresh boronization the appearance of MARFEs can be postponed to higher densities (NGW-bar=e/nGW=1.7). At those high densities e-bar=8x1019 m-3 the Zeff is reduced to 1.3. But as important as the reduction of the global impurity content is the suppressed carbon release on the high-field-side (HFS) just after wall conditioning. Modeling of the MARFE onset by the codes B2-Eirene and TECXY demonstrates that an instability on the HFS due to deuterium recycling and radiation only leads to the formation of MARFEs at higher line-averaged densities in comparison to calculations which include carbon impurities. Complete suppression of MARFEs has been achieved by controlled displacement of the plasma column to the low-field-side (LFS), which reduces the local recycling and impurity release at the inner bumper limiter. This resulted in the highest densities achieved at TEXTOR-94 of NGW=2.1 independent of wall conditions. In this case the maximum achievable density is limited by a radiative collapse