Operational limits under different wall conditions on TEXTOR-94

Generally the operational range of tokamaks is limited by the beta 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 (n(GW)) [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 (N-GW = (n) over bar (e/)n(GW) = 1.7). At those high densities (n) over bar (e) = 8 x 10(19) m(-3) the Z(eff) 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 N-GW = 2.1 independent of wall conditions. In this case the maximum achievable density is limited by a radiative collapse. (C) 2001 Published by Elsevier Science B.V.