MHD activity at the beta limit in RI mode discharges on TEXTOR-94

The β limit in radiative improved (RI mode) discharges where energy confinement is enhanced due to an increased radiation level from the edge by impurity seeding of neon has been investigated. Under these discharge conditions spontaneous confinement transitions from higher to lower energy confinement can be observed when the β limit is approached. The empirically determined limit of the normalized toroidal beta, β_N, is 2.2, β_p reaches values up to 1.5 in the circular limiter tokamak TEXTOR-94. The sawtooth activity is often stabilized and replaced by central mode oscillations around q = 1 when high values (≥1) of the confinement enhancement factor f_H93 with respect to the ELM-free ITER-93H mode scaling are reached. Measurements of the plasma current distribution show that already before the confinement transition the central current density decreases, i.e. the current profile broadens and magnetic shear is decreased. The deterioration of energy confinement has been found to correlate with the onset of MHD activity either at the q = 1.5 and/or the q = 2 surface. The enhanced transport after the β drop is caused by the MHD mode activity in the plasma; the favourable stabilization of the ITG mode under RI mode conditions is not altered. The possible role of pressure driven contributions to the destabilization of the observed MHD modes will be discussed.