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

The beta limit in radiative improved (RI mode) discharges where energy conf inement is enhanced due to an increased radiation level from the edge by im purity seeding of neon has been investigated. Under these discharge conditi ons spontaneous confinement transitions from higher to lower energy confine ment can be observed when the beta limit is approached. The empirically det ermined limit of the normalized toroidal beta, PN, is 2.2, pp reaches value s up to 1.5 in the circular limiter tokamak TEXTOR-94. The sawtooth activit y is often stabilized and replaced by central mode oscillations around q = 1 when high values (greater than or equal to 1) of the confinement enhancem ent factor f(H93) With respect to the ELM-free ITER-93H mode scaling are re ached. Measurements of the plasma current distribution show that already be fore the confinement transition the central current density decreases, i.e. the current profile broadens and magnetic shear is decreased. The deterior ation of energy confinement has been found to correlate with the onset of M HD activity either at the q = 1.5 and/or the q = 2 surface. The enhanced tr ansport after the beta drop is caused by the MHD mode activity in the plasm a; the favourable stabilization of the ITG mode under RI mode conditions is not altered. The possible role of pressure driven contributions to the des tabilization of the observed MHD modes will be discussed.