Confinement and beta-limit studies in ASDEX H-mode discharges

The edge localized modes (ELM) that periodically expel particles and energy from the peripheral plasma in normal H-mode discharges can be reproducibly avoided for periods of up to 130 ms by displacing the plasma column a few cm outwards. During these quiescent periods the power flow into the divertor is very small (<10% of the power input). This, together with the steep density and temperature gradients observed at the plasma edge, points to a transport barrier at the plasma boundary. The location and extent of this barrier is shown to coincide with the zone of increased shear specific to the divertor configuration. During the burst-free periods, particle and energy confinement times can reach values well above 100 ms. This improved confinement leads to an accumulation of metal impurities in the plasma centre, finally resulting in a radiation collapse of the plasma. Owing to the good H-mode confinement, the available neutral beam power of up to 4.2 MW allowed β-limits to be tested over quite a large range of magnetic fields (1.2T<=Bsub(T)<=2.7T). The maximum toroidal β-values obtained obey a law βsub(T,max)=g.I/(a Bsub(T)), with g lying between the theoretical values derived by Troyon et al. (g=2.8) and Sykes et al. (g=4.0) on the basis of ideal MHD stability. The maximum β-values are obtained in a transient state, and decay to a lower equilibrium value on a time-scale of the order of 1 s. This is interpreted as being a consequence of changes in the current density profiles (affecting the β-limit for ballooning modes) that adjust to the H-mode Tsub(e) profiles on a resistive time-scale. (author)