O. Gruber

Beta limits in long-pulse tokamak discharges

The maximum normalized beta achieved in long-pulse tokamak discharges at low collisionality falls significantly below both that observed in short pulse discharges and that predicted by the ideal MHD theory. Recent long-pulse experiments, in particular those simulating the International Thermonuclear Experimental Reactor (ITER) [M. Rosenbluth et al., Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1995), Vol. 2, p. 517] scenarios with low collisionality nu(e)*, are often limited by low-m/n nonideal magnetohydrodynamic (MHD) modes. The effect of saturated MHD modes is a reduction of the confinement time by 10%-20%, depending on the island size and location, and can lead to a disruption. Recent theories on neoclassical destabilization of tearing modes, including the effects of a perturbed helical bootstrap current, are successful in explaining the qualitative behavior of the resistive modes and recent results are consistent with the size of the saturated islands. Also, a strong correlation is observed between the onset of these low-m/n modes with sawteeth, edge localized modes (ELM), or fishbone events. consistent with the seed island required by the theory. We will focus on a quantitative comparison between both the conventional resistive and neoclassical theories, and the experimental results of several machines, which have all observed these low-min nonideal modes. This enables us to single out the key issues in projecting the long-pulse beta limits of ITER-size tokamaks and also to discuss possible plasma control methods that can increase the soft beta limit, decrease the seed perturbations, and/or diminish the effects on confinement

Dependence of particle transport on heating profiles in ASDEX Upgrade

The behaviour of the density profiles in ASDEX Upgrade can be described well with the assumption D χ and a pinch of the order of the neoclassical Ware pinch. The latter is estimated from slowly equilibrating density profiles. The assumption D χ has been succesfully tested by varying the heat deposition profile, making use of on-/off-axis ICRH and ECRH: due to the generally observed self-similarity of the temperature profile, such variations in the heat flux profile have a strong effect on the χ-profile and on the D-profile if the above assumption is correct. The corresponding variations in the density profiles have indeed been observed. The model is also capable of describing the decay of the density profile after injecting a train of pellets. The anomalous transport of impurities is also increased with central heating, and the corresponding flattening of the density profile leads to a significant reduction of the neoclassical impurity pinch. Central ICR or ECR heating are therefore now routinely used to control density peaking and its negative effect on the stability of neoclassical tearing modes as well as to control the impurity transport in ASDEX Upgrade.