The formation of Internal Transport Barriers (ITBs) has been experimentally associated with the presence of rational q-surfaces in both JET and ASDEX Upgrade. The triggering mechanisms are related to the occurrence of magneto-hydrodynamic (MHD) instabilities such as mode coupling or fishbone activity. These events could locally modify the poloidal velocity and increase transiently the shearing rate to values comparable to the linear growth rate of ITG modes. For JET reversed magnetic shear scenarios, ITB emergence occurs preferentially when the minimum q reaches an integer value. In this case, transport effects localised in the vicinity of zero magnetic shear and close to rational q values may be at the origin of the ITB formation. The role of rational q surfaces on ITB triggering stresses the importance of q profile control for advanced tokamak scenario and could assist in lowering substantially the access power to these scenarios in next step facilities.
The early phase of the neoclassical (3,2) + (2,2) tearing mode at ASDEX Upgrade has been investigated in order to study the seed island. Sawtooth crashes or fishbones can trigger this mode, and in few cases it appeared spontaneously.
The dependence of core plasma impurity transport on the Z number has been investigated for ASDEX Upgrade H mode discharges. For the elements Ne, Ar, Kr and Xe the diffusion coefficient in the centre is D ≤ 6 × 10 −2 m 2 /s and rises with the radial distance from the centre. With increasing Z number the transport becomes strongly convective with inward directed drift velocities that produce very peaked impurity densities for high Z. The inward drift decreases with decreasing deuterium density gradient. Neoclassical transport of the impurities has been calculated numerically. The calculated diffusion coefficient and drift velocity are close to the experimental values for the lower-Z elements Ne and Ar. However, for high Z, the calculated diffusion coefficient is smaller by a factor of up to 2.5 and the inward drift velocity is too small by a factor of 10. Toroidal rotation of the plasma that leads to an increased impurity density on the outboard side of the flux surfaces is not taken into account by the neoclassical calculations. Inboard/outboard asymmetries are not present for Ar and Ne with toroidal Mach number Mtor around 1. However, for heavier elements than Kr with Mtor ≈ 2 and an outboard/inboard ratio of ≈1.5, poloidal variation of the impurity density is important and might account for the discrepancy between the measured and calculated transport coefficients.
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