Experimental and numerical investigation of power exhaust in the tokamak ASDEX upgrade with magnetic perturbations

One of the currently most promising concepts to confine high temperature fusion plasmas is the divertor tokamak. In devices of this type a large part of the power which is exhausted from the magnetically confined main plasma is deposited on divertor targets, wall components which are specifically designed to sustain high heat and particle fluxes. The width of the channel in which the power flows to these targets, the so called power decay length, is typically only a few millimeters. Hence, in future fusion devices, such as ITER, the material limit of 5-10 MW/m² of the targets will be exceeded substantially, if no measures to mitigate the heat flux are undertaken. The strategies to reduce the target heat flux efficiently, e.g. by injecting impurity atoms which radiate in the divertor region, have been investigated in numerous experimental and numerical studies. These studies largely assume an axisymmetric magnetic field geometry. However, in recent years 3D Magnetic Perturbation (MP) fields have been increasingly applied in several tokamaks, in order to mitigate or even suppress edge localized modes, harmful bursts of power and particles from the plasma edge. MP fields break the toroidal axisymmetry of the magnetic configuration and lead in effect to a change of the radial plasma transport. If they find application in future divertor tokamaks, their impacts on power exhaust and the heat flux onto the divertor targets need to be understood. MP fields might have harmful as well as beneficial consequences for power exhaust. A possible harmful effect is the toroidally asymmetry of the heat flux pattern. It has been speculated that this may lead to a burn-through, i.e. the existence of toroidally localized high heat flux regions under conditions which would lead to a low heat flux everywhere without MP field. If this problem arises in ITER, it would be necessary to apply countermeasures, such as rotating the MP field, which would entail substantial engineering efforts. On the other hand, MP fields could lead to the beneficial effect of an increased toroidally averaged power decay length. This effect would not only lead directly to a decreased target peak heat flux due to an increased plasma wetted area, but also increase the volume in which impurities can radiate efficiently. In this thesis, the effects of MP fields on the plasma transport in the divertor tokamak ASDEX Upgrade (AUG) were studied experimentally and numerically. Moreover, the implications of the results were discussed in view of ITER. Based on the experimental measurements of the heat flux onto the divertor target it is argued that a burn-through event is unlikely in ITER, since toroidal asymmetries are smoothed out by diffusive transport at low divertor plasma temperatures. In addition to that, the comparison of the experiments with simulations by the transport code EMC3-EIRENE revealed several strong indications that the MP field is screened by currents induced in the plasma. Due to this, the impact on both, the toroidally averaged power decay length and the radiation, is rather small and barely measurable with the present diagnostics in AUG. In ITER, however, where the power decay length is predicted to be of the same order as in AUG, while the radial perturbation of field line paths by the MP field is much larger, a substantially stronger effect is predicted.