Doctoral Thesis
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This thesis delves into some very important scientific challenges for the stellarator concept as a whole and W7-X in particular, namely, how one effectively interfaces the hot plasma with the material walls of the experiment, in special how the plasma heat and particle fluxes are controlled. The fundamental concept that will be used in W7-X for particle and heat exhaust is the island divertor. A number of theoretical and numerical studies have been performed to guide the design of the divertor components. The actual divertor components are in series production at this time, and are largely compatible with the expected heat loads. However, with the sophisticated codes now available, it has become clear that there are some, otherwise very attractive, operational scenarios that could lead to overloading of the W7-X divertors. At least one mitigation strategy was proposed but was until now not analyzed in sufficient detail. In this thesis, state-of-the-art codes are used to analyze this previously proposed mitigation strategy; they are also used to develop several alternative mitigation schemes, which may in the end be advantageous. The work performed here shows not only that it is conceivable to solve this already identified problem in new and arguably better ways but also that the W7-X coil set has enough degrees of freedom that many important long-pulse plasma effects can be effectively mimicked in short-pulse operation. This opens up a rich research program in the early phases of operation and may therefore lead to a significant acceleration of the scientific program to control and optimize the divertor operation in W7-X. The main scientific challenge for the island divertor operation in W7-X is that, since the divertor geometry is now fixed, the magnetic field structure must be adjusted to the divertor geometry, or additional plasma-facing components must be manufactured and installed. Well before this thesis work was done, such additional plasma-facing components were proposed. These are called scraper elements (SEs). As a part of this work, computer simu- lations were performed in order to obtain a better knowledge base regarding the SEs. To analyze the effect of the SE, edge plasma physics simulation code EMC3-Eirene, was used, in combination with state-of-the-art magneto hydrodynamic (MHD) equilibrium codes. This combination was computationally non-trivial and new, and it has led to important insights. One main result of this study is that the SEs significantly reduce the particle exhaust capabilities in steady state operation; this is a concern for W7-X. To test and further quantify this deleterious effect, physics experiments with a prototype SE should be performed as soon as possible, ideally in the first operation campaigns before the approximately two-year break needed to complete W7-X for steady-state operation. In 3 this first operation phase, however, the necessary combination of plasma parameters, heating power, and achievable pulse length is not accessible. This means, on the one hand, that the problem described will not be present in the first operation phase; on the other hand, the physics implications of installing an SE would appear not to be experimentally testable in that phase. One major finding of this thesis is that the coil system of W7-X is flexible enough to allow such an early experimental test. Different stages of high performance long-pulse discharge can be effectively mimicked in the experiment by a targeted use of the available coil sets. Thus, even in the early phases of the W7-X program one can assess both the protection capabilities of the SEs and their effects on particle exhaust and plasma performance in general. These mimic scenarios also have the potential to test other possibilities for divertor pro- tection besides the SE. Such strategies are addressed in this thesis. The two most promising strategies identified here can be classified as plasma shift and iota control. Both adjust the edge magnetic field to better fit the divertor geometry. This is done slowly but dynamically — i.e. during a long plasma discharge.