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In this work, studies with respect to the exhaust problem were performed
in the stellarator experiment Wendelstein 7-X with different target concepts and different magnetic field geometries. Different infrared cameras were used to study the heat flux from the plasma onto the PFC. In the first publication, the limiter set-up was used with a simpler magnetic topology in the plasma edge. The radial fall-off of the parallel heat flux for inboard limiters in W7-X shows, similar to inboard limiters in tokamaks, two different radial fall-off lengths, a short (narrow) one, characterizing the near-SOL, and a long (broad) characterizing the far-SOL. For the far-SOL, the heating power and connection length have been identified as the main scaling parameters, while for the near-SOL, the electron temperature close to the LCFS has been identified as the main scaling parameter. The two fall-off lengths differ by a factor 10, and the found scalings for both regimes differ from known models and experimental scalings in tokamaks. A turbulent-driven feature was discussed in the publication as a possible explanation for the behavior of the fall-off length in W7-X.
The gained information and data have been further used to support many
other publications, covering the symmetry of the heat loads, the
energy balance of the machine, and seeding experiments.
The heat exhaust in W7-X with an island divertor was studied in the second
and third publication. Definitions of parameters such as peaking factor and
wetted area were applied for the heterogeneous heat flux pattern on the
W7-X divertor. It was shown that the island divertor concept is capable
of spreading out the heat efficiently, resulting in large wetted areas of up to 1.5 m2. The reached values for the wetted area are comparable to the ones of the larger tokamak JET but with a much smaller ratio of wetted
area to the area of the last closed flux surface. Furthermore, a positive
scaling of the wetted area with the power in the SOL was observed. This
scaling is beneficial for future reactors but needs further investigation of the involved transport processes. The peaking factor (discussed in the second publication) describes how concentrated the heat load is within the region of the strike line. It was shown that this factor is decreasing for increasing densities without affecting the wetted area. The present work paves the way for further analysis of the transport processes of the heat flux towards the island divertor of Wendelstein 7-X.
AbstractFluctuations of electron cyclotron emission (ECE) signals are analyzed for differently heated Wendelstein 7-X plasmas. The fluctuations appear to travel predominantly on flux surfaces and are used as ‘tracers’ in multivariate time series. Different statistical techniques are assessed to reveal the coupling and information entropy-based coupling analysis are conducted. All these techniques provide evidence that the fluctuation analysis allows one to check the consistency of magneto-hydrodynamic (MHD) equilibrium calculations. Expanding the suite of techniques applied in fusion data analysis, partial mutual information (PMI) analysis is introduced. PMI generalizes traditional partial correlation (Frenzel and Pompe Phys. Rev. Lett.
99 204101) and also Schreiber’s transfer entropy (Schreiber 2000 Phys. Rev. Lett.
85 461). The main additional capability of PMI is to allow one to discount for specific spurious data. Since PMI analysis allows one to study the effect of common drivers, the influence of the electron cyclotron resonance heating on the mutual dependencies of simultaneous ECE measurements was assessed. Additionally, MHD mode activity was found to be coupled in a limited volume in the plasma core for different plasmas. The study reveals an experimental test for equilibrium calculations and ECE radiation transport.
The non-renewable energy sources coal, oil and natural gas that contribute the major share of the world's energy, will be running out in the next 40-80 years. With the growing energy demands especially in developing countries, which is likely to surpass that of the developed countries in next 50 years, an alternate energy source is the need to the hour. The nuclear fusion energy is foreseen as one of the potential candidates to solve the current global energy crisis. One of the major challenges faced by the fusion community is the problem of power exhaust. With the larger fusion devices to be built in the future, the heat load on the plasma facing components are expected to grow higher. The present work explores two numerical studies performed on the Wendelstein 7-X, the world's largest stellarator type fusion device, to cope with this problem.
The first project on `'Numerical Studies on the impact of Connection Length in Wendelstein 7-X'' identifies magnetic configuration with long connection lengths, which could bring down the peak heat fluxes onto the divertor to manageable levels, by greater role of cross-field transport which may assist to get a wider heat deposition profile. The second project on `'Development of Heating Scenario to Reduce the Impact of Bootstrap Currents in Wendelstein 7-X'' advocates a novel self-consistent approach to reach high plasma density at full heating power without overloading the divertor during the transient phase of the evolution of the toroidal plasma current, by controlling two parameters; density and power. The aim of both the projects is to contribute to tackling the challenge of the tremendous power exhaust from fusion plasma which, if solved, will be a large step closer to a fusion power plant.