Open Access

Experience in the construction of optimized stellarators shows the coil system is a significant challenge. The precision necessary allow the generation of accurate flux surfaces in recent experiments affected both cost and schedule negatively. Moreover, recent experiments at Wendelstein 7-X have shown that small field corrections were necessary for the operation of specific desired magnetic configurations. Therefore, robust magnetic configurations in terms of coil geometry and assembly tolerances have a high potential to facilitate swifter and less expensive construction of future, optimized stellarators. We present a new coil optimization technique that is designed to seek out coil configurations that are resilient against 3D coil displacements. This stochastic version of stellarator coil optimization uses the sampling average approach to incorporate an iterative perturbation analysis into the optimization routine. The result is a robust magnetic configuration that simultaneously reproduces the target magnetic field more accurately and leads to a better fusion performing coil configuration.

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.

This thesis describes how the data of the Langmuir probes in the Wendelstein 7-X (W7X) Test Divertor Unit (TDU) were evaluated, checked for consistency with other diagnostics and used to analyse plasma detachment. Langmuir probes are an electronic diagnostic, and were among the first to be used in plasma physics to determine particle fluxes, potentials, temperatures and densities. W7X is a large, advanced stellarator, magnetic confinement fusion experiment, operated at the Max-Planck-Institut for Plasma Physics(IPP) in Greifswald, Germany. Its TDU is an uncooled graphite component, shaped and positioned to intercept the convective heat load of the plasma. Detachment describes a desirable operation state of strongly reduced loads on this component. The evaluation of Langmuir probe data relies heavily on models of the sheath, formed at the interface between plasma and a solid surface, to infer plasma parameters from the directly measured quantities. Multiple such models are analysed, generalised, and adapted to our use case. A detailed comparison is made to determine the most suitable model, as this choice strongly affects the predicted parameters. Special attention is paid to uncertainties on the parameters, which are determined using a Bayesian framework. From the inferred parameters, heat and particle fluxes are calculated. These are also indirectly measured by two other, camera-based diagnostic systems. Observations are compared to test the validity of assumptions and calculations in the evaluation of all three diagnostics by checking their results for consistency. The first comparison, with the infrared emission camera system, shows good agreement with theoretical predictions and reported measurements of the sheath transmission factor, for which we derive and measure a value in W7X. Parameter dependencies in the quality of this agreement hint at remaining issues. The second comparison, with the Hydrogen alpha photon flux camera system, shows significant discrepancy with expectations. These are argued to originate from systematic differences in the measurement locations, which are quantified and related to the magnetic topology. Langmuir probe observations of individual discharges are analysed to discuss conditions under which detachment occurs, transition into that state and fluctuations observed prior to and during it. A spatial parametrisation of the data is developed and used to facilitate this. These observations contribute to the larger aim of understanding particle balance control and fusion plasma edge processes.

This work presents the first experimental investigation of the gas balance on the optimized modular stellarator Wendelstein 7-X (W7-X). A balance of all injected and removed particles and a measurement of internal particle reservoirs allows inference of the bound particle reservoir in the wall, which is of interest due to its effects on plasma density control and fuel retention. Different scenarios of the gas balance are presented with data from the operation campaign 1.2 with an inertially cooled graphite divertor. Both net outgassing and net retention scenarios are presented and W7-X is found to operate stable in a wide range of scenarios with varying wall conditions. Since fusion experiments are conducted in ultra-high vacuum, suitable gauges are required for total and partial pressure measurement. The challenges and opportunities of the operation of pressure gauges in the steady magnetic field extending beyond plasma pulses are discussed. The performance of newly improved neutral pressure gauges, based on crystal cathode emitters is quantified. These provide improved operational robustness since they can be operated for long periods of time in strong magnetic fields. A crystal cathode setup and and its operation performance is presented along with a fast calibration scheme. Partial pressure measurements provide additional important information complementing the total neutral pressure measurements, and allowing additional physics insights. As part of this thesis work, a new diagnostic of this kind was implemented on W7-X, the so-called diagnostic residual gas analyzer (DRGA). It provides a wealth of information on various neutral gas species, with a relatively high time resolution - of order a few seconds. The diagnostic setup and its first results are presented in this thesis.