Abstract
Wendelstein 7-X (W7-X), the largest advanced stellarator, is built to demonstrate high power, high performance quasi-continuous operation. Therefore, in the recent campaign, experiments were performed to prepare for long pulse operation, addressing three critical issues: the development of stable detachment, control of the heat and particle exhaust, and the impact of leading edges on plasma performance. The heat and particle exhaust in W7-X is realized with the help of an island divertor, which utilizes large magnetic islands at the plasma boundary. This concept shows very efficient heat flux spreading and favourable scaling with input power. Experiments performed to overload leading edges showed that the island divertor yields good impurity screening. A highlight of the recent campaign was a robust detachment scenario, which allowed reducing power loads even by a factor of ten. At the same time, neutral pressures at the pumping gap entrance yielded the particle removal rate close to the values required for stable density control in steady-state operation.
Highlights
Creating an interface between a high-performance fusion plasma and material surfaces is crucial in fusion energy research
In the recent campaign, several experiments were performed to prepare for long pulse operation [11]
A fusion reactor based on a stellarator design has the advantage of easier access to long pulse scenarios
Summary
Creating an interface between a high-performance fusion plasma and material surfaces is crucial in fusion energy research. This ratio should be as high as possible to provide high neutral pressure near the entrance to the divertor pumping domain and keep a low concentration of neutrals near the upstream region to avoid sputtering from the first wall due to charge-exchange of neutrals In tokamaks, this is often achieved in the so-called ‘high recycling regime’ [9], where charged plasma particles are adsorbed by the first wall and are re-released as atoms or molecules [10], many times over. In recent Wendelstein 7-X (W7-X) stellarator experiments, a set of large, magnetic islands in front of dedicated surfaces areas were used to define the plasma–material interface This so-called ‘island divertor’ geometry provides a very efficient way to exhaust heat and particles, which is crucial for steadystate operation. In the second part of this paper, the main results from detached plasmas are presented
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