Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation

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.

Plasma–surface interaction in the stellarator W7-X: conclusions drawn from operation with graphite plasma-facing components

“…This uncertainty for attached divertor conditions needs to be taken into account in predictions for long-pulse operation. Note that detached divertor operation shows in experiments symmetric particle and heat loads to all HMs and minimises the modelling uncertainties with this respect [71].…”

“…The simulations will help to explore the capability of these configurations for long pulse operation from the PSI and plasma exhaust perspective. Moreover, the range of ERO2.0 simulations in standard divertor configuration will be expanded to plasmas with impurity seeding (Ne or N 2 ) and detached divertor conditions [46,71] once suitable EMC3-EIRENE simulations are available. Impurity seeded plasmas are one of the most promising candidates to comply with the power load restriction to the target plates with enlarged input power in W7-X.…”

Detecting Plasma Detachment in the Wendelstein 7-X Stellarator Using Machine Learning

“…In OP1.2 [14] a more expanded set of PFCs, including a test divertor, which already had the exact same shape as the water cooled divertor presently being installed, were installed enabling a significant increase in plasma density and heating power. Thanks to the new set of PFCs, feedback controlled divertor gas fuelling capability and a surface conditioning [15], the circumstances were satisfactory to reach complete power detachment in the device [11].…”

Reciprocating probe measurements in the test divertor operation phase of Wendelstein 7-X

“…of carbon in case of graphite erosion due to overheating is usually well tolerated by the plasma in W7-X [49]. However, a graphite body makes insulating the probe tips quite challenging, as arcs between the often electrically biased pins and the probe body must be avoided.…”