The Wendelstein 7-X (abbreviated W7-X) reactor is an experimental stellarator built in Greifswald, Germany, by the Max Planck Institute for Plasma Physics (IPP), and completed in October 2015.

[9] The research facility is an independent partner project of the Max-Planck Institute for Plasma Physics with the University of Greifswald.

[11] The coils (NbTi in aluminium[11]) are arranged around a heat insulating cladding with a diameter of 16 metres, called the cryostat.

[17] The German funding arrangement for the project was negotiated in 1994, establishing the Greifswald Branch Institute of the IPP in the north-eastern corner of the recently integrated East Germany.

[11][18][19] A three-laboratory American consortium (Princeton, Oak Ridge, and Los Alamos) became a partner in the project, paying €6.8 million of the eventual total cost of €1.06 billion.

The end of the construction phase, which required more than 1 million assembly hours,[22] was officially marked by an inauguration ceremony on 20 May 2014.

[25] The aim for the OP 1.1 was to conduct integrated testing of the most important systems as quickly as possible and to gain first experience with the physics of the machine.

[24][26] More than 300 discharges with helium were done in December and January with gradually increasing temperatures finally reaching six million degrees Celsius, to clean the vacuum vessel walls and test the plasma diagnostic systems.

Experimental observations confirmed 3D modeling predictions that showed heat and particle flux deposition patterns on the limiters in clear correlation with the lengths of the open magnetic field lines in the plasma boundary.

[28][29] Such tests were planned to continue for about a month, followed by a scheduled shut-down to open the vacuum vessel and line it with protective carbon tiles and install a "divertor" for removing impurities and heat from the plasma.

[38][39][40][41] In 2021 an analysis of X-ray imaging crystal spectrometer data collected in the 2018 experiment substantially reduced troubling neoclassical transport heat loss.

This exceeded the original budget estimate, mainly because the initial development phase was longer than expected, doubling the personnel costs.