This beautiful picture by Matthias Otte shows the flux surfaces in Wendelstein 7-X. In order to visualize the surfaces, an electron beam is injected in the vessel, which moves along a magnetic field line. It leaves behind a tracer, which is caused by collisions of the electrons with the residual gas. In addition, a fluorescent rod is moved through the vessel cross section, which causes light spots when the elelectron beam hits the rod. In the camera recording, the entire cross section of the magnetic field then gradually becomes visible.
The figure combines the tracers of an electron beam with the image points left behind by the fluorescent rod. The rod is not visible in the figure because it is moved quickly and the exposure time is around 60 seconds.
If you want to know more about the flux surface diagnostic, please check out:
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