Hacker News Reader

Comment by saganus | Hacker News Reader

Does anyone have any idea how do they take pictures of the plasma?

That seems quite hard considering the temperatures and the strong magnetic field.

Good question - I worked at the ASDEX Upgrade (AUG), also operated by the IPP but a tokamak instead of a stellarator: At AUG similar images and videos [0] are recorded by IR cameras. These cameras require strong cooling and a magnetic / electric shielding as described in [1]. Per plasma discharge (~10s) around ~8GB of video data is recorded. There are also fast frame cameras for dust tracking [2] and similar things. I assume the same techniques are applied at Wendelstein.

[0] https://www-ncbi-nlm-nih-gov.eaccess.ub.tum.de/pubmed/266281... [1] https://www.youtube.com/watch?v=QCK51vqWunU [2] http://iopscience.iop.org/article/10.1088/1741-4326/aa4e56

Wow...that's pretty cool!

I operate a fast camera at W7-X. It's a normal high-speed camera that records light in the humanly visible range, and the plasma emits in that range and reflects off the walls. IR cameras are most often used to gauge the temperatures of vessel wall components.

Our camera looks through a pinhole in the vessel wall, but it sits a few meters away from the machine and gets that view through a bundle of optical fibers. There wouldn't be enough space to place the camera right at the pinhole because of the magnets and their cooling systems, and the magnetic fields would be pretty high. The camera needs to be shielded from the fields for its electronics to work properly, and the shielding box perturbs the magnets' field, so moving the camera far away is a good idea. We don't worry about neutrons, because W7-X plasmas are fueled with stable helium and hydrogen (no deuterium or tritium so far, mainly due to onerous nuclear regulations in Germany), and these fuels don't produce many neutrons at all.

> Our camera looks through a pinhole in the vessel wall, but it sits a few meters away from the machine and gets that view through a bundle of optical fibers.

Wait, so does this mean that you have a camera obsucra with an array of optical fibres at its back, and then you have an ordinary CCD camera imaging the other end of the fibre array?!?!

I guess the word pinhole is misleading here, I think it's a few cm in diameter. Behind it there's an array of lenses that projects the view onto a fiber bundle, then a lens at the other end of the bundle projects that view out. That light goes through a beam splitter which shares it between our camera and another one in the shielding box.

Most imaging in fusion is done like this because of space constraints, magnetic fields, and neutron fluxes.

That's just awesome, thanks for the explanation!

As a side note, you gotta love HN... where you ask a question about some obscure thing and often get an answer straight from the source.

There is a video available here as well:

https://www.youtube.com/watch?v=Gtf-1JibORg

Those videos show surprising 'hotspot' lines - I would expect the plasma to be far more evenly distributed.

Because really hot plasma doesn't emit in visible range, it's all x-ray, so what you are seeing is it's (relatively) cool edge. It might have colder regions, but that doesn't necessarily mean that the main volume is unstable.

I can imagine that this is just the neutron flux picture. The neutron streams should pass the walls pretty well (unfortunately), so registering it should not be hard.

I am obviously very uninformed about this, but in the picture I'm referring to [0] you can see the walls in pretty good detail, so I assumed it was an optical picture.

No idea if a neutron flux picture would provide these details or not.

[0] https://www.ipp.mpg.de/4550362/original-1543230147.jpg?t=eyJ...

I doubt that. High energy neutrons are difficult to diffract (so no lenses), reflect more like light on fog than light on tin foil (so no mirrors) and pass through material too easily for a pinhole lens to be plausible.

I think you have to go to quite low energies (per neutron, not flux) for neutron optics to be a thing. Neutron cameras use collimated neutron sources to get around this, but that option isn’t available here: https://en.m.wikipedia.org/wiki/Neutron_imaging#Neutron_came...

You don't need to diffract the neutron flux. Instead, you can wrap (low-res) neutron-sensitive material around the stellarator tube. IDK now well would conventional photodiode or CCD work under high magnetic field, though.

That would just record the total neutron flux. It won't form an image unless you have a lens, aperture, or something similar.

It's almost as if you posit that the flux through every piece of surface is exactly the same. I suspect it's not.

No, total flux at a given location, not total flux in the whole device. The point is that it doesn't identify which direction the neutrons are coming from. That's why lens or apertures are needed.

Reply on news.ycombinator.com