[sballin]

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.

[saganus]

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.

[adrianratnapala]

> 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?!?!

[sballin]

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.