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Comment by lwigo | Hacker News Reader

Alright, I'll be that guy. What materials do we have that can contain something that hot?

The main reason the plasma needs to be kept away from the walls is that they will conduct the heat away from the plasma and the hot gas will cease being a plasma.

It has analogies with static electricity. Static electricity might have a voltage of tens of thousands of volts, but it does little damage when you touch someone as the number of electrons (current) is so small that not much energy is involved.

Similarly with a plasma. The temperature (energy per particle) is very high but the number of particles is small, so the total energy is low. Touching something will cause the energy to leak away (cf. static electricity discharge) converting the hot plasma back to cool gas.

None. dboreham has it right -- all the confinement is done magnetically or photon pressure. Stars do it through gravitation.

So what happens if it touches something? Meaning, somehow, the magnetic field fails due to equipment. Would it be like highly corrosive acid going through material or a much more violent reaction?

The heat capacity of the plasma is actually fairly low. So while it might take a but of wall material, it would cool very quickly, electrons would get captures, and you are left with a bit of gas at rather low pressure.

Non-rhetorical: So what's the point of this experiment? Seems like harnessing its energy will not be very effective based on how quickly it dissipates

Think of it just like fire. Fire will also give you almost no energy if you physically scoop it up. Instead you leave the fire where it is, continually generating heat, and you capture what pours off. Or you pump in fuel, ignite it, and exhaust it over and over at a high speed, like in an engine.

There is little energy stored in the gas, but it produces lots of energy. I.e., every second the gas might produce a megawatt second of energy, but only have the heat capacity to store 1 kilowatt second. Numbers may not be to scale but you get the idea. I'm also not sure how this stays stable. I guess that if the gas gets hotter the process breaks down? Mayb the volume expands within the confinement and the reaction rate goes down as a result?

But surely the whole point of this is to eventually create a lot of steam, right? So hopefully it will have enough energy to do that.

What is actually the plan to harvest the energy (how to get the heat to a steam engine or whatever and generate electricity?)

Deuterium/Tritium fusion produces neutrons that will escape the magnetic fields and hit the surrounding machine producing heat but also damage. This shield around the machine absorbing the neutrons will have a circulating fluid that will remove the heat to run a steam or other type of generator.

We don't really know how to build this part of the reactor right now, but some metals have the ability to self heal and people think this shield should be possible to build that has a decent lifespan. This is a part of fusion reactors that could be researched right now in parallel with other problems, but there has not yet been funding to build a high level neutron source facility to do the research. This is one fact of many that shows, I think, that national governments are not really interested in fusion energy succeeding.

Are the SNS or research reactors not able to simulate the necessary neutron flux?

My father was a plasma physicist that researched controlled fusion and he told me that current neutron sources were not useful for this. I think was a combination of the energy spectrum of the neutrons, the flux power, and the physical size/shape of the target area.

It'll probably be minor to medium damage to the containment vessel, but the amount of gas actually at that temperature is very small, and will cool down very quickly once it expands and dissipates.

Not sure about KSTAR specifically, but a lot of these devices use the field both for containment of the plasma and for starting/maintaining the fusion reaction in the first place. If the field fails, the reaction stops.

If it didn't (hypothetically), I imagine it would do some pretty severe damage to the reactor parts specifically. At some point, the vacuum would be broken, air would rush in, and the reaction would _definitely_ stop.

Most of the larger fusors (as far as I'm aware) seem to be refinements of the basic Farnsworth-Hirsch fusor design (https://en.wikipedia.org/wiki/Fusor), which is a relatively simple device. If you have the motivation, you could build one at home with common-ish tools.

Details for some of the power supplies used for the superconducting magnets in the first plasma experiment: https://ieeexplore.ieee.org/document/5226386

And details about the magnets: https://www.osti.gov/etdeweb/servlets/purl/20261539

> The vertical load on coil system is estimated to be 330 tons for the coil weight and 320 tons for vertical disruption load.

With some incredible forces that can be generated by the plasma.

> The lateral loads in tokamak could be generated by plasma disruptions and localized halo current flows through the plasma facing components (PFC) or vacuum vessel. The estimated peak lateral load in KSTAR is about 1.3 MN

Some designs for fusion walls consider molten sodium walls. Liquid walls have the advantage that they can’t crack, and you can use the absorbed energy to heat water and run an engine.

Huge electromagnets. Perfectly tuned. With the power budget of a small town.

KSTAR uses superconducting confinement coils. It takes virtually no power for the confining field.

I work on a research machine that uses copper coils. It uses 16x 1000 HP train traction motors with 2000 lb flywheels for energy storage to power the confinement field for one second. :)

16x 1000HP is 12MW, so yes a small town of ~10,000 people. For a few seconds at least :-)

My point is that superconducting confinement coils don't take any power to maintain their field. You just spin them up and leave them there. The power to keep cryogenics running is a hundred-fold less than the electrical power needed for confinement coils.

JET and DIII-D are the largest magnetic confinement fusion research devices made so far and they both use copper coils. They have confining fields of 3.45 T and 2.2 T, respectively. My machine operates at a modest 1 T (for now) and is much smaller. I'm sure the electricity bill that the confining coils that those large machines racked up is substantial.

Forgive my ignorance, but aren't cryogenics and plasma temps at complete odds to function in near proximity?

You're not supposed to let them touch. You won't go to space^W^W^W have fusion today if the magnetically confined plasma somehow escapes its confinement and touches the walls of the plasma chamber, let alone the cryogenics of the magnets.

Besides creating the pressure, that purpose of the magnets is to keep the _few grams_ of plasma away from the walls of the otherwise vacuum chamber.

Look up magnetic levitation, where you keep an object up using just a magnet. Now imagine huge magnets that push against an object from all sides, creating a ton of pressure. (Don't forget to include a whole bunch of fancy math to prevent the object from slipping away like a wet bar of soap you squeeze with your hands.) At no point should the object touch any of the magnets.

Also keep in mind that when you (don't) want to heat something else up it's not the temperature that matters, but the total energy of the object. And the total energy is basically mass x temperature difference. So 1 gram at a million degrees is equivalent to 1 Kg at a thousand degrees, which is a lot less scary.

Thanks for the correction! I did wonder whether it is wise to comment when I don't know anything about that model.

Crazy strong EM fields to make sure that the plasma doesn't get anywhere near anything!

A magnetic field?

Just buy a bunch of totinos pizza rolls.

In the original spider-man 2, this is what Doctor Octopus is trying to achieve with his metal arms before he goes insane (because... the arms were evil or something?). Perhaps Deep Mind could try outfitting self aware human exo-skeletons to control the magnetic containment of fusion reactions now that they're done with protein folding.

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