Gave The Astra Planeta Overview Article A HUGE Rework. Like... 2000 Words' Worth Of Rework. Have A Look!

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Astra Planeta

An overview of Astra Planeta from a metacanonical perspective.

Gave the Astra Planeta overview article a HUGE rework. Like... 2000 words' worth of rework. Have a look!

Time for another science post! As usual, you can read more about fusion reactors under my #fusion tag! Let’s talk about…

Tokamaks vs Stellarators!

These devices work on the same general principle: take a very hot plasma, and confine it in a loop using powerful magnets. Heat up the plasma enough and it starts to fuse, then harvest the resulting neutrons to generate electricity. But there’s a catch!

You can’t just have the plasma move around in a circle. If you did that, negative and positive particles would drift in opposite directions, collide with the walls of the chamber, and fizzle out. Instead, you need to get everything moving in a helix.

The difference between a stellarator and a tokamak lies in how they generate that helix.

Tokamaks

This is a tokamak! The vast majority of fusion reactors out there use this design. They work by having a torus (donut-shaped) plasma, with spiraling magnetic fields traveling through it. Within the toroidal plasma, particles follow the helical field lines.

The way you generate this helical field is pretty dang clever. What you do is take two perpendicular magnetic fields — one poloidal (looping in and out of the donut hole), and one toroidal (around the donut in a circle) — and add them together. The resulting combined field is a helix!

The toroidal field is “easiest” to generate. You have a series of powerful magnets that form a ring around the torus. They create a powerful magnetic field in the toroidal direction.

The poloidal field is trickier. For that one, you use a big transformer coil that sits in the middle of the “donut hole.” By ramping up the current in the transformer, you induce a corresponding current within the plasma itself. That plasma current in turn produces its own magnetic field, in the poloidal direction. Toroidal + poloidal = helical.

Stellarators

This is a stellarator! Or rather, one kind of stellarator. Rather than having a torus-shaped plasma with a helical magnetic field running through it, you just make the plasma itself into a helix! You use toroidal magnets (weird blue shapes above) to contain it and drive it along, without the need to generate any plasma current at all. This results in some incredibly wacky geometries!

So which is better?

That’s not an easy question to answer. Let’s compare pros and cons.

Tokamak pros:

Simple geometry, symmetric all around.

Design is forgiving to small errors.

We’ve been building them for 50 years and have gotten pretty good at it.

Every toroidal magnet is identical. Manufacturing the machine is relatively cheap.

Extremely dense plasma that produces a ton of energy.

Tokamak cons:

Plasma current induced by a transformer is inherently transient. If you want to run “steady state” (I.e. for more than 30 seconds), you need to have other ways of keeping that current going. This is doable with various techniques called “current drive,” but it’s tricky. KSTAR and EAST are two tokamaks that are very good at this.

If the plasma collides with the walls of the machine, the entire plasma current gets dumped into one spot. You get what’s called a “halo current,” where several mega amps of electricity blast through an area maybe 10 centimeters wide in a few milliseconds. This is called a “disruption event.” They happen a lot, and they give the machine a hell of a beating.

Stellarator pros:

No need for plasma current! This means you can just run steady state pretty much indefinitely.

No plasma current means that disruptions are relatively gentle, when they happen at all.

Stellarator cons:

As opposed to a tokamak, there are a lot of potential stellarator configurations. Before modern computers, trying to calculate a good stellarator magnet geometry was all but impossible.

They are extremely difficult and expensive to build. Every magnet is unique, and the Seussian vacuum chamber itself is a nightmare to engineer.

Stellarator plasmas are generally colder, and not as dense.

Given how difficult they’ve been to design, we just haven’t built many stellarators. Stellarator research is 30 years behind tokamak research.

So which is best? Nobody really knows! Judging by the trends in research, the first fusion power plant will almost certainly be a tokamak. But hey, it’s possible that eventually, stellarators will become the industry standard.

And now to finish it off, here are some pictures inside the two biggest stellarators in the world: LHD (the Large Helical Device) in Japan, and Wendelstein 7-X in Germany! They are confusing and hard to look at, and have very different geometries.

Here’s W7-X:

And here’s LHD:

soundscape of young green martian playing with pvc pipes

if we've talked regularly for more than like six months? I love you now. not sorry, that's just how it is. philia. I cannot help but be filled with love and compassion and affection and in a world that seems to demonize those things I hold them tighter as an act of rebellion. we are human, we are made to care for each other, we have been for two million fucking years and we will until the end of time!!! for such small creatures as we, the vastness is bearable only through love!!!

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opinion on crispr/cas9 + favorite application of it?

Oughhhh it has literally been so long since I read up on CRISPR. I used to have a much more detailed understanding of the technique, but I don't really deal with genetics on much more than surface level these days, so I'm very behind on the state of that field.

Generally speaking, I think it's a great tool... for now. So far I haven't heard of any major issues with it; scientists can add or remove any section of a cell's genome with little to no additional complications observed. It's admittedly pretty ingenious and has catapulted the entire field of genetics forward by decades (if not centuries). But let's not forget that CRISPR is really one of our earliest direct genetic engineering tools! As time progresses we're bound to discover and/or invent far more sophisticated and even safer methods of genetic tinkering.