Quote:
Originally Posted by HBrutusH
Resource link for the Polywell?
How is the other design flawed? From what I've read it just puts immnense amounts of magnetic pressure on hydrogen (?) in an atmosphere of boron(?) and smushes them together.
Explain please 
I really like the idea of fusion, the power that could be made is immense, with no radiation (besides heat+ light?) or harmful waste. |
Well, there are two parts to the tokamak problem.
The first is simple engineering. A power generating tokamak is basically a big torus (donut) with three layers. The inside is the core, which is emptied to a vacuum and contains the plasma that should (in theory) be reacting. The core then sits immersed in molten lithium deuteride (which is kept at around 1000K). Then outside that are your superconducting magnets, which need to be kept at around 5K. The first problem is your have your supercooled magnets at 5K sitting next to molten lithium deuteride at 1000K, so it's a challenge to keep everything at the right temperature, and a lot of energy. The second problem is that the lithium deuteride and its containment can't interact with the magnetic field. The third is that molten lithium deiterode is quite reactive and likes to explode when exposed to air, and it produces highly toxic fumes when it burns.
Now you need LiD for two reasons. The first is that tokamaks use what's called a Deuterium-Tritium reaction; tokamaks react deuterium and tritium (two isotopes of hydrogen), and when they fuse they make Helium-4 and a fast neutron. Deuterium is common enough (it can be extracted from sea water) but tritum is unstable (radioactive) so it has to be made... this is what the molten lithium deuteride is for- the neutrons generated react with the lithium deuteride to produce lithium. They also HEAT the lithium, a lot. So the lithium needs to be cycled, have the tritium extracted (which is then pumped into the reactor core) and run through a heat exchanger. The heat exchanger then drives a normal thermal power plant, which will convert the heat into electricity with about 50% efficiency. And a lot of that power has to be cycled back to the electromagnets and the cooling systems. Plus, not all the neutrons are captured by the lithium deuteride, and they will make the reactor vessel radioactive.
So that's a tremendous engineering challenge, which is largely why ITER is supposed to cost about $15 billion dollars to build.
Now for the HARDER problem, the physics of the tokamak.
You can't really squeeze a plasma with a magnetic field. Magnetism is a right-hand force, which means that it acts on particles at right-angles. An ion or an electron whizzing through a magnetic field gets spun around in a circle, it doesn't get squeezed or anything. Tokamaks use two magnetic fields to contain their plasma... the first makes the particles travel in tight spirals. The second loops the spiral around into a circle.
Now there are two main forces at work within a fusion reactor- the first is electromagnetic, which is the dominant force. The magnets use the electromagnetic force to guide the particles around the inner chamber, and the atomic nuclei in turn electrically repell each other (since they're all positively charged). The other force is the nuclear force, which is extremely strong over very short distances, but loses strength very quickly as the distance grows. Normally nuclei in the reactor don't interact at all... the magnetic containment fields dominate all other forces, but occasionally two nuclei get close enough that they interact with each other. Now as hot as the plasma is, the plasma's energy has a maxwell-boltzmann distribution, so only a tiny fraction of all the particles in the plasma have enough kinetic energy to fuse. Even when the particles interacting have enough energy, there's still only a 0.1% chance that they'll get close enough to fuse. The vast, vast, vast majority of interactions are failed interactions, and in failed interactions one of the particles tends to get kicked a little bit further out of the particle stream. It's a random process, but the tendency is for the flow of particles to slowly drift away from the center of the reactor to the walls. Magnetic containment isn't true containment; there's no restoring force to push the particles back to the center, so there are constantly particles impacting the reactor wall. This erodes the vessel wall and bleeds tremendous amounts of energy out of the reactor.
Here's the kicker... you can minimise this effect somewhat by increasing the strength of your magnetic fields, but that increases bremsstrahlung radiation. The more powerful your containment fields, the more bremsstrahlung radiation is generated by the reactor. A plasma in a tokamak will quickly bleed all its energy away through bremsstrahlung radiation.
Unlike the engineering challenges, these are basic design flaws due to the physics of the tokamak. There's a reason you don't see any donut shaped stars.
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