What is the issue in creating a self-sustaining fusion reaction?

[Galacticruler]

I would simply like to know if it is based on the size of the reaction or fuel intake?

because the sun is a highly efficient nuclear fusion reactor, so why is it so difficult to create a mini-star on Earth?

[RulerOfNothing]

The main problem is that in order to get a fusion reaction occurring, you need to have the atomic nuclei coming rather close to each other, which requires a combination of temperature and pressure. In the Sun, pressure is very high so fusing temperatures are reduced (There is also the small matter that the fusing matter in the Sun is kept in place by gravity, which is not a containment option available here). Since we cannot generate such high pressures, much higher temperatures are required (about 100 million K compared to the Sun's core temperature, which apparently is estimated at only 15 million K)

[Galacticruler]

Ah, but wouldn't forcing them together with magnets work?

are Deuterium and Tritium even magnetically influenced?

[Kryten]

They are when they're plasma, which they inevitably will be at those kind of temperatures, and that's how all fusion reactor prototypes work-trying to physically force it together with solid objects (if that makes sense...) would contaminate the plasma and rapidly make fusion impossible. Completely containing plasma with magnetic fields is a lot easier said than done, of course.

[WestAir]

Don't magnets put equal force on both polar opposites? So the force that is crushing the core together will also be the same strength of force trying to force the magnetic chamber apart? At fusion pressures, such a contraption would just explode apart, right?

[Kryten]

The pressure on the reactor doesn't tend to be in issue-it's the forces that are equal rather than the pressure, and the forces required to produce fusion pressure is relatively (very relatively) low because there's only ever a small amount of plasma being acted upon, a few grams at most.

[Mr Shifty]

For magnetically confined plasma, a puff of gas (H2 or H3 or He) is released into a donut shaped vacuum chamber called a Tokamak. The gas is heated up using a variety of methods, principally large neutral ion beams that irradiate the gas with many megaWatts of energy. At high temperatures (100 million Kelvin) the atoms in the gas are energetic enough that the electrons are stripped off the nuclei; this is what characterizes a plasma. So there's a bunch of negatively charged free electrons and positively charged free ions in the vacuum chamber. At that point you create a 3 dimensional time varying magnetic field profile using huge coils that carry megaAmps of current. The magnetic fields cause the electrons and ions to rotate around the Tokamak in opposite directions; they follow helical paths that converge as they flow around the torus to form a solid mass of plasma, with a definite edge where denisty and temperature drop off dramatically. If you can get the ions streaming around the torus fast enough, they'll start colliding with each other, causing fusion reactions. Each fusion reaction produces a heavier ion that is more energetic than the combined energy of the ions that created it, so makes the plasma hotter, causing more fusions in a chain reaction that will engulf the plasma.

That's how it's supposed to work. Roadblocks are plasma electrodynamics is still being studied; it's somewhat well understood but there are characteristics of the plasma that are still be probed and researched. For instance, it's not well understood how to best contain plasma disruptions. In high energy plasmas, simply collapsing them could cause damage to the tokamak surface. There are state transitions in the plasmas being investigated. The formation of instabilities called edge localized modes (ELMs) are still being investigated for their cause, mitigation, and utility. Materials research is ongoing. Current experiments have to be scaled up to produce any usable amount of power, and scaling up produces differences in kind, not just scale. There's a lot to do.

There are also inertially confined fusion experiments, where high powered lasers are fired into small pellets of material to try to cause fusion reactions. But I only work on the magnetic experiments, so I don't know anything about that.

[DerekL1963]

That's how it's supposed to work. Roadblocks are plasma electrodynamics is still being studied; it's somewhat well understood but there are characteristics of the plasma that are still be probed and researched. For instance, it's not well understood how to best contain plasma disruptions. In high energy plasmas, simply collapsing them could cause damage to the tokamak surface. There are state transitions in the plasmas being investigated. The formation of instabilities called edge localized modes (ELMs) are still being investigated for their cause, mitigation, and utility. Materials research is ongoing. Current experiments have to be scaled up to produce any usable amount of power, and scaling up produces differences in kind, not just scale. There's a lot to do.

A friend of mine who also studies high energy plasmas (but over on the astrophysics side of the house) describes fusion research as "the science of finding new ways for plasma to misbehave".

[drakesdoom]

As Mr shifty has said in much better words it is more a problem of keeping them running and non critical.

I have heard of a pulse fusion rocket in the works that magnetically fires bands of metal at a pellet, the combined force of the magnetic fields from the bands driving a small fusion explosion. Supposedly everything is working separately and funding for a test bed was secured.

[Scotius]

Yes, but it is not sustained fusion. It's more like detonating minuscule thermonuclear bombs inside reaction chamber - and we are quite proficient at thermonuclear explosions