# Why not just collide particles for fusion?

1. ### chhitiz

221
why do scientist insist on confinement as the way of producing power, not something like colliding accelerated particles together?
and is polywell really the best possible method to produce fuion power as the new trend suggests?

2. ### RocketSci5KN

162
Because it's been tried and simply doesn't work well enough to achieve net energy gain. Too few particles will fuse and the energy given to all the others is lost. You can easily get fusions and lots of neutrons, but you cannot get net gain.

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4. ### chhitiz

221
ok i get it. but i have some questions about the polywell.

http://en.wikipedia.org/wiki/Polywell

how does the same magnetic field that confines electrons be used to concentrate positive ions, they most probably have the same path towards the center. so, shouldn't the field be repelling the positive ions?
also, stopping ions completely by deceleration, and then reaccelarating again, wouldn't this have high braking radiation?

5. ### mheslep

3,318
It has been shown that electrons so confined set up a virtual cathode in the middle of the device. Thus positive ions constantly accelerate toward the cathode in the middle - from all directions.

No, as i recall the theory is that the higher mass species (ions) don't radiate significant power due to acceleration. However, the electrons do radiate if they get hot (many KeV), and it has been shown that that case they will radiate away the equivalent of a significant fraction of the fusion power output.* In an accelerator device like the Polywell, as opposed to a thermal device like NIF's laser implosion or a Tokamak, escaping radiation is a problem. One of the debates about the Polywell involves the temperature the electrons will reach, with advocates taking the position that electrons can stay relatively cold as you might imagine.

*This is another reason high atomic number fusion materials present more problems (Li, Boron,...) for practical reactor: for every ion there are even more electrons (in a neutral plasma) to radiate away even more energy.

6. ### chhitiz

221
i understand that. but my question is- the magnetic field won't be exerting any fprce on the electrons, unless they move. so i guess they need to be introduced at some velocity such that magnetic force on them makes them end up in the center. now positive ions when moving towards this center, can't all be parallel to the magnetic field(which is pretty complex and varying in itself), so wouldn't they experience a force opposing them?
can't we use the escaping radiation for power? isn't that how tokamaks work?

7. ### RocketSci5KN

162
With the energy from the radiating ions and electrons (photons), the most from this you can recover is about 40% from a standard steam cycle. It's a losing game, energy wise.

8. ### mheslep

3,318

Here's the paper on the limitations with acceleration based fusion schemes.
Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium

A hint of the content, from the author's acknowledgements:
Yet each man kills the thing he loves,
By each let this be heard,
Some do it with a bitter look,
The coward does it with a kiss,
The brave man with a sword!

- Oscar Wilde​

Well the problem lies in using the energy put into the system to effectively create fusion in the plasma, and not to introduce energy solely for the purpose of throwing it against a wall as heat, most of which must then be lost.

9. ### chhitiz

221
you know, unless you are yoda you could directly answer my question.

10. ### mheslep

3,318
You've been down there, Neo. You already know that road (simple answers without understanding). You know exactly where it ends. And I know that's not where you want to be.

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So, the ion's have a V|| and a V_|_, the ion's would experience a force in a direction orthogonal to both the magnetic field B and V_|_.

What do you mean by "a force opposing them"?

12. ### chhitiz

221
i mean that since negatively charged ions are going to the center under the particular direction of motion , positive ones should pretty much go in the opposite way

25
I think the polywell operates on a principle somewhat similar to a Penning trap. My understanding is that the field coils generate a cusp and have a positive charge, an electron cloud is contained within the cusp. This configuration has a negative charged center and positively charged field field coils and forms a potential well. The plasma is injected and ions oscillate in the well. I'm interested to see how well the theory behind the design works in practice.

14. ### mheslep

3,318
The electrons are held by containment inside a magnetic field produced by current coils, which have no electrostatic charge themselves. The motivation for the Polywell idea is that theoretically it should be much easier to contain electrons via magnetic field than relatively massive ions as is done with Tokamaks, since the spiral radius of motion is so much smaller for a given field strength and particle charge.
$$$R = \frac{{mv}}{{\left| q \right|B}}$$$

25
Perhaps the 2nd para. of the WP article is in error. I'm no expert on the Polywell myself.

### Staff: Mentor

The magnets of the Polywell confine Electrons in the center of the device. This containment is far from perfect however. Many electrons will escape at the cusps of the device. IE in the center of the magnets and where the magnetic fields of each magnet meet. However, the grid itself (The shell that contains the magnets) is charged to a high positive charge that attracts any Electrons that escape. Depending on the design, a large amount of these electrons can be attracted back inside so that the losses of the electrons are very minimal.

Now, this mass of electrons inside the device forms a virtual cathode. This cathode attracts positive ions which are the fuel to the center of the device where they constantly move back and forth through the center until they collide and hopefully fuse. The fused material is captured and converted into electricity.

Note that there are many things to overcome here, no matter how optomistic the researchers are that are building it. We'll just have to wait and see if it works.

17. ### ZapperZ

30,523
Staff Emeritus