Fusion reactors: why is bigger better?

[rabbit44]

I can't actually find an explanation of this? I think that bigger plasma volumes suffer less energy/particle loss (and therefore have a greater confinement time), but what is the physical reason for this?

Thanks

 

[theCandyman]

It probably has to do with the surface area to volume ratio. Plasma on the outside can lose its energy to the confinement walls.

 

[mheslep]

It probably has to do with the surface area to volume ratio. Plasma on the outside can lose its energy to the confinement walls.

Yep. Losses tend to scale with the area, power tends to scale with the volume.

 

[Hologram0110]

The energy in a fusion reaction is split between the charged particles and neutrons which result from the reaction. Most of the energy is carried away by neutrons since they have no charge they are not confined. The energy of the charged particles goes to heating (or keeping) the plasma hot. Having more a better volume(heating)/surface(cooling) ratio helps keep the plasma hot without needing extra energy from outside.

Similar results can be achieved with improved confinement so that you can get a higher particle density in the same volume. IE more particles in same volume= more reaction = more heat produced for roughly the same rate of cooling.

 

[mheslep]

The energy in a fusion reaction is split between the charged particles and neutrons which result from the reaction. Most of the energy is carried away by neutrons since they have no charge they are not confined. ...

Yes, for many reactions like D+D and D+T. For others like p + 11B, the opposite is true in that almost no neutrons are released and almost all energy is release in the form of the charged alphas.

 

[Astronuc]

The energy in a fusion reaction is split between the charged particles and neutrons which result from the reaction. Most of the energy is carried away by neutrons since they have no charge they are not confined. The energy of the charged particles goes to heating (or keeping) the plasma hot. Having more a better volume(heating)/surface(cooling) ratio helps keep the plasma hot without needing extra energy from outside.

Similar results can be achieved with improved confinement so that you can get a higher particle density in the same volume. IE more particles in same volume= more reaction = more heat produced for roughly the same rate of cooling.

The limit is the pressure obtained by the magnetic field and the temperature of the plasma.

One can use aneutronic reactions like He³+D and p+B¹¹, but the optimal temperature increases with Z of the reacting nuclei. In addition, He³ is rather rare and expensive.

 

[Hologram0110]

You're right. I was talking specifically about a D + T reaction. This reaction is considered the most accessible for test or even commercial implementation because it has the lowest ignition temperature.

 

[mheslep]

You're right. I was talking specifically about a D + T reaction. This reaction is considered the most accessible for test or even commercial implementation because it has the lowest ignition temperature.

Yes, and the neutron flux from D+T is considered the biggest impediment to creating an economically successful fusion reactor after the net energy R&D problem is solved. The n flux drives minimum size. A [STRIKE]17MeV[/STRIKE] 14.1 MeV neutron requires a given wall thickness to stop, regardless of power design point.