# Fusion, bigger output than input?

John Mcrain
Fusion has bigger output then input, because produce more energy than to initiate reaction.
This is allways case, for example 100kg of wood burning produce much more energy then initiate fire with one match fire.

Can someone explain this statement?

Mentor
Of course, any source of energy must produce more than it consumes, otherwise it is not a source. Fusion has the potential to produce huge amounts of energy for little input, hence its appeal. But current technology is not there yet.

Lord Jestocost
John Mcrain
Of course, any source of energy must produce more than it consumes, otherwise it is not a source. Fusion has the potential to produce huge amounts of energy for little input, hence its appeal. But current technology is not there yet.
So why they emphasize "bigger output than input"?

Mentor
So why they emphasize "bigger output than input"?
Because this is the next milestone in fusion.

Lord Jestocost and vanhees71
Mentor
So why they emphasize "bigger output than input"?

Mentor
Fusing two iron nuclei needs more energy than you get out of it.

If the goal is a power plant then people choose reactions that release energy, of course. In that case the total final energy (not including the rest energy of particles) is always larger than the initial energy (same accounting). That's not enough for a power plant, however. With most reactions you only get heat as output. Converting heat back to electricity has an efficiency of around 1/3. There are other losses in the system, too, and you often need power for cooling, heating and other things. A power plant needs to induce fusion very efficiently to counter all these losses.

artis
Fusion has bigger output then input, because produce more energy than to initiate reaction.
This is allways case, for example 100kg of wood burning produce much more energy then initiate fire with one match fire.

Can someone explain this statement?
It's an easy question but it doesn't have an easy answer.

A short answer would read something like this. Not all nuclear reactions are exothermic (releasing energy) there are also endothermic (taking up more energy to start than they release in the process) so exothermic reactions are "rolling downhill" while endothermic are "pushing uphill".

Nuclear fusion of common isotopes of hydrogen like deuterium and tritium are giving off energy much like Uranium fission does, the energy given away is mostly the kinetic energy of the particles like neutrons, and alpha particles. You can turn this energy into heat. In fact it turns into heat whether we want it or not, it's just a consequence of fast moving particles moving through surrounding matter.

"Fusion has bigger output then input, because produce more energy than to initiate reaction."

This is true only if you phrase the statement correctly.
A fusion reaction where two, say for example, deuterium nuclei fuse releases more energy than the kinetic energy needed to "smash" (overcome the coulumb electric field repelling the nuclei) those two nuclei together, that would be true.
But in order to get those nuclei to such fast speeds we need to add energy to them, we do this by heating the substance they are in (plasma) this heating is not 100% efficient nor are the fusion reactions 100% efficient (nuclei don't always fuse, they can scatter or lose energy by escaping the plasma etc )
So in order to get more final electrical energy out than we put in to heat the plasma we need the plasma to be dense enough, hot enough for long enough, otherwise what we get out is less than we put in because the lower output combined with the energy conversion losses gives a diminished return.

So just because a reaction gives away extra energy doesn't always mean that it will start or continue without the right circumstances.

You can see this by pouring too much diesel on a cold small flame, instead of burning it will extinguish the flame as it it was water. On the other hand put the right amount of diesel fuel within a closed space , apply certain amount of energy to it - by compression which causes rapid heating of the fuel and it ignites giving more energy than was used to compress it.

ohwilleke