# Antimatter through fusion

1. Oct 14, 2012

### Crazymechanic

As much as i have read and understood antimatter can be created artificially when colliding elementary particles at high energy , like two protons in a particle accelerator.
Could antimatter be created in a plasma in a nuclear fusion reactor if the plasma temperature /pressure would be high enough for such an event, and would that antimatter created annihilate with matter in that plasma giving off all the energy to the plasma and heating it up even more +more radiation and so on?

So basically what i'm asking is that if you reach kinetic energy levels in a plasma high enough could that make the plasma a on spot antimater creator and also absorber with all the consequences of that like more heat more radiation and so on?

2. Oct 16, 2012

### Crazymechanic

A little that i wanted to add , as i read that the pp cycle at one point creates a positron that annihilates creating gamma radiation and so on , so would it be fair to say that if we could reach fusion at temperatures/pressures like in the sun then the fusion reaction would be more self sustaining than with the temps and pressure we can achieve now like those planned at ITER facility?

3. Oct 16, 2012

### ofirg

If I understand correctly, you are asking two questions.

1) Will antimatter be created in a plasma with high enough temperatures?

Yes, However in order to produce positrons due to the temperature alone (using the kinetic energy of particles in the plasma) you will need a temperature of the order of 1 MeV (positron mass is 0.5MeV) which is about $10^{10}$ kelvin. Even the center of the sun doesn't have such high temperatures.
The positrons in nuclear reactions ( including the sun) are produced with the help of nuclear binding energy which is of order of MeV.
In the specific process which is thought to be useful for fusion in reactors here on earth,
deuterium+tritium->helium +neutron a positron isn't produced and will not be produced due to the temperature alone.

2) How is the process which occurs in the sun related to the attempts on earth?

In the Sun, Where the pp chain starts with 4 protons you have to produce positrons in order to get the neutrons which are present in helium. This is possible due to the nuclear binding energy and not the kinetic energy of the particles / temperature of the plasma.

However, This process has no relevance for facilities on earth, since it is too slow.
This it why the sun lives so long. It takes a given proton on the order of billions of years to find itslef in a fused helium nuclei. The reason the sun emits so much energy is the it has so much available mass for this process.

For earth a different process is need, which is much faster.

4. Oct 16, 2012

### Staff: Mentor

Fusion reactors on earth have higher temperatures than the center of the sun - but still far away from 1010K.

@Crazymechanic: Antimatter does not violate energy conservation - if antimatter is produced, the production will require as much energy as the annihilation releases. You cannot gain anything.

5. Oct 16, 2012

### Crazymechanic

@mfb , so basically your saying that for example the massive stars in universe output all sorts of radiation as energy and that energy is pretty much equal to the input energy that would be like all the mass via gravity and nuclear binding energy from fusion + matter antimatter annihilation is that right are those the 3 basic steps from which a star get's it's huge amount of energy?

But why I was asking about antimatter creation and collisions is because fusion only releases like some I don' t remember correctly but like 3/4 % of all the mass involved as usable energy for us in the reaction, and antimatter -matter releases a lot more than what typical fusion can yield so wouldn't it be more promising in fusion to go for the fusion to the point where it creates sufficient quantities of antimatter that would annihilate with mater as to increase the energy output and efficiency of a particular reactor/device? Put in simple words you can use gasoline and diesel but for the total usable energy output and efficiency diesel wins.ofcourse there are considerations like rpm and so on but in overall.

Is this close to reality or am I just thinking BS in my mind?

6. Oct 16, 2012

### Staff: Mentor

A star gets energy from gravitational collapse (negligible in the long run) and the release of nuclear binding energy via fusion.
While there are reactions which produce positrons, this production needs energy itself, so "antimatter annihilation" is not an energy source of stars.

And there is no way to change that, unless you find some way to violate baryon number conservation to convert matter to antimatter.

In theory, antimatter can be used as energy storage, but not as a source. Neglecting all engineering issues and inefficiencies:
Use 1.8*10^17 J to produce 1kg of antimatter and 1kg of matter (you have to produce them together). Store the antimatter. You can let it annihilate later with 1kg of matter to release those 1.8*10^17 J.

7. Oct 16, 2012

### Crazymechanic

Thanks for sorting that out mfb.

8. Oct 17, 2012

### hch71

Actually, there is a proposition that in the center of supermassive stars the temperatures can go high enough that 1MeV photons are present and will pair-produce electrons and positrons. This would result in a sharp stellar pressure drop leading to a very bright type of core-collapse supernova.

9. Oct 17, 2012

### Crazymechanic

That sounds interesting, can anyone more competent than me elaborate on this?

10. Oct 17, 2012