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DoubleDot7 t1_iyvfwfj wrote

Reply to comment by Englandd12 in Why not use hydrogen and deuterium in fusion reaction rather than tritium and deuterium? by Curious_user4445

>it has the highest specific reactivity at lower temperatures

Does that mean that deuterium-tritium reactions need less energy for the reaction to start? Is that because tritium is less stable than the other isotopes?

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RobusEtCeleritas t1_iyvicsj wrote

It means that the Coulomb barrier is a little bit lower. It's unrelated to the stability tritium, it's just possible to make this reaction occur at a reasonable rate at lower temperatures.

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financial2k t1_izsx87h wrote

How much lower is this temperature?

Is the main motivation of using Tritium the lower temperature or actually the breeding reaction?

How far apart is the fusion temperature from the fusion ignition temperature and how is each one defined?

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RobusEtCeleritas t1_izt7by2 wrote

>How much lower is this temperature?

Here is the reactivity as a function of temperature for a few candidate reactions.

>Is the main motivation of using Tritium the lower temperature or actually the breeding reaction?

The main motivation is the temperature. Obtaining fuel for DD is not an issue, because there's plentiful deuterium in nature (seawater, for example). It's a nice benefit, and quite important for tritium, which is not found naturally in large amounts. We have to produce tritium somehow, and having the reactor breed its own fuel is a nice way to do that.

>How far apart is the fusion temperature from the fusion ignition temperature and how is each one defined?

Not sure what you mean here.

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financial2k t1_iztmlqb wrote

Thanks.

How far apart is the fusion temperature from the fusion ignition temperature and how is each one defined?

This was answered in a comment below somewhere. perhaps even by you

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[deleted] t1_iyvj5ib wrote

[deleted]

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treefor_js t1_iyvuhla wrote

This is not correct. Temperature is a measurement of the average kinetic energy of the particles - you express plasma temperature in units of eV (energy unit). If one element is heavier then it'll have a slower average velocity. DT reactions require lower temperatures to achieve their highest cross section for fusion reactions. Meaning you need to put less energy into the system.

  • HEDP plasma physicist
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ChipotleMayoFusion t1_iyw8hte wrote

Isn't that exactly what Robus said? The DT reaction is favorable because it reaches high reactivity at lower temperatures. https://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/Fusion_rxnrate.svg/330px-Fusion_rxnrate.svg.png

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treefor_js t1_iyw9ick wrote

That was the conclusion, yes. However, the reasoning was not correct.

Edit: the other thing to note here is not just that it takes a lower temperature to reach higher reaction cross sections but the loss mechanisms that scale with temperature as well. It's a balancing act to keep the plasma warm to use the fusion products to keep burning the fuel without it cooling off rapidly. Bremsstrahlung radiation - x-rays generated by accelerated charged particles, is the main culprit here.

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ChipotleMayoFusion t1_iywmfex wrote

Ok, thanks for the clarification. Maybe I misunderstood what his post was getting at. I have heard that proton-Boron is basically impossible because the brems losses at the temperature where reactivity is sufficient will always be higher, or almost always higher. I think this is what you are saying, you can't just focus on the temperature. Sam Wurzel had a great talk on this at APS 2021, clarifying Qeng vs Qsci and how that changes depending on your recirculating power fraction and other factors.

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treefor_js t1_iywn305 wrote

Oh nice. I didn't get a chance to go to that one. Came down with a stomach bug for a day or two in Pittsburgh. Also wish I had time to go to the commercial fusion breakout this year, but alas. There's always next year.

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ChipotleMayoFusion t1_iywnxfg wrote

I'm glad you were able to attend at all, a lot of the US national labs people were not there due to COVID travel rules.

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treefor_js t1_iywoopg wrote

I sat in on one of the MagLIF sessions and I think there was one live talk with like 10 recorded ones. It was a weird conference. Basically just networked with university folks. So much better turn out this year with national lab folks returning.

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[deleted] t1_iyvl308 wrote

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TinnyOctopus t1_iyvpco3 wrote

As you suppose, it's the sourcing of the tritium that's the problem, but I think you're underselling the difficulty. For the DT fusion, the plasma composition can be mostly D, which is difficult but not impossible to purify out of naturally occurring water (prevalence is generally about 1 in 10,000 to 100,000 hydrogen atoms). Tritium is about 1 to 10^18th hydrogen atoms, which is a million million times less common than even the uncommon deuterium. Which means tritium needs to be manufactured, and at a certain point, the amount of energy being put in to make the tritium fuel will become prohibitive, making the economics of a T-T rector nonviable.

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UWwolfman t1_iz0py64 wrote

>Does that mean that deuterium-tritium reactions need less energy for the reaction to start?

Yes and no.

Focusing on energy "needed to start" misses the key physics and related engineering issue.

When two fusion reactants (such as deuterium and tritium) collide, that vast majority of the time they do not fuse. Instead they scatter off of each other. This is true even if they have enough energy to fuse. The reactants only occasionally fuse when they collide. The specific reactivity is a measure of how often they fuse.

While the scattering collisions conserve energy, they lead to thermal conduction. Without some sort of thermal insulation, a fusion-ing plasma will cool off quickly and fizzle due to this collision induced thermal conduction. In fusion we call this insulation (energy) confinement.

So the key issue for engineering an economical fusion power plant is not providing enough heat to the fuel we can do this, but instead it's about building a good enough insulator to keep the plasma warm.

The more reactive the plasma, the more leaky the our insulator can be. It turns out the deuterium-tritium has the highest reactivity of all fusion fuels.

>Is that because tritium is less stable than the other isotopes?

It's complicated, but I'd argue is has more to do with He-4 being a very stable isotope. There are other factors, such a a relatively low coulomb barrier compared to high-Z reactants.

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