Exploring Quantum Tunneling for Controlled Fusion

In summary: I don't know, some way.In summary, the sun relies on quantum tunneling of protons through their natural repulsive barrier in order to produce the amount of heat we observe. This might be useful in obtaining controlled thermonuclear fusion. However, without quantum tunneling, the sun's core is not hot enough for this to happen.
  • #1
Pooua
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I've heard that Sun is not hot enough to sustain thermonuclear fusion at the level we observe. Instead, the solar output relies on quantum tunneling of protons through their natural repulsive barrier. Might this same effect be useful in obtaining controlled thermonuclear fusion? It seems to me that this would require a large quantity of atomic interactions in a confined space. Might an atom laser be useful in producing a finely tuned beam of atoms that could direct large numbers of atoms into a small enough space for them to fuse?
 
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  • #3
phyzguy said:
Nonsense! models of the sun using fusion cross sections measured here on the Earth are in excellent agreement with the measured solar parameters. See, for example:

http://en.wikipedia.org/wiki/Standard_Solar_Model
http://www.ap.stmarys.ca/~guenther/evolution/ssm1998.html

Oh, I did not know that quantum tunneling is not commonly accepted as the reason solar fusion works as well as it does. I got my information from this video:



In the comments, the author says:

"People have been asking for the math. So here it is. The Sun's core temp is ~13.6 MK. For hydrogen nuclei the Coulomb barrier is roughly 0.1 MeV. This corresponds to a temperature in excess of 1 GK! Luckily, tunneling and the distribution of speeds among nuclei lower the actual temperature required. So without tunneling even the Sun's core isn't hot enough for fusion. To see most of this worked through, check out this link:

http://burro.cwru.edu/Academics/Astr221/StarPhys/coulomb.html" [Broken]

for a less mathematical explanation, try:

http://en.wikipedia.org/wiki/Nuclear_fusion#Requirements"

I do not see that your first link, to the Wikipedia article on stellar evolution, addresses the question of solar fusion in sufficient detail to say whether or not quantum tunneling has any affect.

Your second link, to the Standard Solar Model, says that Sun's core is about 16 million kelvin. I've heard before that terrestrial thermonuclear fusion has to reach 100 million kelvin to 150 million kelvin because our projects are so much smaller than Sun, and I was led to believe that Sun simply is an inefficient furnace. The idea that quantum tunneling is required to account for the temperature difference makes more sense to me than simply stating that Sun is an inefficient furnace.

You might also want to refer to: http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/coubar.html" [Broken]
 
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  • #4
Pooua, yes, pp fusion is dominated by tunneling. The barrier energy and temperatures are quoted correctly. At these energies, you'll only have a handful of protons at sufficient energy to react. (Odds of particle having energy E is exp(-E/kT) ~ exp(-100) ~ 10^-43, and Sol has roughly 5*10^56 protons, so we are talking about 50 trillion protons capable of fusing at anyone time.) But with tunneling, it becomes much more reasonable, since a particle with half of the barrier energy will already have good odds of fusing, and if you run the numbers, there are a LOT of these.

So it's not fair to say that fusion cannot proceed without tunneling at all, but tunneling is how it happens.

There is also a Carbon Cycle, which has lower activation energy, but it's not a dominant cycle in Sun.
 
  • #5
Maybe I misunderstood your initial point. You said that "Sun is not hot enough to sustain thermonuclear fusion at the level we observe". This is clearly false, since the sun is powered by thermonuclear fusion. If your point is that quantum mechanics is required to understand nuclear fusion cross sections, well, I doubt anyone would argue with that!
 
  • #6
But the sun is an inefficient furnace. It only burns ~10-10 of its hydrogen per year. That's far too low to be commercially viable.
 
  • #7
Thanks for the confirmation about Sun's reliance on quantum tunneling.

What I really would like to know is, how would one make a practical fusion generator? I know that the U.S., former Soviet Union and several countries have spent billions of dollars and half a century trying to answer that question. I am wondering if there might be some other way to go about it than the ways commonly attempted?

I suppose the key to exploiting quantum tunneling would be to have many reaction and a lot of material. I don't know how much would be necessary, or how difficult it would be to confine that much material long enough to make it useful.
 
  • #8
Vanadium 50 said:
But the sun is an inefficient furnace. It only burns ~10-10 of its hydrogen per year. That's far too low to be commercially viable.

A furnace that can keep going for 10,000,000,000 years without refueling seems like a pretty efficient furnace to me. What would make it more efficient would be to concentrate all of it's output on the Earth while simultaneously slowing its reaction by a corresponding amount. If this were achievable our 5,000,000,000 year time horizon could be dramatically lengthened.
 
  • #9
mrspeedybob said:
A furnace that can keep going for 10,000,000,000 years without refueling seems like a pretty efficient furnace to me. What would make it more efficient would be to concentrate all of it's output on the Earth while simultaneously slowing its reaction by a corresponding amount. If this were achievable our 5,000,000,000 year time horizon could be dramatically lengthened.
Concentrating all of it's [Sun's] output on the Earth would incinerate the surface of the earth, and in fact melt the Earth's surface. The Sun's mass is roughly 333,000 times the mass of the earth. The mean density of the sun is 1408 kg/m3, or 1.4 times that of water at 4°C. The central density is ~1.622 x 105 kg/m3, something we cannot reproduce on earth. The solar core pressure is ~2.477 x 1011 bar, which again is well beyond the capability of terrestrial technology. The Earth's atmospheric pressure at sealevel is ~ 1.01325 bar.

Ref: http://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html

The sun will do it's thing regardless of the earth. We only need a small portion of the energy output of the sun. During the summer, the Earth gets plenty of warmth from the sun.
 
  • #10
I did throw in that we'd have to simultaniously slow it's reaction. I guess it might be more practicle to build a Dyson sphere.
 

1. What is quantum tunneling?

Quantum tunneling is a phenomenon in quantum mechanics where a particle can pass through a potential barrier even though it does not have enough energy to overcome it. This is possible due to the wave-like nature of particles at the quantum level.

2. How is quantum tunneling related to controlled fusion?

In controlled fusion, two nuclei must overcome a strong electrostatic repulsion to fuse and release energy. This can be achieved through quantum tunneling, where the nuclei can "tunnel" through the repulsive barrier without needing high temperatures or pressures.

3. What is the significance of exploring quantum tunneling for controlled fusion?

By understanding and harnessing quantum tunneling, we can potentially create more efficient and sustainable methods for controlled fusion, which could provide a clean and virtually limitless source of energy.

4. How is quantum tunneling currently being studied for controlled fusion?

Scientists are using advanced theoretical and experimental techniques, such as quantum simulations and high-energy particle accelerators, to better understand and manipulate quantum tunneling in the context of controlled fusion.

5. What are the challenges in utilizing quantum tunneling for controlled fusion?

One of the main challenges is controlling and optimizing the tunneling process, as it is highly dependent on various factors such as particle energy and barrier thickness. Additionally, accurately predicting and measuring the outcome of quantum tunneling events can be difficult due to the probabilistic nature of quantum mechanics.

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