How is Nuclear Fusion Possible?

[lauromuller]

Under extremely high temperatures (over ~3.000K) the electrons and protons dissociate, resulting in plasma. But for hydrogen nuclear fusion to happen we need much higher temperatures than this (order of 10⁷ K). How does this process get "inverted"? I mean... If in temperatures higher than 3.000K the protons and electrons dissociate, how is possible that in much higher temperatures they "re-associate"? How are atoms formed during nuclear fusion? Besides, in the beginning of universe expansion, we had the required temperature to fusion.. Why it didn't happen then?

[Nicholas Kang]

Hello. You misunderstand quite a lot here.

 

Now, there are 2 conditions. One is on Earth, another is on the Sun, more presicely, its core.

 

On Earth, we can have a wide range of plasma, from very cold ones to very hot ones.

 

http://en.wikipedia.org/wiki/Plasma_%28physics%29#mediaviewer/File:Plasma_scaling.svg

 

But, for nuclear fusion to happen, we only need the hot plasma. You can never expect cold plasma to fuse.

 

How hot? As long as it is above the critical ignition point. Refer:

 

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

 

And don`t forget immense pressure too. Now, on Earth, it is very unlikely to find a place that can have such immense pressure, not even earth`s core, so scientists on earth that want to conduct nuclear fusion just have to go all the way up to the temperature. Low pressure means an even higher temperature, as long as the ions are closer than 1 * 10^-15 m. This is the coulomb barrier. Any distance longer than that value will cause fusion fail.

 

So, in conlusion there are different temperature requirements for different conditions. Unlike Sun`s core, we don`t have that much pressure.

 

Secondly, atoms won`t fuse. Ions do. The Sun has no gas, only plasma. They don`t re-associate. After 3000K, the ions, on average, still haven`t break the coulomb barrier, so temperature continue to rise, if the pressure is intense, this help to achieve fusion in much lower temperature. Then, in appreciable temperature, fusion begins. since we say plasma and ions, this means electrons don`t involve in such reactions. That`s why atoms cannot fuse becuase electrons are still bounded to them via electromagnetic force. Famous examples include Proton-proton fusion, CNO Cycle (for heavier stars) and tri-alpha process. Remember that in those reactions, electrons aren`t involve.

 

http://en.wikipedia.org/wiki/Nuclear_fusion

 

How are atoms formed during nuclear fusion?

Thus, the usage of atoms here is wrong.

 

Thirdly, in the begining of the universe, nuclear fusion does occur rampantly. It starts at 0.01s after Big Bang and slows down after the First Three Minutes.

 

http://en.wikipedia.org/wiki/Chronology_of_the_universe#mediaviewer/File:History_of_the_Universe.svg

 

During that epoch, lighter elements like hydrogen, helium, and minor traces of lithium and berrylium are thought to be formed. This is nucleosynthesis. Later only heavier elements form in stars core and continue to be recycled during supernovae nuclesynthesis. Thus, it happens. Nuclear fusion of lighter elements do occur in wide scale and the effect is observable today. Our observable universe today is about 3 quarters hydrogen and one quarter helium with minor traces of other elements.

Addendum; on earth, nuclear fusions are carried out in several places and the progress is quite slow compared to nuclear fission due to technical issues. Don`t confuse nuclear fusion on earth over nuclear fusion in the Sun`s core.

Edited November 6, 2014 by Nicholas Kang

[Sensei]

Under extremely high temperatures (over ~3.000K) the electrons and protons dissociate, resulting in plasma. But for hydrogen nuclear fusion to happen we need much higher temperatures than this (order of 10⁷ K). How does this process get "inverted"? I mean... If in temperatures higher than 3.000K the protons and electrons dissociate, how is possible that in much higher temperatures they "re-associate"? How are atoms formed during nuclear fusion? Besides, in the beginning of universe expansion, we had the required temperature to fusion.. Why it didn't happen then?

 

In typical fusion process, one free proton has to overcome 2nd free proton Coulomb's repelling force.

 

[math]p^+ + p^+ \rightarrow D^+ + e^+ + V_e + 0.42 MeV[/math]

 

For later reactions happening in star see this thread:

http://www.scienceforums.net/topic/85656-solar-fusion-neutrinos-and-age-of-solar-system/

[davidivad]

everything is moving around faster and there is more "chance" of transmutation.

the greater the shadow the greater chance of getting hit.

Edited November 7, 2014 by davidivad