Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: Fusion Reactions
Date: Aug 01, 12:28 pm
Poster: Jim Lux

On Aug 01, 12:28 pm, Jim Lux wrote:

>Hey, so how much energy does it take to make two hydrogens fuse?

Assuming you mean regular old H (not D or T), it's actually a pretty complex series of reactions involving carbon (hence the name the "carbon cycle")

There is also a proton-proton reaction:

Of course, he uses the "billions and billions of degree" thing.... Just divide by 11,000 to get electron volts... 10 billion K = 900 keV (not all that much, eh?) The carbon cycle apparently runs at 10E6 K or about 9 keV (quite low)...

The reaction cross sections can be looked up at a variety of sources (I have a page that has some links: http://home.earthlink.net/~jimlux/nuc/sigma.htm ) That page doesn't give the H(p,d) or the C(p,N) reaction but you can probably find the data at the ENDF site which is linked from my page.

At the low energies cited, the reaction rates are probably pretty low, but don't forget that there is a LOT of stuff to react on the sun. It's not like any significant fraction reacts in a given second. (I hope not... unless the sun goes nova, it's gotta last a few more billion years)

The reaction rates (in a percentage of the ions present that fuse in a given time interval sense) for a fusor is MUCH higher than the sun.
Richard's getting, say, 1E5 n/sec, in a 8" fusor (4 liters) at 1 micron pressure (roughly 1E-6 atmospheres) (i.e. around 1E17 atoms in the sphere)... so he is reacting 1E-12 of the atoms every second. (He'll burn em all up in about 31,000 years)

The sun, on the other hand, should burn for about 10 billion years (easily), or roughly 315E15 seconds.. The fractional reaction rate is, then, around 3E-18.. or about a millionth of what you do in a fusor.

All these calculations are very back of the envelope and may be off by orders of magnitude. But, you should get the general idea... slow reaction rates (small cross sections) don't hurt you if you have billions of tons of reactants...