Oppenheimer-Phillips, and a guarantee of overcoming Coulomb?
Posted: Fri Jul 22, 2011 9:12 am
At the risk of countering my departure message of yesterday, there is one point of issue that I feel I should 'put to bed' before deleteing 'talk-polywell' in my shorcuts, else any newbs (or those uneducated in such matters who look these things up here) might be a bit bewildered, due to the writings of a clown who wrote;
The peak cross-section for, example, D-T is around 64keV. But the Coulomb Barrier is around 1 MeV (I think the exact figure is ~500keV, but I'm talking oom here for light particles). This is because when fusion fuel nucleii meet, they play a sort of lottery game in which they decide whether to fuse. Most don't and bounce away in a Coulomb scatter.
The way in which fusion nucelii fuse is not by overcoming the Coulomb barrier, which is ~1 MeV high for light nucleii fusion, but by digging a tunnel underneath that very high hill of energy.
So you might think [just like the clown above has posited] 'Ah! So if you want ALL your nucleii to fuse, then just drive your nucleii to an energy ABOVE that hill, and truly overcome it!"
Unfortunately not, because light nucleii have very low binding energies and in nuclear terms they are 'weakly bound'. If you pick 1MeV as the collision energy, sure some may fuse, but another reaction begins to dominate for deuterium called Oppenheimer-Phillips, which simply knocks off neutrons like a marble game.
This is why you can never have any sort of certainty [like 'the majority of particles'] that fusion will occur between any two particular nucleii. It is a random game you are unlikely to win, and your game is either the lottery or marbles. The fusion we see in fusion reactors, be it thermal tokamaks or electric-fusion fusors, is just a statistically anomalous tail-end of unlikely reactions that occasionally occurs, instead of the expected Coulomb scattering. The only reason we see fusion is because there are trillions of trillions of collisions, and so we see the millions of fusion reactions. Just like if you bought trillions of lottery tickets, you might expect a few wins out of so many tickets.
The clown has misunderstood that the 'peak cross-section' for fusion is a much lower level than the Coulomb Barrier. In part, this is because he appears to have stated a strange view that there is no such thing as quantum effects in fusion.
In historic terms, it may be interesting to note that when Rutherford set his lab, the Cavendish, on course to split the atom, the idea that all at the time had was that it would take MeV levels of acceleration - because that was known to be the Colulomb barrier height. It was Gamow who visited the Cavendish who came up with tunneling theory and gave hope to Cockroft and Walton to get on with as much acceleration voltage as they could manage to generate. As soon as they accelerated protons into lithium at 100keV, way way below the level expected, that they got strong helium emissions - they'd split the atom with tunelling! It is historically noteworthy that many research labs around the world at that time could have already accelerated to those levels with relative ease, but no-one bothered looking because everyone saw the 'Coulomb barrier' height to be the accelerators' target, and it was, in effect, Rutherford accepting Gamow's work (that few else accepted at the time - and seemingly some still don't) that lead them to running the experiment anyway, in case he was right. It paid off, and all involved got their Nobel prizes.
to which I repliedJoseph Chikva wrote:So, one beam of particles should transit through another beam and their relative speed (velocity) should be sufficient for that the majority of particles could overcome the Coulomb barrier between the reacting particles.
(new highlights/parentheses are mine)... points to note;
...
... the required energy to overcome the Coulomb barrier is many MeV of energy. At this level ['many MeV'], if such particles meet they simply destroy themselves in a process called 'Oppenheimer-Phillips stripping'. There is no energy to be gained by this process, but if your objective is to get a pile of nuclear particles then it does that.
... At energies below O-P, energy-releasing fusion doesn't occur just because you get two nuclei to a high energy. The nucleii must undergo tunnelling. This is a probabilisitic process. Only one in a million of any such reactions would end in fusion - even if they were spot-on, head-to-head.
The peak cross-section for, example, D-T is around 64keV. But the Coulomb Barrier is around 1 MeV (I think the exact figure is ~500keV, but I'm talking oom here for light particles). This is because when fusion fuel nucleii meet, they play a sort of lottery game in which they decide whether to fuse. Most don't and bounce away in a Coulomb scatter.
The way in which fusion nucelii fuse is not by overcoming the Coulomb barrier, which is ~1 MeV high for light nucleii fusion, but by digging a tunnel underneath that very high hill of energy.
So you might think [just like the clown above has posited] 'Ah! So if you want ALL your nucleii to fuse, then just drive your nucleii to an energy ABOVE that hill, and truly overcome it!"
Unfortunately not, because light nucleii have very low binding energies and in nuclear terms they are 'weakly bound'. If you pick 1MeV as the collision energy, sure some may fuse, but another reaction begins to dominate for deuterium called Oppenheimer-Phillips, which simply knocks off neutrons like a marble game.
This is why you can never have any sort of certainty [like 'the majority of particles'] that fusion will occur between any two particular nucleii. It is a random game you are unlikely to win, and your game is either the lottery or marbles. The fusion we see in fusion reactors, be it thermal tokamaks or electric-fusion fusors, is just a statistically anomalous tail-end of unlikely reactions that occasionally occurs, instead of the expected Coulomb scattering. The only reason we see fusion is because there are trillions of trillions of collisions, and so we see the millions of fusion reactions. Just like if you bought trillions of lottery tickets, you might expect a few wins out of so many tickets.
The clown has misunderstood that the 'peak cross-section' for fusion is a much lower level than the Coulomb Barrier. In part, this is because he appears to have stated a strange view that there is no such thing as quantum effects in fusion.
In historic terms, it may be interesting to note that when Rutherford set his lab, the Cavendish, on course to split the atom, the idea that all at the time had was that it would take MeV levels of acceleration - because that was known to be the Colulomb barrier height. It was Gamow who visited the Cavendish who came up with tunneling theory and gave hope to Cockroft and Walton to get on with as much acceleration voltage as they could manage to generate. As soon as they accelerated protons into lithium at 100keV, way way below the level expected, that they got strong helium emissions - they'd split the atom with tunelling! It is historically noteworthy that many research labs around the world at that time could have already accelerated to those levels with relative ease, but no-one bothered looking because everyone saw the 'Coulomb barrier' height to be the accelerators' target, and it was, in effect, Rutherford accepting Gamow's work (that few else accepted at the time - and seemingly some still don't) that lead them to running the experiment anyway, in case he was right. It paid off, and all involved got their Nobel prizes.
