[Arvid]

MTd2 wrote: Do you know a book with tables about cross sections for different elements, like proton or deuterium + element? It might be that D+B10 cross section is much higher than D+D at its peak value.

This is a little 30-page chapter from a larger book that discusses a number of reactions relevant to controlled fusion and fusion in stars. (Warning: PDF.) Unfortunately, nothing about D + B 10. Interestingly, it shows p + B 11 with a huge, but very sharp resonance at 146 kEV, before its main peak over 500. Could some machine be designed to take advantage of that?

[Arvid]

Since the heading is “Newbie Questions” maybe nobody will mind me sticking in a couple of mine, rather than starting an identical thread….

A) On the subject of neutrons created by the boron-11 occasionally fusing with a helium nucleus: Why is any helium sticking around? I can see where this would be a problem in a tokamak, but the alpha particles created in the focus fusion device are going to be heading for the other end of the machine at about 12,000,000 m/s; couldn’t you peel them off after they’ve passed through the coils and given up their kinetic energy and sequester them so they don’t go back and come in contact with the fresh fuel? And….

2) When I first started reading about proton-boron fusion 40 years ago, they never mentioned that side reaction with helium—it had probably only been done in colliding beams at that time, so I suppose it never came up. They did say, however, that every once in a grand while when a boron-11 nucleus absorbed a proton, it would manage to get rid of enough energy by emitting a gamma-ray photon that it could stay together as carbon-12, but in an excited state that would still emit another shakedown gamma—kind of like cobalt-60, only with a half-life of 12 hours instead of 60 years. Does that really happen, or would the kinetic energy with which the reactants come together in the focus fusion machine be too high for that?

[Arvid]

Aeronaut wrote: Besides, that kind of mobility will open up the space travel and vacation industries. Been waiting for that all my life.

First, belated congratulations on your 1000th post. Yeah, I’ve been waiting for that my whole life too. I was 17 (well, almost) when we landed on the moon for the first time. Now I’m 58, and we’re farther from being able to land on the moon than we were in 1961. Our only sign of movement is SpaceX reinventing the wheel and taking us back to 1960—and that’s considered progress!

I did a little ballpark arithmetic on my plan—if you had a 1000-tonne spacecraft powered with a D-D reactor, only using the proton for thrust, you’d need about 16 tonnes of deuterium to transport several hundred tonnes of payload from low earth orbit to low mars orbit and back, and in the process produce about 12 tonnes of helium-3 (well, 6—you’d have to wait for the 6 tonnes of tritium to decay). That should be enough to produce (with some added deuterium) about 200 GW-years of electricity. That’s got to be worth something on the open market.

As always, getting rid of waste heat is the real problem. If you took two years for each leg of powered flight, you’d need at least 40 acres of radiators, and there goes your payload. Maybe you could take longer—I’m assuming this is cargo-only, people would have to go much faster because of the cosmic ray exposure. It would be one way of making space travel pay for itself. You’d need to build essentially this interplanetary spacecraft even if you were parking it in orbit to burn deuterium into helium-3. You’d have to use some of that thrust for stationkeeping. Why not go somewhere with it?

[Arvid]

Well, that’s most of what I meant by decrying the Bohr atom, was this picture of the proton in particular, as a hard, positively-charged billiard ball, that must be made of positively-charged parts that would fly apart if given a chance.

Here’s a picture that helps me visualize the whole thing—I can’t pretend it’s justifiable from a quantum-mechanical standpoint. Think of trying to stuff some cotton batting into your couch cushion. It’s really good stuff—so fluffy that it forms a cloud 40 or 50 miles across on this scale—but I’ve got big hands, so I just keep squeezing and squeezing and squeezing until I’ve got it compressed so small that it’ll fit inside the cushion, so I jam it in and zip it up. Problem is, just like compressing a spring, I’ve stored a tremendous amount of energy in it by squeezing it so tight, and it’ll just burst right back out.

So why doesn’t it just combine with the proton to form a neutron? I’ve undoubtedly given it enough energy for it to do so. I think you have to look one layer deeper and realize that the proton is composed of three quarks, staying fairly close together in the center of what we call the “radius” of the proton. Just like the atom, most of the proton is empty space too. The quarks are kept from wandering apart by the color force, but since it has three different kinds of charge, it’s even harder to explain than electric charge, so just take it as read.

It helps me to think of the hydrogen atom as formed of 4 quarks—3 colored quarks that form the proton, and one “colorless” quark that’s only held to them by the electric force, so it’s not bound nearly as tightly. If I force it into proximity with the other quarks, some “magic” has to happen to change the consistency of my cotton batting so it’ll stay inside my couch cushion. We call this magic the weak force. It can change the identity of the electron into a down quark and therefore the proton into a neutron. If you sit around waiting for magic to happen, though, it might take a while—most times when you force an electron into a proton it won’t.

Well, now we’ve mentioned the color force and the weak force, so it appears that we’ve made the problem worse. Maybe it’s elephants all the way down! It may help people to realize that there’s more than one kind of charge, though. Gravitational charge is called “mass” and there’s only one kind. Color charge has three kinds. It’s just a fact of life at the quantum level that if we add enough energy to do something like turn a proton and electron into a neutron, we may just create new particles out of the vacuum that can do unexpected things. The world is a much more interesting place than Lord Kelvin thought it was!

[Arvid]

I’ve had people ask me this question, too. It’s amazing that the public is still fixated on the Bohr atom, which lasted 12 years over 85 years ago, but there it is. That’s the answer everybody has in their mind—the electron is whirling around really fast like a miniature Solar System, so it doesn’t crash into the nucleus.

I don’t think it’s possible to avoid quantum theory altogether. Just remind people that energy—light, in particular—comes in discrete packets, proportional to the frequency. Then just tell them that particles, too, have a wavelength associated with them inversely proportional to their momentum. It may be hard to understand or visualize, but it’s an observed fact.

Then the question of “Why doesn’t the electron crash into the proton?” answers itself—it did! You don’t need to talk about anything more complicated than a ground-state hydrogen atom, because after that it gets messy in a hurry, but in the hydrogen atom the electron is sitting exactly on top of the proton—it just can’t be localized nearly as closely because of its much longer wavelength.

Then if you think that worked, you can remind them that that “cloud” that represents the electron is really just a graph of the probability of finding it if you look for it, and it could be inside the nucleus at any given moment—it’s just impossible to say. If they haven’t run away yet, tell them that if you gave the electron enough energy that you could say for sure it was in the nucleus, it would be far, far away very soon, because it would be moving too fast for the nucleus to hang onto it.

I find if you take it step by step like that, most people seem to grasp the underlying point. Whether it sticks long-term or not, I don’t know. That pesky Bohr atom is still with us—but that’s a minor annoyance compared with the mystical mumbo-jumbo called the “Copenhagen Interpretation”!

[Arvid]

vansig wrote: And therefore there is no plan to breed tritium, and no good reason to want to, if you can just burn boron.

That was my point. D-D produces helium-3 and tritium, which you allow to decay to He-3 before shipping it dirtside for use in (aneutronic) power reactors.

EDIT: I should perhaps make this even clearer: this scenario is predicated only on the eventuality that p-B11 doesn’t work, nobody wants to burn D-D or D-T on earth because of the neutrons they produce, and we’re reduced to trying to obtain helium-3 somewhere for D+He-3. (Why not He-3+He-3? No side reactions there.) I just think it would be enormously cheaper to produce our own with D-D reactors in space than ship mining and extraction equipment to the moon.

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