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Comment by rkagerer | Hacker News Reader

This is probably another ignorant question, but why can't we "aim" and shoot protons at each other to make them smack together without requiring all that heat and pressure?

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Say you shoot the deuterium and tritium ions at each other in beams. Making two ions fuse isn't quite the same as hitting a fixed bullseye. Typically, physicists consider a 'cross section' for a reaction between two particles as a function of their relative velocity: a notional area that allows one to compute the probability of a reaction occurring, which may be thousands of times larger or smaller than what we typically think of as the physical size of the particles.

The cross sections for fusion reactions are significantly less than 1000th those of elastically bouncing off, even at the optimal velocities. Fusion reactions release about 1000 times[2] the typical energy needed by the reactants in order to get close enough for there to be any significant probability of reaction. So a one-pass beam system is always going to lose out to elastic scattering.

[1] https://en.wikipedia.org/wiki/Nuclear_cross_section [2] D + T releases 17.6 MeV of energy; typical fuel temperatures are 15 keV.

Like hydrogen-boron fusion?

https://newsroom.unsw.edu.au/news/science-tech/laser-boron-f...

https://revolution-green.com/breakthroughs-make-commercial-l...

Other: Google for "LENR". Some people claiming (for years) they are near to success.

I've wondered, aren't there any fusible elements that can be solids in vacuum? Then shoot pellets at each other at a sizable fraction of c. Way better density than a particle beam.

For instance, lithium-6 is a stable solid metal with the right nucleon composition and binding energy to fuse into carbon-12. I have no idea if it actually would to a useful degree.

Solid in a vacuum at what temperature?

Whatever's practical for your giant pea-shooter and reaction chamber. But lithium is a soft metal at room temperature, so this part doesn't seem too constraining.

If you want to sustain a fusion reaction for generating power through heat, you're not going to do it at room temperature, and there're not many materials that will remain solid at fusion temperatures.

I said "shoot pellets at each other at a sizable fraction of c". The point is to hold together before the splat, so that by high density enough nuclei actually hit each other. This is a bang-bang kind of thing, not a continuous reaction.

We don't have any trouble creating fusion explosions. The whole trouble of fusion is sustaining a reaction. Fusion bombs already exist, and what you're describing isn't that different from an Ulam-Teller device.

Existing fusion bombs need a fission bomb for ignition. This lower-bounds both the bang and the unpleasant byproducts. A smaller bang can be used much more practically. For example, 20th-century cars were also powered by explosions. Somehow they outcompeted the previous dominant technology of continuous reactors, steam engines.

To dismiss my half-assed idea by pointing to another, I'd bring up something more comparable, like inertial-confinement fusion.

https://en.wikipedia.org/wiki/CERN

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