I apologize for my lack of physics knowledge, and I won't be gaining much any time soon, since I'm focusing mostly on work and physical exercise. I can't help but wonder about this:
Questions:
What are the main barriers preventing physicists from slowly pressing two hydrogen atoms together to fuse them into helium? (as opposed to very many high-speed collisions)
Is such a device that would do that physically impossible to construct, or just very much harder to construct than one of those magnetic-confinement, toroidal, plasma trap, fusion reactors?
I won't ask any more ignorant questions for a while (maybe about 6 months).
Best Answer
It takes a lot of force to fuse hydrogen. Nuclear fusion requires the atoms be brought close together, causing them to experience very high electrostatic repulsive forces. Only once you get them close enough together do you see the quantum effects which draw the nuclei together into fusion. The atoms have to have enough energy to push past this barrier, known as the Coulomb barrier, before they can engage in fusion. In chemistry, the analogue of this is the "activation energy" required for a reaction to occur. For a typical deuterium/tritium reaction, this energy barrier is 0.1MeV.
If you "pushed" two atoms together, you would have to provide all of the force required to overcome the electrostatic forces. This is quite a lot of force, because the electrostatic forces increase proportional to the square of the distance between the nuclei, and they have to get very close. This is also a very unstable repulsive system, so the nuclei would like to escape. It's like trying to press two billiard balls into each other.
Instead, it is much easier to accelerate the hydrogen atoms over a very long time period to a high speed, and let kinetic energy do the hard part of overcoming the Coulomb barrier.