Update

After a few more years of thinking about this idea, I have made several significant changes to my original design. The first is that the reactor would be made out of a carbon nanotube instead of diamond, because carbon nanotubes have excellent thermal conductivity and are extremely strong. They are also the ideal shape for this application. The second is that the hydrogen atoms would be fused by being crushed between two large ions, instead of being hit by one. This would function like an atomic scale piston, since the repulsive electrostatic force of the larger atoms would be stronger than that of the hydrogen atoms (due to the former having more electrons), forcing the hydrogen atoms together. I also think that it may not be possible to generate energy in the way that I previously proposed. Most of the energy from the reaction would be released in the form of  the helium atoms, which would have a very high kinetic energy. This energy would either be transferred to the large atoms, which could be collected directly, or transferred to the reactor walls as heat. This could be harnessed in multiple different ways including turbines and thermocouples.

The most significant change is related to the fuel. I have found that the reaction of tritium and deuterium releases free neutrons, which decay after about 8 seconds. Despite this short life, the neutrons are nearly impossible to contain, since they have no charge, and they would travel large distances due to high speed. The neutrons have the potential to cause significant harm to anything that they hit by making them radioactive. The reaction of deuterium with helium 3 produces a very small number of neutrons, therefore, it might be a better choice. However, the amount of energy required to fuse these elements is higher, and would need a more powerful reactor.