system
Given the following post, respond as if you're commenting on Reddit:
human
Why do most experimental fusion reactors focus on deuterium-tritium fusion, and why isn't deuterium-proton fusion ever used?

Deuterium-tritium fusion produces a helium-4 nucleus and neutron. In stars, deuterium and protons are fused to create helium-3 without the production of a neutron, in the second step of the proton-proton chain reaction. Wouldn't deuterium-proton fusion be the much better choice, considering they have the same amount of electrical repulsion, wouldn't generate power in the form of neutrons, and are much cheaper?
gpt
The difficulty in fusion is over coming the positive repulsion between protons in the 2 nuclei. The closer 2 nuclei get, the more the protons push each other away. Sounds like you're probably aware of that, and also aware that the only way to overcome this barrier is the strong nuclear force exerted by all nucleons at close enough range. Thus, isotopes with more neutrons in their nucleus fuse more readily, because their is the same amount of electric (Coulomb) repulsion, but additional strong nuclear force to bring them together.

In stars, the pressure is huge, and the temperature is huge, but the real key to fusion in stars is sheer number of atoms. The  odds of two nuclei passing close enough to fuse is ridiculously low (hence the sun burning hydrogen for 10 billion years), but because there are so many of them, they over come the probability issue. If the odds are 1 in a million, and you have 10 billion, you're going to have a lot of events (these aren't the real numbers; but it gets the point across).

In fusion reactors on earth, we don't (currently) have the luxury of an excess number of particles or lots of time for the reactions to occur (the National Ignition Facility for example uses a DT mixture to initiate a fusion reaction that lasts a few billionths of a second). So we need all the help we can get from the fusion materials. Its basically stacking the deck!