Can electrons ever collide with other electrons during nuclear fusion? Or do they repel each other because of Coulomb's law?
Let us clear up the physics.
Nuclear fusion happens when two nuclei on the left side of the rising binding energy per nucleon curve scatter off each other with enough energy to overcome the coulomb barrier, so that the stronger attractive nuclear forces can act.
This means that the nuclei are bare of electrons, and the way that this can happen is in a plasma , a gas of ions and electrons which happens at very high temperatures.
This means that there are energetic electrons in a fusion plasma, but electron+electron can only give interesting results at very high energies, creating new particle-antiparticle pairs in the process. No fusion as lepton number is conserved. Electrons just scatter off each other according to the coulomb solution of the quantum mechanical scattering .
How to manipulate plasma to bring it to the energies needed for enough fusion of nuclei to get a positive result in energy spent versus energy output is a research project the world over.
Hmm good question. My intuition would be 'almost never'.
When all species in the plasma are in thermal equilibrium, they have the same energies, the same Coulomb wall to overcome as protons do, but they cannot tunnel into any potential minimum.
So as anything in the universe, they will have a nonzero probability to collide, but this will be quenched by the non-existent Gamow factor.