[ISL]

This is the key observation that isn't obvious about fusion. The capture cross-section is smaller than the scattering cross-section, so essentially any beam design is doomed to fail.

I'm an experimental physicist who recently looked hard at jumping into the fusion game. My approximate conclusion is that anything that doesn't involve some form of adiabatically heated plasma is extremely unlikely to work.

Fusion is an efficiency and engineering problem: how can you ensure that each particle gets enough energetic collisions to have a prayer of fusing before it loses energy?