However, I think there is a lot of interesting physics for accelerators beyond colliders. For instance, the linac coherent light source (LCLS) is a huge x-ray laser out in California [2]. They are in the middle of a big upgrade and employ a large number of accelerator physicists. There is also still a huge amount we need to learn about how free electron lasers work and how we can improve them.
Another big topic of research is energy recovery linacs (ERLs). We are just on the cusp of being able to implement a technology that could save something like 90% of the wall plug power of current accelerators. They draw a huge amount of power, so the benefit of this saving is pretty clear. The first machine to demonstrate this new technology will come online this summer (hopefully). [3]
[1] https://physicsworld.com/a/disappointment-as-japan-fails-to-...
Relevant "I saw this on YouTube" video:
- "Should we build a bigger particle collider? - Sixty Symbols"
- https://youtu.be/-cD66O01E4E
- TL;DR: the LHC has only found one new particle, and it was looking for it. Before spending 30 years and 10-20 billion pounds on a 4x bigger collider, maybe we should wait until we have an idea of what we'd be looking for, as it's not clear what the larger collider would be looking for. The downside of not building a new collider soon is that the people who know how to build a collider now won't sit around waiting until we decide to build one, so starting from scratch in the future will presumably take longer & be more expensive.