The LHC is much larger than its predecessors, and proposed successors much larger still. Today, particle accelerators seem to be the main source of new discoveries about the fundamental nature of the world.
My lay interpretation is that particle accelerators like the LHC are essentially the only viable apparatus for performing experiments in particle physics, passive detectors of naturally energized particles notwithstanding. Experiments vary by configuration, sensors and source material, but the need for an accelerator is constant, and more powerful accelerators are able to perform experiments which are out of reach to less powerful accelerators. For the most powerful accelerators, "more powerful" seems to imply "physically larger". In these ring-shaped accelerators, for a given type of particle, its maximum power appears to be (very) roughly proportionate to circumference. I use the word "power" in a loose sense here, reflecting my loose grasp of its meaning.
Technology upgrades can make an accelerator more powerful without making it larger, e.g. the planned High Luminosity upgrade to the LHC. One imagines that an upgrade would be cheaper to build than a colossal new accelerator, yet larger accelerators are still built, so it would seem to follow that the upgrade potential of a given accelerator is limited in some way - that there is, in fact, a relationship between the size of an accelerator and its maximum power.
The first part of my question is this: what is the nature of the relationship between the size and power of a modern particle accelerator? Are there diminishing returns to the operating cost of making an accelerator more powerful? Or are there fundamental physical constraints placing a hard limit on how powerful an accelerator of a given size can be? Or is technology the main limiting factor - is it conceivable that orders-of-magnitude power increases could be efficiently achieved in a small accelerator with more advanced technology? Is it likely?
The basic premise of these experiments seems to be that we observe the collision byproducts of energetic particles, where "energetic" presumably refers to kinetic energy, since we used an "accelerator" to energize them. To create interesting collision byproducts, the kinetic energy in the collision (measured in eV) must be at least as large as the mass of the particle (also measured in eV) we wish to create. Thus, we can observe particles of higher mass with a higher powered accelerator.
The second part of my question is this: are particle accelerators the only way of pushing the boundaries of experimental particle physics? Is it conceivable that there is a way to produce these interesting byproducts in an experimental setting without using high-energy collisions? If not, is it conceivable that there is a way to energize particles other than by accelerating them around a track? If not, is it impossible by definition or for some physical reason? If either of these alternatives are conceivable, then assuming they're not practical replacements for large accelerators today, is it possible that they will be in the future? Is it likely?
In a sentence, my question is this: is the future of experimental particle physics now just a matter of building larger and larger particle accelerators?