You might find some
interesting stuff here. Humphries is pretty good.
Of course, you're not going to make one with anywhere near the same order magnitude energy that the big labs are getting, but depending on what you want to do, might not care about that.
There are plenty of things one can do in the sub-megavolt range.... I think billions, and certainly trillions of eV are going to be out of range for even an advanced experimenter.
It's more a question of are you wanting to do nuclear physics, or subnuclear, the latter being pretty much out of reach for the citizen, even with the new plasma wakefield designs (that take one heck of a laser...) - and even then have much too much energy spread to do most interesting work.
And of course, detectors need to be up there - and with a good detector, you can work with lower rates which is what you get with lower energies...
In fact, you need good detectors to even measure your accelerator build and diagnose things - maybe even better ones than you need at run time.
John Futter here does some very interesting things well below a megavolt, for example.
And of course fusors do as well - and by my own measurements, due to various effects of space charge and shielding in the plasma, are doing it at achieved energies in the handful of kV, even when we're applying 10 times that. They work due to statistics - with enough attempts, sometimes a few particles get close enough for long enough to tunnel into fusion. We can't really manipulate or overcome the short-range forces that do this, but we can bang the rocks together enough to get the odd reaction and measure the products.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.