In my experience, the majority of people at an accelerator don't work on instrumentation. But that's because the majority are not accelerator physicists. There's usually one or several departments/divisions whose task is to develop, maintain and do the R&D work for the next generation of accelerator work. I've spent a lot of time at Fermilab over the years and the people who do this kind of work are those in the Accelerator Division (AD). Most students (summer or otherwise) aren't affiliated to the AD, but to a specific experiment or theory group and thus don't do actual accelerator physics. But, that doesn't mean that no one does instrumentation and accelerator work.
If you're interested in working on the machines needed for particle or nuclear experiments, I'm guessing that you could study EE with something related to resonance cavities, for instance. But if you know that you want to work on particle instrumentation, your best bet would be to apply to an accelerator physics department. Most places don't have an accelerator physics program (they have high energy experiment programs where people analyze data from the experiments at the accelerators, they don't actually design the accelerator), so you need to do the necessary research to find the schools that do. bfollinprm has given some good starting points.
Most people at accelerator complexes aren't accelerator physicists. Those that are tend to do the hands-on accelerator R&D.