Browsing on Google Earth around Aberdeen Proving Grounds in Maryland, you can see a very obvious ring-shaped lack of trees. I saw this and thought it might be a particle accelerator, but to my knowledge APG has no particle physics functions.
Some cursory looking around on Google found that it is the Edgewood Arsenal Tandem Van de Graaf Accelerator, built in 1969, and (I presume) now inactive.
My questions are:
What was this facility used for, and were any important discoveries made there?
Why was it built in coastal, forested Maryland instead of somewhere drier and more open?
Was this run-of-the-mill technology at the time, or was it highly advanced?
Why was it shut down?
What became of the accelerator after being shut down?
What is the facility used for now?
And also an interesting bit of trivia I discovered upon looking into it: the accelerator was designed by a company at which Donald Trump's uncle was a principal designer. So was the president's uncle involved in building this accelerator?
It seems you've lead yourself astray by jumping to the conclusion that this treeless ring constitutes a particle accelerator. While there indeed was a van de Graaff accelerator there, that was located in this spot on the southern peninsula (building E5951). The circular thing you see is is presumably here. Through Medusa you can find that it i the "Army Pulse Reactor Facility (APRF)". (also referred to as the Aberdeen PRF as well) and the ring is one of three concentric rings of fences around the facility and nothing to do with the function in-itself.
This report has a brief overview of the facility, which was active from 1967-2003. It was a pulsed nuclear reactor. Such a reactor brings together fissile matter to criticality (a self-perpetuating nuclear reaction) very briefly by inserting a 'pulse rod' of highly enriched uranium. According to this report there was also a criticality excursion (essentially a meltdown) during preoperative tests in 1968 as more fission occurred than expected and the reactor parts melted and were damaged.
The point of a pulsed reactor here is to give off the neutrons and gamma radiation produced in a reaction. One of the main research aims, according to the former report, was to study the effects of neutron and gamma radiation on military equipment (in particular electronics), in order to harden them against it.
Results on military equipment are not likely to be unclassified at this point in time. But civilian research was also done there for radiation-hardening components for space as shown by this Sandia research report and it was also used for other research purposes, as shown by this late 1970s NASA research report on research into nuclear-pumped lasers conducted there.
As for the tandem Van de Graaff accelerator, the Maryland Historic Trust writes:
The Truex Laboratory was constructed in 1968, and originally housed the Tandem Van de Graaff Nuclear Accelerator. The Arm) Nuclear Defense Laboratory operated the accelerator from 1968 to about 1974. Research into nuclear matter and nuclear reaction; with an emphasis on nuclear structure and force investigations took place at the laboratory (EAI Corporation 1991:E5951). By 1974, the Flame Physics Research Branch controlled the building and performed research into the development of ignition bomblets and ways to thicken simulants. From 1977 to the present, the building has served for research into smoke/obscuration methods, concentrating on ways to mask personnel and equipment on the battlefield, and ways to detect enemy movements through aerosol obscurants.
Neither of these facilities were unique; Los Alamos had a pulsed reactor before this, and many van de Graaff generators had existed. (By the 1960s they were fairly obsolete as particle accelerators, by cyclotrons and then synchrotrons etc)