The Prototype is a Target Fuel collider, the little "marble" is presumably the target fuel, made of tellurium (take this with a grain of salt) and other alloys (overflow document as a source)

the Prototype was tested on May 10th, 1982

It was meant to demonstate a "never before seen phenomenon"

This phenomenon is of heavy speculation, but I assume that it is to demonstrate a smaller-scale version of the threshold, compressing a really small amount of plasma compared to the full scale threshold to demonstrate that igniting and compressing plasma would lead to a connection to the Complex, as we can see severe camera interference, like the interference we see in Found Footage 3, when Ravi enters his closet