During the Electric Propulsion Lab tour, the Academies got to see the vacuum facilities. Since launch testing is usually prohibitively expensive, NASA does ground testing as much as possible, and the vacuum facilities are a great help in simulating space environments.
VF5 is a vacuum chamber with very high pumping speed, where they were testing a xenonfueled Hall thruster. The thruster had the nice characteristics of not requiring much fuel (about 900 kg of xenon over its lifetime) but still providing high thrust given enough time.
VF5 can also be made very cold, so they can use it with simulated lunar regolith to act like the environment of the Moon. VF6 was not being used during the tour, so we were able to take a walk inside. Its big feature is an aperture that emits heat and radiation to simulate a solar environment. (Thankfully, it never turned on while we were inside!)
We also got to hear about upcoming projects, such as the unfinished Glenn Extreme Environment Rig, meant to simulate the high temperature and gaseous atmosphere of planets like Venus.