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Well, the site page says that it can operate at "very low voltages, down to 1 MV, to accommodate requirements of the astrophysics program." However, the history page has the following quote as well, "After some modifications to the original equipment, a terminal potential of 25.5 MV, the highest in the world, was achieved while accelerating a 58Ni beam."

I used to run a delivery route to the facility, had to go through special training for being in the building in case of a pressure vessel rupture - they use SF6 (sulfur hexafluoride) to insulate the systems. And you could always tell when the system was in operation - a background noise that you couldn't hear (hard to explain it) and just the feeling that something very big was hovering overhead.

Well, the site page says that it can operate at "very low voltages, down to 1 MV

1 MV = 1,000,000 volts. That ain't what I call very low voltage!!

a background noise that you couldn't hear (hard to explain it) and just the feeling that something very big was hovering overhead.

I would think that 1MV to 25.5MV would cause a "hum" that you could "feel".

site page says that it can operate at "very low voltages, down to 1 MV, to accommodate requirements of the astrophysics program." However, the history page has the following quote as well, "After some modifications to the original equipment, a terminal potential of 25.5 MV

Van de Graff generators -- you can make them at home too. First you need a big sphere. Your maximum surface-to-air breakdown voltage is determined by diameter -- about 35,000 volts per inch of diameter. So to reach a million volts you need about a 30 inch sphere.

Next, since surface-to-surface breakdown voltage is about 20,000 volts per inch, you need your sphere up on an insulated post at least 50 inches above preferably flat or smoothly curved base surfaces.

A PVC pipe would make a good pole. Big spheres could be foil covered styrofoam -- whatever. It doesn't have to be strong, just have a conducting surface.

Finally inside the pole you need a rotating belt to carry the charge. The belt is non-conducting rubber, like neopreme. At the base you spray charge onto the belt with a small comb or tuft of wire. You can boost this with a high voltage source -- but smaller units just use a non-conducting pulley and the action between the pulley and the belt causes the tuft of wire which is hooked to ground, to spray a charge on the belt.

Up at the top, the pulley is typically metal. The tuft of wire is hooked to the sphere surface. Charges always seek the surface, so when the charge on the belt approaches the tuft or comb, it jumps to the comb and flows to the surface.

The belt will keep transporting charge to the sphere until the breakdown voltage is reached or leakage on the sphere matches the rate of current delivered.

Since sphere size and smoothness determine breakdown voltage and leakage, it is the sphere physical dimension that determines the ultimate voltage it can store -- the belt system only determines how fast it can reach that voltage.