Van de Graaff Generator : Limiting factor on Voltage

  • #1
Many sources talk about the field inensity (and therefore arcing/breakdown) on the outside of the dome as a limiting factor on the highest voltage that can be possibly reached on a VDG machine. There are also references to possible dielectric breakdown along the belt itself.

I am wondering if there could be another phenomenon at play.

As the belt runs downwards after delivering its charge to the upper roller + brush, it has to pass the inner edge of the window at the bottom of the globe. However carefully we smooth out this edge, there will be a strong field between the aperture and the belt. As the voltage builds up on the shell, there will be an increasing tendency to spray some charge back onto the downward running part of the belt, which will then carry the charge away as fast as the upward motion brings it in. (This process would be stable and continous at some point, and is not the same as breakdown of the belt material).

Is this mechanism really a practical factor to consider in VDG design? If so, are there cases where this process would kick in well before the breakdown on the outer surface?
 

Answers and Replies

  • #2
Simon Bridge
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There are many factors at play due to technological limitations. Even if everything were ideal, there is a maximum charge you can get on the dome with a constant rate of mechanical work.

Whether a particular factor is important to the design depends on what you want the generator to do.
My understanding is that edge effects by the belt are, in practice, small compared with the breakdown - which is achieved quite quickly in most designs. However, I imagine it is possible to rig insulation and other dimensions so that they become significant.
You know it is not "really" a practical factor in most designs because none of your sources mention it.
 
  • #3
e.bar.goum
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The biggest VDG ever built, as far as I know, went to 25 million volts - the 25 URC at Oak Ridge. It shut down operations a few years ago. There are about half a dozen machines around the world that go beyond 10 million volts still in operation. Coronal discharges, sparks, and leakage currents (all resulting in terminal instability) limit the maximum voltage, but this is more an engineering challenge than a physics one. You're also limited in how big you can reasonably build it. VDGs aren't compact accelerating structures!

The trick is to run them with the terminal inside multiple atmospheres of an insulating gas - SF6 to suppress sparks, and to not use a belt, but a chain of metal pellets, connected by nylon links.
 

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