Van de Graaff Genrator Tune-up

[George Schmermund]

The beam on target system now under construction will be using a Van de Graaff generator to produce the HV needed to do useful experiments. The VdG that has been sitting on the shelf for years has been re-enlisted into service and will need some attention if it's going to work at peak performance. The first step is to disassemble the major parts for a through inspection and to make improvements where possible.

The belt and rollers will get the first tests and design improvements. Since these parts are the ones that will determine the HV terminal polarity and maximum charging current rate, I'll describe a means of testing and maximizing their combined potential (pun). The belt I'll be using was purchased decades ago from a commercial source that specialized in conveyor belts. Having tried a large assortment of possible belt materials and splicing methods, nothing has worked better or lasted longer than this custom made nylon one.

I'll confess to having fabricated more VdGs over the years than memory can recall. It seems that HV heads choose early on in life if they're going to inoculate themselves with the VdG or Tesla coil virus. My kink was with the VdG strain.

Anyhow, these belt/roller tests start with an evaluation of the belt's ability to transfer any charge that develops on its surface. Measuring the surface resistivity of both the front and back of the belt will indicate how well it's going to perform. When the surfaces are clean and dry they should have a resistivity of 10^12 ohms/square or better. The surface resistivity meter tells the tale on this account.

One of the fortunate aspects of triboelectric charging is that the rollers need only come into contact with a dissimilar material in order to produce a charge on the surface. This property works well with just the selected surface material attached onto what otherwise might be a conductive roller. The photos show how this charging test can be accomplished and measured. Using a Monroe Faraday Cup and Nanocoulomb Meter it is easy to evaluate the magnitude of the contact charge and its polarity. With this information a choice of roller cover materials can be chosen.

As the photos indicate, the cylindrical aluminum blanks are covered with different clear packaging tape and rolled over the belt by using a polyester string. The charged sample can then be transferred to the Faraday cup for evaluation. Several of the tapes look quite similar in appearance, but have vastly different charging properties. It is now an easy matter to chose the most triboelectrically opposed coverings on each roller for achieving the maximum charging rate.

There are other things that need to be considered for an overall VdG tune-up and the particulars will follow.