WHAT IS A VAN DE GRAAFF GENERATOR?

The physicist and inventor Robert Van de Graaff created an electrostatic generator in the early 1930s that became a wonderful science-lesson demonstration of the effects of very high voltages. By "going-large", it also became a work-horse for the development of nuclear physics, right up until the 1980s and is still in use today both in the classroom and in smaller nuclear physics labs.

What's it all about?

Most Van de Graaff generators consist of a tower, on top of which is a metal dome. A non- conducting belt is driven between the inside of the dome and the bottom of the tower as shown in the accompanying diagram. The belt is carried by two rollers, one inside the dome and the other, a powered roller, at the base of the tower allowing the belt to rotate. The height of the tower can range from half a metre or so for classroom experiments, to 70 metres for the tallest Van de Graaff accelerator built for nuclear research in the UK.

At the base of the tower is a high-voltage supply, which applies a large positive or negative potential to a comb of thin metal points positioned very close to, but not touching, the bottom of the belt. This voltage creates a corona discharge in the air from the comb's teeth and positively charged ions and negatively charged electrons are produced between the teeth and belt. Another comb like structure is suspended from the interior of the dome and located just above the belt.

What does it do?

A Van de Graaff generator can produce a positive or negative charged dome, depending on the charge on the comb at the base of the tower. For table-top units, electric charges of between 100 kilovolts and 500 kilovolts can be created on the dome depending on the height of the tower and size of belt. Typically, the voltage on the lower comb required to produce ionisation in the gap between the comb's teeth and belt is 5 Kilovolts to 10 Kilovolts depending on size of tower and dome. At the top end of the range - the monster 70 metre accelerator, built in the 1970s - generated 20 million volts on its dome!

So the main function of a Van de Graaff is to create high voltage static electricity on the metal dome.

To Produce a Positive Dome

To produce a positive charge on the dome, the electrode at the bottom of the belt is made positive, creating a plasma of positive ions and electrons in the air between the teeth of the comb and belt. The positive ions are driven towards the belt by repulsion from the electrodes and pull electrons out of the molecules of the belt surface creating positive charges in the belt as the ions in the air are neutralised. The overall result is that positive charges are transported upwards by the belt.

At the upper comb, the positive charges on the belt pull free conductance-band electrons from the dome and the charges on the belt are neutralised and the belt then continues downwards, where the process is repeated.

If you recall, from A Journey into Modern Physics, given sufficient thermal energy, the outermost electrons of the atoms in a metal can escape from the nucleus and freely wander the conductance bands of the metal. It's these electrons that are pulled from the dome to neutralise the charge on the belt.

Pulling electrons off the dome results in there being more positive ions in the metal lattice of the dome than free conductance electrons. These remaining electrons distribute themselves evenly around the metal structure revealing an even distribution of positively-charged ions within the metal lattice. The dome is now a source of static, positive electricity.

To produce a negative dome

The electrode at the bottom of the tower is made negative and still creates ionisation between the comb and the belt. Free electrons from the plasma are repelled by the electrodes towards the belt and deposit themselves onto the belt to which they stick and are driven upwards.

When the electrons 'stuck' to the belt arrive at the dome, their combined electric field repels electrons on the comb, creating regions of positive charge on the points that pull electrons off the belt allowing them to move into the metal's conduction bands. The result of this migration from belt to dome, neutralises the belt's charge and the belt heads on downwards.

The growing population of dome electrons distribute themselves evenly around the structure. The end result is that the free, conductance-band electrons outnumber the positive ions of the metal and the dome now becomes a source of negative, static electricity.

Does the Dome's Static Charge Keep on Increasing?

Once the belt is started up, the charge on the dome starts to increase, but it doesn't increase for ever. There is a charging current being delivered to the dome by the moving belt and there is always going to be leakage of charge from the dome.

The most spectacular form of leakage is a spark between the dome and something close by when the voltage on the dome is high enough to create a corona discharge. If the air is very humid, then sparking to nearby objects is more prone. The efficiency of the charging current can be controlled by changing the speed of the belt.

At some stage, an equilibrium can be achieved between the charging rate and the leakage current from the dome. Even in very dry conditions, there will still be a small leakage of charge through the tower and surrounding air.

A stable dome voltage can be achieved by modifying belt speed until the charging current on the belt and leakage current reach equilibrium. However, it doesn't take much to create a discharge!

Science Lab Experiments

There are myriad experiments that can be performed with a desk-top version of a Van de Graaff generator: from creating impressive sparks, to making your hair stand on end! There are loads of videos on YouTube demonstrating and explaining the range of experiments that can be performed. If you are not familiar with these demonstrations, just head for YouTube and have a look at this amazing piece of kit in action – it's well worth it!

The Van de Graaff Accelerator

The Van de Graaff concept also went large! A typical nuclear physics Van de Graaff accelerator might have occupied a two story building. In the top half, the large dome was given a very large positive charge, typically 5 or 6 million volts. The belts were made very wide, the ionisation points were large, comb-like, ionisation structures, operating across the whole width of the belt. To stop the dome sparking to anything around, it was encased in a large, detachable chamber that was filled with a spark suppressant gas to keep the voltage stable without sparking.

An ion source, usually hydrogen or one of its isotopes, is introduced into an evacuated pipe inside the dome, vertically inclined. This leaves a positively charged proton sitting inside a +5 million volt dome and, thanks to repulsion between dome and proton, off it speeds down to the ground floor.

On reaching the lower level, very powerful electromagnets bend the beam into a horizontal, fast-moving stream of protons, which are directed into a horizontal, evacuated beam-line, along which they travel until they smash into a static target. Detectors arranged around the target then pick up the debris created by the collisions and the data produced reveals information about the target nucleus.

The largest Van de Graaff built

Situated in the north west of England in the UK, the Daresbury Van de Graaff, built in the 1970s, was housed in a concrete tower 70 metres in height. This monster generated 20 million volts on the dome, but actually accelerated ions through 40 million volts in what was termed a 'tandem' configuration.

The dome was located, not at the top but half-way up the tower. The source of ions to be accelerated was placed at the top of the tower and consisted of negatively charged ions. For example, hydrogen can be made to foster an extra electron, making it a negatively charged ion (one proton and two electrons).

This negative ion could then be accelerated down the tower, in an evacuated pipe, to the positive 20 million volt dome at the half way point. On arrival inside the dome, the ion's electrons were stripped away by passing the beam through very narrow foils or gas strippers. As the base of the tower was maintained at ground potential (0 volts) the suddenly naked proton continued to accelerate away from the positive dome down to the earth, where very powerful magnets directed the beam into a horizontal beam-line to smash into static targets.

In summary; the proton was accelerated through a two stage process. First, negative ions were accelerated from the top of the tower to the +20 million volt dome, on arrival, the ions were stripped of their electrons to become positive ions and then accelerated down to 0v volts at the bottom of the tower. The tandem configuration accelerated protons through 40 million volts in total.

An Early Pioneer of Nuclear Physics

The Van de Graaff accelerator was one of the pioneer instruments in the development of nuclear physics from the early 1930s up to the 1980s, but can still be found performing nuclear physics experiments. As well as pure research, Van de Graaff accelerators have been used to develop techniques in applied physics in numerous disciplines: for example in metallurgy for determining how CO2 diffuses into metal alloys used in turbine blades or gas cooled reactor conduits. They can also be found in medical physics labs where radiation damage is studied.

The 70 metre high nuclear-physics, research Van de Graaff, situated at Daresbury, closed down in 1993 and may well have been the tallest facility of its type in the world.

Conclusion

Robert Van de Graaff developed a versatile, high-voltage, static-electricity generator that has been used ever since its creation for educational purposes and in its 'go-large' incarnation as a real work-horse helping to further our understanding of the atomic nucleus. In this day and age of the super high-energy accelerators, like the LHC, it's easy to forget what an important contribution the Van de Graaff accelerator made to physics.

If you find this article interesting, part 3 of a Journey into Modern Physics goes up a gear and looks at the high-energy accelerators at CERN in detail, including the Large Hadron Collider. CERN's accelerators are ring-based and provide vastly increased energies compared to even the 70 metre Daresbury Van de Graaff, resulting in a transition from low-energy, nuclear research to high-energy, particle research.

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