For years now I've been using the 'salad bowl accelerator' demonstration and I'm frequently asked for advice on how to build your own. Well, the wait is finally over!
First up let me say my own version was inspired by that of Todd Johnson from Fermilab, which I've blogged about previously. A few years ago I realised I could do the same thing with a Van de Graff generator and a salad bowl, so I did just that. Interest in my little demo spread quickly and before I knew it it had been filmed at the Royal Institution and the accelerator bowl had been included in the 'Explore your Universe' project which distributed equipment to around 20 science centres around the UK. It seems everyone wants a piece of this demo!
Feel free to use the demo and please let me know if you make your own, have any feedback, additions or awesome extensions by leaving a comment or popping me an email.
What is it?The ‘accelerator bowl’ is a model of a particle accelerator that can be used to explain the workings of real machines like Diamond, ISIS or even the Large Hadron Collider. Although it’s a very simple model, by discussing the similarities and differences between this and a real accelerator we can understand a lot of the science involved in these intriguing machines.
How I present the demo
Filmed at the Royal Institution in London (the accelerator bowl segment is straight up after the intro)
Materials and suppliers
- Aluminium tape, wire & banana plugs - www.maplin.co.uk
- Nickel screening compound - RS online
- Ping Pong Balls - your sports shop, or Argos, these are easy
- Custom blown acrylic dome - mine came from HLN supplies
Choice of bowl:Whichever bowl (or other shape) you use as the confining mechanism for your 'accelerator', you need to make sure it is insulating. People have made these models with plastic, wooden and even some types of glass bowls. (Glass is not recommended as it's dielectric properties can mean it fails to let the strips hold charge - I learned that one the hard way). A more expensive but reliable option is a custom blown acrylic dome from eg. HLN supplies or Sunlight Plastics.
Make sure it is smooth and has a shallow curve to the sides of it, allowing the ball to roll around it and up the sides unimpeded. It can also help to have a flat spot in the centre so the bowl sits stably on a surface; if not you'll need a stand or supporting structure. However, make sure there is a smooth transition between the flat spot and the curved sides, any bumps are likely to make the ball lose momentum and affect the demonstration. If you have one custom-blown make sure to ask for it to be a squished shape, not a full hemisphere (think half a Smartie, rather than half a Malteaser)
- Coat the ping pong balls with the screening compound. It is best to do this outside in a well-ventilated area. Let them dry according to the paint instructions.
- When you’re ready to build the ‘accelerator’, first check the amount of charge that your Van de Graaff (VdG) generator usually builds up. (Calculate the voltage on the dome as 30kV per centimetre of spark in air). You don't need a big VdG to make this work, in fact too much charge will ruin the demo. (The charge that builds up in the bowl is likely to be quite a bit lower than on the dome and depends on the overall surface area of the metal strips.)
- Cut and attach the sticky Al foil strips. There is no exact blueprint for how many or the design of the strips, but there are some guiding principles. The minimum distance between strips should leave enough space so charge can’t travel from one strip over the ball and to ground (or the ball will short-circuit the demo and it won’t work). We recommend tapering the ends of the grounded strips and reducing the width of the charged strips toward the centre in order to maximise this distance. Make sure any tapering or cutting is done neatly and with rounded corners, any sharp corners will have a stronger charge density and be more likely to spark. (Note: once ‘stuck on’ the strips are very hard to get off!)
- Next join the four grounded strips around the outside of the bowl with a long continuous piece of Al tape.
- Attach one wire to the edge of the charged strips (strip 1-2cm of the plastic sheath from the wire and attach using more Al tape or croc clips) and attach another to the grounded strips.
- Using banana plugs or croc clips, attach the charged strips to the high voltage terminal of the VdG and the ground plug into the ground connection. (Make sure to keep the grounding wand connected also during operation!)
- Place the ball near the centre of the bowl and switch the VdG on. If nothing happens, try giving the bowl a gentle nudge to get some initial movement happening. (If nothing happens still, check your VdG is operating properly!)
How does it work?The Van de Graaff generator builds up a high voltage by generating static electricity. This means there is a high voltage of over 30,000 V on the metal strips but it is rather safe as even if it sparks, there is only a tiny current flowing. Inside the bowl are two sets of metal conducting strips. One set crosses over in the centre of the bowl, and these are attached to the high voltage terminal of the Van de Graaff generator. The rest of the strips are connected to ground, or 0 V.
A ping pong ball coated in a conducting paint is placed in the bowl. When the voltage is switched on, the ball moves around a little because of induced charges on the ball. Soon it comes into contact with a charged strip and picks up that charge – so now it has a like charge to that of the strip. This causes repulsion and gives the ball a push along. When it rolls over a grounded strip, the ball becomes neutralised and loses its charge. But it doesn’t lose its momentum and keeps rolling around the bowl. The next time it comes across a charged strip, it picks up the charge again, gets repelled in the same direction as before and once again gets a little kick along. Every time the ball crosses a charged strip it gets accelerated.
What happens in a real accelerator?In most real particle accelerators this is how subatomic particles such as electrons, protons or ions are given their energy; they see a voltage and get pushed along by it. In circular accelerators the beam of particles is bent around in a circle so voltage gets re-used again and again, with the particles gaining a little bit of energy each time. The difference is that in most modern accelerators the voltage isn’t static like the one from a Van de Graaff generator. You might have noticed that in this demonstration the ‘particle’ is changing it’s charge every time it gets a kick. But real particles have a fixed electric charge, so instead the voltage has to change very quickly from positive to negative and back again. That way, every time the particle goes past it will see an accelerating voltage rather than a decelerating one! To do this, we use radiofrequency cavities. These cavities resonate with electromagnetic waves and play the trick of providing a rapidly varying voltage. If the frequency of the wave is timed correctly, every time the particle goes through it will be accelerated. For more information on this see here.
The highest energy we’ve reached by accelerating particles this way is at the Large Hadron Collider (LHC) at CERN, where the protons are travelling at 99.999999% of the speed of light. At this speed they whizz around the 27km ring more than 11,000 times per second.
But that’s not the only difference between this demonstration and a real accelerator. Real particles are much smaller than a ping-pong ball, it’s hard to define a size of something as tiny as a particle, but the classical radius of a proton is around 200 trillion times smaller. To see the beam we have developed tools called diagnostics, which act like our eyes and ears when it comes to seeing the position of the beam, it’s charge, current and size.
The accelerator bowl only uses one ‘particle’, but real accelerators have billions or trillions of them at the same time, all with the same electric charge so they repel against one another. Controlling them at the same time is tricky, but it’s very important that we don’t accidentally lose the beam! Even though in the LHC each proton only has as much energy as a fast-flying mosquito, the whole beam combined has enough energy to melt tonnes of solid metal!
Salad bowls…Unfortunately we can’t roll particles around in salad bowls either. They travel in a beam pipe, which has all the air taken out so it’s under ultra-high vacuum. If there were any air left in there, the particles would scatter off it and get lost, which we want to avoid. To make the particles bend around a corner we use magnets. Dipole magnets do the bending, while more complicated magnets called quadrupoles, sextupoles and even octupoles do the job of beam focusing and other effects. The higher the energy of the beam, the stronger those magnets have to be. In the case of very high-energy machines like the LHC the magnets are often superconducting. This means that they can create very high magnetic fields, but it also means they have to be cooled down to cryogenic temperatures – in some cases just 1.8 K, that’s -271° C!
A bendy problem…In high-energy accelerators, bending a beam around a corner does something rather strange, it gives off synchrotron radiation. In an electron accelerator like Diamond this is exactly what they want to happen. The intense beams of radiation (mostly X-rays) are used to conduct experiments. They actually add in extra devices called wigglers and undulators in order to make as much synchrotron radiation as they can. But in an accelerator like the Large Hadron Collider this is an unwanted side effect, as it makes the beam lose energy. This energy loss can limit the maximum energy of a circular accelerator – we reach a point where the ‘kick’ from the radiofrequency cavities is just replacing the energy lost by synchrotron radiation.
The challenge of being able to do all these things at once means particle accelerators are always on the cutting-edge of technology. We’re constantly seeking to go faster, higher and better than before!
Teaching and extension activity ideas
One of the wonderful things about this demo or activity is how many areas of physics it involves all in one go. The other key thing to note is the deeper physics concepts you can introduce by discussing the difference between this 'accelerator' and one like the Large Hadron Collider at CERN. Here are a few ideas of what you could explain with the help of this demo or workshop, but feel free to add your own!
- Insulators & conductors
- Induced charges, transfer of charge
- How the Van De Graff generator works
- Static electricity, sparks and electrical breakdown of air
- The charge of fundamental particles
Taking it further...
- How a cyclotron works
- How a synchrotron works
- Magnetic fields bending particles
- Static voltage vs oscillating voltage
- Coulomb repulsion between particles or "space charge"
- Special relativity and velocity change with energy (which translates to frequency change with energy)
- Magnetic focusing - weak focusing and strong alternating gradient focusing
- How the LHC works
Thanks to Oxford Physics and JAI for supporting the APPEAL teacher days that this material was prepared for.
This demo was originally inspired by Fermilab's Todd Johnson who created a version using a 15kV DC power supply. You can read his advice and see his demo video, which I blogged about earlier.
Thanks to ASTeC/STFC for financial support for original development materials. Thanks also to Alom Shaha and Jonathan Sanderson of sciencedemo.org for the enthusiasm and discussion on the teaching possibilities of this particular demo.