Accelerators history 3
 In the early 1900s, Ernest Rutherford had found out about the structure of the atom by firing alpha particles at it. Nowadays, physicists probe the structure of sub-atomic particles and make new particles using huge particle accelerators. These accelerators led to the description of the structure of protons and neutrons because physicists developed the theory of the quarks to explain all the particles that have been made in them. But how do accelerators work?

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 Particle accelerators A particle accelerator works on the same principle as the cathode ray tube (like the ones in a television). Electrons are negatively charged so they are repelled from the negative electrode and attracted towards the positive one. The voltage across the electrodes is usually a few thousand volts. This will give the electron enough energy to make a small glow on the fluorescent screen of a TV tube. However, in physics experiments, we want the electrons to have more energy. So we need a bigger voltage to push the electrons harder and increase their kinetic energy. Turning up the voltage

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 Figure 1. The air between the lectrodes can withstand a voltage of about 10 000 volts for each centimetre of the gap. Once the voltage gets bigger than this, there will be sparks and then the air will break down.
 Unfortunately, if the voltage goes above a few thousand volts the air between the electrodes breaks down and we get sparks (figure 1). We cannot keep increasing the voltage to give the electrons one huge push. Instead, we have to give the electron a sequence of smaller pushes with manageable voltages. The simplest way to do this is in a linear accelerator. A huge electron gun In a linear accelerator, the electrons pass through a series of electrodes. If the electrons are to keep accelerating then they must always be leaving a negative electrode and heading towards a positive one. Therefore the voltages on the electrodes have to be switched over as the electrons pass through. Look at figure 2.
 Figure 2. The principle of a linear accelerator. A sequence of electrodes keeps on pusshing a beam of particles.
 Electrode A is negative and B is positive so the electrons accelerate to the right. Before they pass out of B, it is made negative and C becomes positive to keep pulling them to the right. The voltage is switched back again to make D positive and C negative so they are pulled towards D. The electrons are travelling close to the speed of light (300, 000 km/s). So the voltages have to be switched over very quickly. The frequency of the alternating voltage is a few hundred kilohertz (a radio frequency). Notice that electrode D is longer than electrode B. This is because the electrons are travelling faster by the time they reach D. So, if they are to spend the same time in D, it has to be longer.
 More tunnels The longest linear accelerator is at Stanford in California in the USA. It is 3 km long and has an effective accelerating voltage of 30 GV (thirty thousand million volts). To get more acceleration would require even longer accelerators. Alternatively, the linear accelerator has to bent around on itself. This kind of accelerator is called a synchrotron.

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The story so far
 • Charged particles can be accelerated using a large voltage • There is a limit to how big a voltage can be used • Linear accelerators use a series of electrodes that keep bumping up the particles' energy • Linear accelerators are limited by their length
 Question H3

a) Particle physicists want to get particles going faster and faster. Why can't they simply increase the voltage between two electrodes?

b) In a linear accelerator, the voltages on the electrodes have to be swapped over as the particles go down the tube. Why is this

c) Why do the electrodes get longer as you go down the tube?

 The story so far The atom has a positive nucleus The nucleus contains protons and neutrons Physicists want to know what the protons and neutrons are made of