Electrons are Accelerated in a Copper Structure
| Bunches of electrons are accelerated in the copper structure of the linac in much the same way as a surfer is pushed along by a wave. |  |
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In the linac, the wave is electromagnetic. That means it is made up of changing magnetic and electric fields. Think of a magnetic field as a region of space where magnetic effects can be detected - one magnet pulling or pushing on another, for example. Similarly, an electric field is a region of space where electric effects can be detected. You can make an electric field by removing electrons from one substance and putting them on another. The region of space between the two substances then contains an electric field. An example is rubbing an inflated balloon on your hair. The effect is to make your hair stand on end. |
The electromagnetic waves that push the electrons in the linac are created by higher energy versions of the microwaves used in the microwave oven in your kitchen. The microwaves from the klystrons in the Klystron Gallery are fed into the accelerator structure via the waveguides. This creates a pattern of electric and magnetic fields, which form an electromagnetic wave traveling down the accelerator. |
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The 2-mile SLAC linear accelerator (linac) is made from over 80,000 copper discs and cylinders brazed together. Inside the accelerator structure, the microwaves from the klystrons set up currents in the copper that cause oscillating electric fields pointing along the accelerator as well as oscillating magnetic fields in a circle around the interior of the accelerator pipe. The trick is to have the electrons or positrons arrive in each cell or cavity of the accelerator just at the right time to get maximum push from the electric field in the cavity. Of course, since positrons have opposite charge from electrons, they must arrive when the field is pointing the opposite way to be pushed in the same direction. |
| The size of the cavities in the accelerator is matched to the wavelength of the microwaves so that the electric and magnetic field patterns repeat every three cavities along the accelerator. |
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This means, in principle, there could be electron bunches following one another three cavities apart, and positron bunches half way in between. Usually the spacing between the bunches is kept somewhat larger (though always in multiples of three cavities for the same sign particles).
Notice how far the bunches have moved
after just 1/20,000,000,000 of a second!
