After electrons leave the electron gun they enter the linear accelerator, or linac. Here the electrons are accelerated to close to the speed of light before they enter the booster synchrotron.
In theory this could be achieved be passing the electrons through a very strong magnetic field. However in practice it is not possible to generate high enough voltages so a linac achieves the same goal by acclerating electrons through a series of smaller fields.
The electrons pass through a series of chambers with electric fields which alternate as the electrons pass through. This way the electrons always see a positive field ahead of them, which they accelerate towards. You can use AUTO FLIP to alternate the electric fields automatically, or use the FLIP button to try this for yourself!
You will notice that the tubes get longer as they go along: this is so that all the tubes can be powered by a single oscillator. When the electrons are travelling slowly they only travel a short distance before the polarity changes. The faster they go the further they travel before the polarity changes again.
At very high energies the electrons approach the speed of light, and increasing their energy no longer increases their speed. Instead, relativity means that their mass increases instead.