The linear accelerator, or linac, is the electromagnetic catapult that brings electrons from a standing start to relativistic velocity--a velocity near the speed of light. Here is a photo of the ALS linac.

The linac is four meters long--not a great distance in which to get even an electron from zero to almost 300,000 kilometers per second. How is it possible? Here's a simplified drawing that shows how a linear accelerator works:

The major parts of a linear accelerator are:

  • The electron gun
  • The buncher
  • The linac itself
Each part is responsible for a stage in the acceleration of the electrons.

The Electron Gun

The electron gun, located at the left in the drawing, is where electron acceleration begins. The electrons start out attached to the molecules in a one-cubic-centimeter chunk of barium aluminate. This is the cathode of the electron gun. A cathode is a surface that has a negative electrical charge. In linac electron guns this charge is usually created by heating the cathode. Barium aluminate is a "thermionic" material; this means that it's electrons tend to break free of their atoms when heated. These electrons "boil" near the surface of the cathode.

The gate is like a switch. It consists of a copper screen, or "grid," and is an anode. An anode is a surface with a positive electrical charge. Every 500 millionth of a second the gate is given a strong positive charge that causes electrons to fly toward it from the cathode in tremendous numbers. As these electrons reach the gate, they become attracted even more strongly by the main anode, and pass through the gate.

Because the gate is pulsing at a rate of 500 million times per second (500 MHz), the electrons arrive at the anode in loose bunches, a 500 millionth of a second apart. The anode is a torus (a doughnut) shaped to create an electromagnetic field that guides most of the electrons through the hole into the next part of the accelerator, called the buncher.


The Buncher

The purpose of the buncher is to accelerate the pulsing electrons as they come out of the electron gun and pack them into bunches. To do this the buncher receives powerful microwave radiation from the klystron. The microwaves accelerate the electrons in somewhat the same way that ocean waves accelerate surfers on surfboards. Look at the following graph:

The yellow-orange disks are electrons in the buncher. The curve is the microwave radiation in the buncher. The electrons receive more energy from the wave--more acceleration--depending on how near they are to the crest of the wave, so the electrons riding higher on the wave catch up with the slower ones riding lower. The right-hand wave shows the same group of electrons a split second later. On the front of the wave, the two faster electrons have almost caught up with the slower electron. They won't pass it though, because they are now lower on the wave and therefore receive less acceleration.

The higher electron on the back of the wave gets just enough acceleration to match the speed of the wave, and is in the same position as it was on the left-hand wave. This represents the last electron in the bunch. The lower electron on the back of the wave gets too little energy to keep up with the bunch and ends up even lower on the right-hand wave. Eventually it will fall back to the electron bunch forming one wave behind.


The Linac

The linac itself is just an extension of the buncher. It receives additional RF power to continue accelerating the electrons and compacting them into tighter bunches. Electrons enter the linac from the buncher at a velocity of 0.6c--that's 60% of the speed of light. By the time the electrons leave the linac, they are traveling very close to the speed of light.
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