Door to the linear accelerator. Radiation inside means that access is restricted.

The linear accelerator, or linac, is where all of CERN’s particles begin their travels. In this simple and elegant machine, particles are accelerated by an electric field. This field, which is actually radio waves, moves along a straight tube that is under a vacuum of about one billionth of an atmosphere of pressure.

A view of the linac.

Because the electric field occurs in waves, it has peaks and valleys. The particles are accelerated by the peaks, and would be decelerated by the valleys. But in the linac, a set of drift tubes shields the particles from the valley portion of the waves: the particles move peak to peak through the drift tubes, bypassing the valleys.

The peaks and valleys of the accelerating radio waves occur in a regular pattern over time. But with each pass between drift tubes, the particle is being accelerated, and covers more and more ground per second. Therefore, the drift tubes must get longer and be placed farther apart.