# What is a linear accelerator? How does it work?

SLAC 2 February 1995

• Your TV is an accelerator too!

The simple answer is: An accelerator works the same way as your television tube, which is a small electron accelerator with a built-in electron detector -- the screen.

In a TV, electrons escape from a hot metal cathode in much the same way as water molecules escape from the surface of a hot pot of water. The electrons are accelerated to a small speed by the effect of an electric field, due to the potential difference between the cathode and an anode a few centimeters in front of it. They are then steered by magnetic or electric fields transverse to their motion to hit a particular spot on the screen. The screen is covered by a material which gives off light when struck by an electron, thus producing the picture. To see a figure, click here .

Most TVs use electric potentials between parallel plates for steering, rather than magnetic fields from electromagnets. We also mention the possibility of using magnets because that gives a closer parallel to the way we steer electron beams at SLAC.

To obtain a "polarized" electron beam a different electron source is used: Polarized light from a laser is aimed at a carefully-prepared gallium arsenide surface. Electrons are knocked out of the surface by the laser light and collected into the accelerator by accelerating them across a small potential difference. Polarized electrons have their spins (internal angular momentum) aligned in a particular direction, in the same way that polarized light corresponds to an alignment of the angular momentum of the photons in the travelling electromagnetic wave we call light.

• Electric Fields Increase the Energy of the Electrons

The electrons at SLAC are produced in much the same way as in a TV. After the first acceleration by the electric field due to the potential difference between the cathode and the anode, they enter a long copper pipe-like structure which is the accelerator itself.

This accelerator pipe is pumped out to produce a very good vacuum, so that there is nothing inside it to deflect electrons travelling down the accelerator. It contains discs every 3.6 cm with a small hole in the center (2 cm in diameter). The beam travels through these holes. The discs make the pipe into a series of resonant cavities, each about the size and shape of a tuna-fish can. To see a figure of a cutaway view of the accelerator structure, click here .

By feeding microwave power into this structure we make pulses of electric field which effectively travel along the structure and provide further acceleration for the electrons travelling with them. The electrons ride the microwaves like surfers on a water wave, gaining energy from the force on them due to the electric field. The electron "beam" is actually a series of bunches of electrons timed to enter each cavity just when the electric field in that cavity is close to its maximum and pointing in the right direction to accelerate the electrons down the accelerator. The bunches sit a little ahead of the peak of the wave. Any electron that falls a bit behind the bunch feels a greater accelerating field and hence catches up with the bunch. Conversely any that get a bit ahead feel a weaker field and hence are less accelerated, so the bunch catches up with them. To see a figure, click here .

To describe the same thing in a slightly more technical fashion: The cavities respond to the microwave input by resonating in a particular mode of microwave oscillation that has an electric field component along the axial direction (i.e. along the accelerator) and a cylindrical magnetic field around the inside of the pipe. This electric field is just what is needed to accelerate electrons or positrons down the accelerator. The magnetic field provides an inward force towards the center of the accelerator that compensates for the electrical repulsion among the many electrons in a bunch. The relative phase of the wave is opposite in neighboring cavities, but the spacing of the cavities and the frequency of their oscillation is chosen so that the electrons travelling at close to the speed of light are only in a cavity when the electric field is pointing in the right direction to accelerate them down the pipe.

• Positrons can be accelerated too.

The positron is the antiparticle of the electron. It has the same mass and the same magnitude of electric charge, but positive instead of negative charge. When positrons are accelerated the positron bunches are timed to enter the cavities when the electric and magnetic fields are reversed compared to the electron case, so they too are accelerated down the pipe by the electric field and bunched together by the magnetic field.

The positrons are produced by extracting accelerated electrons from part way down the accelerator, and colliding them with a target. One of the possible processes that can occur is that the incoming electron is deflected by collision with a target atom and then radiates an energetic (virtual) photon which produces an electron and a positron. The positrons are collected using electric and magnetic fields to control their motion, and are sent back to the beginning of the accelerator.

• Magnetic Fields are used to steer electrons.

At the end of the accelerator we use large magnets to produce a magnetic field perpendicular to the direction of the beam. An electron moving in a magnetic field is subject to a force that is perpendicular to its motion and hence will be deflected. The magnets are used to steer the electrons to the various experimental areas. For some types of experiments, electrons and positrons are fed into storage rings in which they circulate many times. In the storage rings dipole magnets (i.e. magnets with the two opposite pole tips arranged above and below the beam) are placed periodically around the ring to produce a magnetic field perpendicular to the beam. This gives the centripetal acceleration needed to make the electrons follow the curved path. More complicated magnets, such as quadrupoles (with four pole tips) and sextupoles (with six) are placed at regular intervals along the beam pipe. These act on the beam like lenses in a beam of light, and are used to keep the bunches well focussed and travelling in the center of the storage ring structure as desired.