Fermilab

Bremstralung

To Fermi Lab Physicists:

I was told that in particle accelerators, not linacs, energy is released at distinct points along the circumference of the accelerated particle's path, where magnets are placed. I am not talking about 'kicker' or 'wiggler' magnets that DO increase the energy of the particle, but the permanent magnets that are placed in order to change the path of the particle.

I was also told that many accelerator labs take advantage of this phenomenon and set up laboratories beside the points where the energy is released. I was also told this energy was in the form of x-rays. I know work cannot be done on the particles because the magnetic force acts perpendicular to the particle's motion, so where do the high energy photons come from?

Tarek

Dear Tarek,

You have almost got it, but you are confused on some basic facts.

1. Charged particles radiate electromagnetic energy (synchrotron radiation) in response to acceleration.

2. There are two types of acceleration: acceleration parallel to the direction of motion increases the energy (or speed) of a particle, but acceleration perpendicular to the direction of motion changes its direction only.

3. Magnetic fields only bend particles (perdendicular acceleration), they do not change the energy of the particle.

4. Electric fields can increase (or decrease) the particle energy. The can also be used to bend particles, but, except in special circumstances, it is easier to use magnetic fields for bending.

5. The radiation depends on the mass of the particle: in high energy accelerators electron radiation is a dominant design consideration, proton radiation is small, usually negligible.

The following more or less follows from the above facts: Linear electron accelerators do not produce much synchrotron radiation. Circular electron accelerators and storage rings produce copious synchrotron radiation in all the bending magnets, especially when the magnetic fields are high or the beam energy is high. In a circular machine, the radiation is caused primarily by the bending magnets. When the particles radiate, they loose energy. The particle energy is replentished by a radio frequency cavity that uses an electric field to increase the energy of the particles. (Recall that magnetic fields do not change the beam energy, you need an electric field to do that). Synchrotron radiation is a huge nuisance when the goal is to accelerate electrons. However, a variety of scientists including physicists, earth and environmental scientists, chemists, and biologists can (and do) use synchrotron radiation in their research. Special machines called "light sources" have been built to provide a source of synchrotron radiation for these researchers. (The term "light" is meant in a general sense, synchrotron radiation is not necessarily visible to the human eye. Most light sources produce high frequency, short wavelength radiation known as x-rays). These researchers have a voracious appetite for synchrotron radiation. It is possible to produce even more synchrotron radiation with the introduction of special magnets called "wigglers." These magnets consist of an alternating set of bending magnets. The net bend in a wiggler magnet is zero. But all the acceleration back and forth (the wiggling) throws off synchrotron radiation like crazy. (You need more radio frequency power to compensate for the energy lost in the wiggler magnets). The intensity and frequency of the light is determined by the parameters of the wiggler magnet. Most synchrotron light sources look at the "light" coming from the bending magnets and from special wiggler magnets that are installed in the ring. You can get more details from the web pages of the many synchrotron light sources around the the world.

John Marriner

Electron storage rings

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