COUR Project

An upcoming experiment is to measure coherent off-axis undulator radiation (COUR). Ultimately we hope to produce a non-interrupting bunch length diagnostic, as described below.

What is COUR?

COUR is a type of radiation that is produced when an electron beam passes through an undulator. Most interest in undulator radiation involves observing the radiation that is emitted only in the forward direction, or on the beam axis. In this experiment we measure off-axis undulator radiation by looking at radiation which is emitted at angles to the beam axis.

As for the "C" in "COUR", radiation whose wavelength is longer than the electron bunch will be radiated coherently. That is, the waves emitted by the electrons have roughly the same phase. In contrast, when the emitted wavelength is shorter than the electron bunch, the radiation is incoherent. In this case, the waves emitted by the electrons interfere with each other and do not have a definite phase relationship with each other. Coherent radiation is more intense than incoherent radiation by a factor equal to the number of electrons in the bunch. Since a bunch typically has on the order of 109 electrons, the difference between coherent and incoherent radiation is significant.

Since a range of wavelengths are emitted when an electron bunch passes through the undulator, some may be coherent and some may be incoherent, depending on their wavelength. The total amount of coherent radiation present depends on the length of the electron bunch.

Why do we want to measure COUR?

There are two purposes to this experiment. The first is to measure COUR and show that it exists and has the properties that theory predicts it has.

The second purpose is to try to use this radiation as a diagnostic tool to measure bunch lengths. As stated above, the total amount of coherent radiation emitted depends on the bunch length. Specifically, the shorter the bunch length, the more radiation with a wavelength longer than the bunch length, and thus the more coherent radiation there is. So, more radiation is produced by shorter bunch lengths. We can measure the off-axis radiation emitted from an undulator and see if the bunch length is increasing or decreasing by seeing if the radiation decreases or increases.

This diagnostic is non-interrupting because we are observing the radiation off-axis and therefore do not need to block the electron beam. Furthermore, the undulator that is used is short and therefore will disturb the electron beam only negligibly. This is a useful characteristic which will allow us to measure bunch lengths while still using the electron beam for other applications.

When the DUV-FEL is operating, it is important to know what the bunch length is. If we could adjust the bunch length while the DUV-FEL is operating, we would optimize performance of the FEL. The COUR bunch length diagnostic will be able to do this. We will be able to measure changes in the bunch length as we adjust it or as we adjust other parameters. Since the diagnostic does not interrupt the beam, we can optimize the FEL while it is operating!

This doesn't look like physics – Where are the equations?

If you'd like to know more about the theory, please see our publication in the online journal Physical Review Special Topics – Accelerators and Beams.

How do we measure the radiation?

The experimental setup is shown below. The electrons pass through the undulator and the radiation is reflected by a mirror into a light pipe. The light pipe directs the light into the detector. The detector is a liquid-helium-cooled bolometer detector that operates at a temperature of 4K (-269° C) so that it can be sensitive enough to detect the radiation emitted.

Who is working on this?

Of course, the whole SDL team is working on this. This project is the basis for the doctoral dissertation by Charles Neuman, a graduate student affiliated with Duke University. His advisor is Dr. Patrick O'Shea, who is an accelerator physicist at the University of Maryland.

What practical applications does this project have?

First of all, the "non-practical" aspects of this project are interesting and may be very useful. We hope to learn more about undulator radiation in general. This will add to our knowledge of a subject that is both interesting and useful. To read more about free electron lasers, visit the Virtual Library on Free Electron Research and Applications.

The main practical application of this project is to help the DUV-FEL, or other similar devices, operate better. To see why the DUV-FEL is interesting and useful, you can read more about it.