Edge Radiation Project

The NSLS has established itself as the world leader in storage ring based infrared science. The source of the infrared radiation for this work is the incoherent emission of synchrotron radiation by electrons in the body of the dipole magnets of the NSLS VUV ring. Interest has arisen in developing new infrared sources of radiation with different spectral and angular characteristics.

Edge radiation is the name given to the light emitted by an electron as it traverses the fringing fields of a dipole magnet. In the simplest model of edge radiation one can treat the fringe field as a “sharp edge transition” from zero field to the full field value. This impulsive acceleration gives rise to the radiation, which for low frequencies, has the same spectral and angular distribution as transition radiation when viewed in the far field. Loosely, one might think of edge radiation as “magnetic transition radiation”; the electrons don’t hit a foil, but they do receive an impulse acceleration when going from zero field to a finite value.

In this simple model the far field radiation spectrum is “white” and all the radiation is emitted within an opening angle of 1/g. Clearly the radiation spectrum does not extend to infinite frequencies, a cutoff depending on the fringe field length must be introduced. There has been considerable discussion in the literature about the potential advantages of this type of infrared source, which could possibly yield radiation with a much smaller opening angle than is possible with the synchrotron radiation from the body of the dipole magnets.

In a typical experimental setup it is difficult to isolate radiation from a single edge; usually edges occur in pairs with some intervening free space and perhaps other magnetic elements such quadrupoles, sextupoles, etc. With a pair of edges, there is the potential for constructive or destructive interference which can modify both the spectral and angular distribution of the edge radiation.

In the SDL Coherent Edge Radiation Experiment we propose to measure the infrared edge radiation emitted by the electrons in the pair of dipole magnets in the NISUS bypass section. Since there are two dipoles in close proximity we will be able to explore the interference effects of the two edges. Because the electron bunches are generated by a photocathode gun they will have a very short bunch length of 100 fs (30 microns). When electrons are packaged in such very short bunch lengths they radiate coherently, proportional to the number of electrons in the bunch, for wavelengths on the order of p times the bunch length and longer.

The experiment to characterize the coherent edge radiation should provide insight into the usefulness of this radiation as a source of infrared radiation for NSLS users. In addition, edge radiation plays a key role in several models of the transient effects of coherent synchrotron radiation that is a concern for bunch compressors in numerous short pulse FEL experiments. A better understanding of edge radiation may have an impact on these important experiments.