Wavelenghth switching in an optical klystron

Abstract A symmetric optical klystron consists of two identical undulator sections separated by a dispersive region. For a device of a fixed length, an optical klystron is capable of producing more bunching, and therefore more gain, than a traditional undulator. It is also possible for the gain spectrum of an optical klystron to have multiple positive peaks. If the strength of the dispersion section is adjusted to provide nearly equal gain on the largest of these peaks, then they will compete, and the FEL may switch wavelengths based on noise, cavity length, or other perturbations. We provide the first observations of this behavior, using the FIREFLY system at the Stanford Picosecond FEL Center. In FIREFLY, relative wavelength switching by more than 3% - more than twice the laser linewidth - has been observed by varying dispersion section strength. Stable switching has also been observed as a function of cavity length.