Abstract
We report on experimental studies on the harmonic interaction between an optical laser and a relativistic electron beam in an undulator up to the 15th order. In this experiment, a significant energy modulation is imprinted on the beam longitudinal phase space through the electron-laser interaction when the laser frequency is the 3rd, 5th, 7th, 9th or 15th harmonic of the fundamental resonant frequency of the undulator. The experimental results are in good agreement with theory, and indicate that high harmonic interactions in undulators with large $K$ values and small phase errors can be quite efficient. The results confirm the basic physics of harmonic interaction with a goal toward ushering forward the development of many high harmonic based applications in free-electron lasers.
Highlights
When a relativistic electron beam goes through an undulator, monochromatic radiation at discrete frequencies is generated
The sinusoidal energy modulation produced by the laser-electron interaction is measured with a high resolution energy spectrometer
The measured peak energy modulations for the 3rd, 5th, 7th, and 9th harmonic interactions are shown in Fig. 6 and they are in good agreement with the theoretical values calculated with Eq (4) assuming an ideal undulator
Summary
When a relativistic electron beam goes through an undulator, monochromatic radiation at discrete frequencies is generated. In free-electron lasers (FELs), the radiation interacts resonantly with the electron beam in longer undulators, leading to an instability that amplifies the radiation exponentially while packing electrons into microbunches These density-modulated electrons radiate in phase and emit coherent radiation with orders of magnitude higher power than the spontaneous radiation in synchrotrons. In this paper we report on observation and measurement of harmonic interactions up to n 1⁄4 15, a regime where numerous undesirable effects (e.g. undulator phase errors [25]) start to play important roles in high harmonic applications In this experiment, an electron beam energy was varied incrementally from 120 MeV down to 54 MeV to examine the 3rd through the 15th odd harmonics with an 800 nm laser in a Nu 1⁄4 10 period planar undulator with u 1⁄4 55 mm and K 1⁄4 2:76. This suggests a promising path toward harmonic-based applications in modern FELs and IFELs
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