First observation of laser–beam interaction in a dipole magnet
Free electron lasers (FELs), which use relativistic electron beams to generate short-wavelength radiation with high brightness and ultra-fast time structure, are cutting-edge instruments in many fields, such as biology, nonlinear physics, and material science. As a new generation light source, unlike synchrotron light sources, the amplification of FEL pulses comes from the strong and continuous interaction of electromagnetic waves and relativistic electron beams in a periodic lattice of alternating dipole magnetic fields, which is known as an undulator.

The three most critical processes in FEL physics include energy modulation, bunching, and power amplification. In the FEL process, the interaction between electromagnetic waves and electrons results in energy modulation of the electron beam. The energy modulation evolves into a longitudinal density modulation in the FEL wavelength range, known as bunching. The bunching contributes to the growth of the FEL power, and the amplified FEL power further enhances and speeds up the bunching. This positive feedback loop is the basis of the modern XFELs. With the tremendous development of FELs in the past three decades, the power amplification has been well measured in each FEL. While all the previous experiments are based on tens of undulator periods at least, the snapshot of energy exchange within the sub-period of an undulator still lacks direct measurement.

Fig. 1. Interaction between laser and electron beam in a dipole magnet.

Fig. 2. Experimental characterization of the laser-beam interaction in the dipole magnet. (a) Longitudinal phase space of the electron beam after the interaction. The red dashed line represents the central energy of the electron beam. The orange box contains areas that are altered due to the laser-beam interaction. Beam head is on the left. (b) Measured coherent radiation intensity and fitted curves after the electron beam passes through the first chicane under different laser pulse energies. (c) Calculation results of the energy modulation amplitude and the initial slice energy spread using the coherent radiation generation method.

Researchers from the Shanghai Advanced Research Institute and the Shanghai Institute of Applied Physics of the Chinese Academy of Sciences recently experimentally demonstrated the interaction between an ultraviolet laser and a relativistic electron beam in a pure dipole magnet. Based on the Shanghai soft X-ray FEL test facility, a 266-nm laser was used to modulate an 800-MeV electron beam. The energy modulation of the electron beam was observed directly by an X-band transverse deflecting structure and measured as 40 keV. Further, the use of such energy modulation to drive a seeded FEL has also been experimentally demonstrated, that is, lasing at the sixth harmonic of the seed laser in a high-gain harmonic generation scheme.

This work completes the last indispensable experimental measurements of FEL physics. The experiment reveals the most fundamental process of the FEL lasing and open up a new direction for the study and exploitation of laser-beam interactions. The results prove that a short dipole magnet can serve as an effective tool for introducing energy modulation of relativistic electron beams, which opens a new path for the development of compact laser heater systems for high-brightness XFEL, stable energy modulators for plasma accelerators based XFELs, and even novel radiators for future coherent light sources.

The article about the first observation of laser-beam interaction in a dipole magnet has been published in Advanced Photonics on July 29, 2021. Yan Jiawei, PhD candidate is the first author. Deng Haixiao, Researcher, and Zhao Zhentang, Academician, at Shanghai Institute of Applied Physics, Chinese Academy of Sciences are the co-corresponding author. This research was supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, the Chinese Academy of Sciences, and Shanghai.


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Translated by English Translation & Edit Group, SINAP