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Fundamentals of X-ray free electron laser

Fundamentals of X-ray free electron laser

  • 2020-05-26
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Fundamentals of X-ray free electron laser

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  • 2020-05-26
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Unlike traditional lasers, FEL is produced without gain media and without the need for particle number inversion. Instead, coherent laser pulses of high brightness are produced by converting the kinetic energy of an electron beam accelerating to near the speed of light into photon energy. The wavelength of the FEL can range from far-infrared to X-ray wavelengths, and the wavelength of the output laser can be changed by changing the electron energy, magnetic field period and intensity. The characteristics of FEL are that the laser wavelength and pulse structure can be designed as needed and continuously adjusted over a wide range. According to the amplification gain, FEL can be divided into low gain and high gain FEL devices. Oscillator FEL is a typical FEL device with low gain. Due to the lack of reflective material suitable for vacuum ultraviolet and shorter wavelength, oscillator FEL is mostly used in THz and far infrared band. High gain FEL devices generally include electron gun, linear accelerator and wave oscillator. At present, the XFEL devices in operation and construction in the world mainly include SASE and HGHG. Since the amplified spontaneous emission is the basic principle of the electron linear accelerator is accelerated to nearly the speed of light, and the relativistic electron beam under the action of periodic transverse magnetic field (undulator) to approximate sinusoidal trajectory, and spontaneous emission in the tangent direction of the electron beam movement, the initial spontaneous radiation is low energy irrelevant and uniform distribution in the electron beam; The spontaneous radiation energy along the direction of the electron beam is continuously coupled with the relativistic electron beam in the oscillator. Specifically, the emitted photons interact with the electrons during each oscillator period, causing the electron beam density to be modulated periodically by the spontaneous radiation and to form a microbeam in the oscillator of sufficient length. The microbunched beam in turn amplifies only photons of some energy, reinforcing the spontaneous radiation and amplifying the positive feedback until the system reaches saturation.

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