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keywords: Free Electron Laser, FEL
Taiwan is Developing a New Generation Light Source – Free Electron Laser


The NSRRC R&D team and the FEL facility.

A Free Electron Laser (FEL) is radically a new type of light source that uses electron beam as lasing medium. The electron beam is accelerated close to the speed of light by a radio frequency linear accelerator (LINAC). Bursts of intense coherent radiation can be generated from the beam as it propagates through a periodic magnetic field. FELs offer radiation of extremely high intensity and good coherence so as to explore matter in greater detail at atomic and fast time scales. This new type radiation source is sometimes called the 4th generation light source that opens up new frontiers in various areas of science.

National Synchrotron Radiation Research Center (NSRRC) is now developing key technologies for future FELs. An exciting breakthrough has been achieved on July 20th, 2017 when the high quality electron bunches were successfully brought to relativistic energies (about 30 MeV) and produced coherent undulator radiation in the THz region for the first time. This new radiation source is also known as “THz Prebunched FEL”. The NSRRC team now continues to optimize the parameters for higher THz output power. As a major technical challenge in the near future, quality electron beam of higher energy will be produced for Vacuum Ultraviolet FEL (VUV FEL) or even Extreme Ultraviolet FEL (EUV FEL) operation.

A critical hurdle to realize a VUV FEL is to design a “High Brightness Electron LINAC System” that delivers high energy electron beam with excellent beam quality. “Bunch Compression” is an indispensable technique to produce sub-picosecond (10-12 sec) electron bunches for higher beam current. One unique part of the VUV FEL is the design of a “Nonlinear Bunch Compressor” that can be used to shorten the electron bunch duration down to the few tens femtoseconds (10-14) level to enhance beam current without the use of an extra harmonic microwave linac system. Thanks to the invaluable experiences accumulated from the construction of Taiwan Photon Source (TPS), this VUV FEL will be designed and assembled completely in house by the NSRRC professionals. If successfully implemented, it will be the first of such sub-system in the world being used in FELs.

TPS is able to deliver broadband radiation ranging from infrared to hard X-ray which could accommodate more 40 specially designed beamlines for various experiments at the same time. In comparison to TPS, the VUV FEL is a tunable coherent radiation source with expected peak brightness that is ten million times (108) higher in its frequency range and the pulse duration is a thousand times shorter. Such capabilities permit scientists to explore the ultrafast dynamics of matter at femtosecond time resolution. The complementarity of the two radiation sources offers various opportunities for future research teams.

Currently USA, Europe and Japan have completed their FEL facilities while Switzerland, South Korea and China are in the phases of construction or commissioning. FELs and synchrotron radiation are both considered to be the most important light sources in the following decades and allows us to revolutionize our understanding of matter in fields of semiconductor, physics, chemistry, life sciences, medical applications, energy science, environmental studies etc. in the future.