The world’s second X-ray Free Electron Laser (XFEL) recently went online in Japan, hot on the heels of the first, the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in the US, which began operating in the hard X-ray region in 2009.
The advent of lasers in 1960 led to a fundamental change in photon science and technology due to the unprecedented intensity, high degree of coherence and narrow pulse width of the light that lasers can emit.
Since then, tremendous efforts have been made toward creating shorter-wavelength lasers in the hard X-ray region, with expectations for fundamental changes in X-ray science and technologies similar to those seen in the infrared, visible and ultraviolet spectral regions. As the spatial resolution of observations using light is directly related to the wavelength of light used, one of the biggest advantages of using shorter wavelength, X-ray light is the significant resolution enhancement is provides—allowing observation of subnanometer-scale structures such as atoms and molecules.
X-ray lasers cannot be built with the same technologies used to create conventional, longer-wavelength lasers. Accelerator-based free-electron lasers, however, using a self-amplified spontaneous emission (SASE) scheme, are able to generate coherent electromagnetic radiation in the X-ray region. The SASE X-ray Free Electron Laser consists of a high-performance electron linear accelerator (LINAC) and a long undulator, in which high-energy, high-density, low-emittance electron bunches are alternatively deflected in a periodic magnetic field, causing them to emit quasi-monochromatic X-rays at an energy determined by the electron energy, the magnetic field strength and the magnetic period.
The interaction between the electromagnetic field of the emitted X-rays and the electron bunch as it travels through the long undulator eventually aligns the electrons in the bunch with the period of the X-ray wavelength. The principle of SASE is that the aligned electrons move coherently in the magnetic field of the undulator to emit coherent X-rays.
Construction projects for SASE XFEL facilities began to be discussed in the US and Europe around 2000, and later materialized as the LCLS and Euro-XFEL projects. At that time, an 8 GeV electron storage ring for synchrotron radiation facility, SPring-8, was being commissioned in Japan. SPring-8 was one of three large-scale synchrotron radiation sources in the world at the time, alongside European Synchrotron Radiation Facility (ESRF) in France and the Advanced Photon Source (APS) in the US. As the technologies developed for the construction of SPring-8 are very similar to those necessary for an XFEL, the concept of the SPring-8 Compact SASE Source (SCSS), a prototype of an SASE XFEL, emerged.
To support this prototype, an in-vacuum undulator technology, higher-frequency accelerator tubes in the C-band (5,712 MHz) and a combination of a classical thermionic electric gun with a CeB6 single crystal as cathode were adopted. Based on this concept, an SASE XFEL with an 8 GeV LINAC capable of emitting coherent electromagnetic radiation at a wavelength 0.06 nm was designed. This combination of technologies allowed the facility to be constructed at a third of the length of the LCLS or Euro-XFEL facilities, measuring just 700 m.
Following the prototype SCSS project, the construction project for the XFEL was launched in FY2006 as one of the Japanese government’s ‘Key Technologies of National Importance’. At the end of FY2010, all the hardware was in place, and the facility was named the SPring-8 Angstrom Compact Free Electron Laser, or SACLA.
One of the unique features of the facility compared with the LCLS and Euro-XFEL, is its co-location with the SPring-8 synchrotron radiation facility. A new building was set up to allow the XFEL and SPring-8 X-ray beams to intersect at a sample, which will make it possible to use SPring-8 rays to observe how materials relax following an instantaneous impact from the XFEL beam. An electron beam transport from the XFEL LINAC to the SPring-8 storage ring was also built to allow the XFEL LINAC to be used as an injector for SPring-8. Electron beam commissioning for this brand-new SACLA facility began in March 2011.
On June 7, the first SASE lasing was observed at SACLA. Plans now call for reaching SASE saturation and for the initiation of X-ray optics commissioning and end-station commissioning in FY2011. In March 2012, the facility will be opened to public users from around the world.
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