Compact X-ray free electron laser by transverse gradient undulator

The X-ray free electron laser (XFEL) is a new type of light source, which can provide coherent, high-flux, ultra-short photon pulses in the soft and hard X-ray energy region. Until now, a long linear accelerator as well as long linear undulators have been thought indispensable, because the principle is based on the self amplified spontaneous emission (SASE). Indeed, both FLASH at DESY and LCLS at SLAC, which are the world's first X-FEL facilities in soft and hard X-rays, respectively, are facilities on a huge scale. Recently, Dr. Z. Huang (SLAC National Accelerator Laboratory, USA) and his colleagues have published a very interesting idea for a compact XFEL facility that uses an ultra-short pulse laser instead of an ordinary linear accelerator. It is known that laser-plasma accelerators can produce high energy electron beams with low emittance, high peak current but a rather large energy spread, which makes it difficult to consider XFEL applications. Their main strategy is the introduction of a transverse field variation into the FEL undulator. In their calculation, such a transverse gradient undulator together with a properly dispersed beam can greatly reduce the effects of electron energy spread and jitter on the performance of XFEL generation. For more information, see the paper, "Compact X-ray Free-Electron Laser from a Laser-Plasma Accelerator Using a Transverse-Gradient Undulator", Z. Huang et al., Phys. Rev. Lett. 109, 204801 (2012).