Next generation advanced light source science

Recent advances in accelerator science make feasible the provision of XUV and harder X-ray FELs that will generate short (fs regime) pulses of light that is broadly tuneable and >106 times more intense than spontaneous undulator radiation. Energy recovery technology offers the promise of short pulse, high peak flux spontaneous radiation, with particular advantages in the IR and THz parts of the spectrum. The new science enabled by these 4th generation sources is reviewed. A key feature is dynamic measurements. Pump-probe experiments will allow real-time measurements of reaction pathways and short-lived intermediates. The high intensity of FEL radiation will allow very high resolution in imaging applications. The very high field intensity of the XUV radiation will lead to the creation of new states of matter, while at the highest X-ray energies, the goal is to achieve single molecule diffraction. Illustrations are provided of some of the experiments proposed in the Science Cases for the major world 4th generation projects. Some of the science already undertaken using IR and UV FELs, and results obtained from new XUV sources (such as FLASH at DESY) are discussed.