We discuss recent results on soft and hard X-ray free electron lasers (FELs) and how they can be used to design and optimize the next generation of these sources of high brightness, coherent photons, with femtosecond pulse duration, or very narrow linewidth. In particular, we consider the experimental and theoretical progress in the electron beam generation and manipulation. These results, when combined with the successful development of powerful simulation codes, can be used to design optimized, high intensity sources of coherent photons, and to reduce their size and cost.
Systematic analysis about the polarization in thermal radiations of magnetars