Fourth generation light sources

As remarkable as this performance improvement has been, even higher brightness and laser-like coherence appear achievable and are needed scientifically, particularly at soft and hard X-ray wavelengths. Reaching higher performance levels is the goal of fourth generation sources which we may define as sources which exceed the performance of previous sources by one or more orders of magnitude in an important parameter such as brightness, coherence, or shortness of pulse duration. The most promising directions for fourth generation sources in the wavelength range from the UV to hard X-rays are storage rings with even lower emittance than third generation rings, and short wavelength free-electron lasers (FELs) which offer sub-picosecond pulses with full transverse coherence.