China is joining the elite club of countries that have equipped researchers with the potent sources of high-energy photons called free electron lasers (FELs). The Dalian Coherent Light Source, whose completion was announced today in Beijing, has a twist that makes it unique: It is the only large laser light source in the world dedicated to the particular range of short-wavelength light called vacuum ultraviolet, which makes it “a new tool for the detection and analysis of molecules undergoing chemical reactions,” says Alec Wodtke, a physical chemist at the University of Göttingen in Germany.
Scientists around the world have rushed to build FELs over the past decade because they produce vastly brighter light, in shorter pulses, than synchrotrons, the particle accelerators that have been the workhorses of protein crystallography and cell biology and materials science. In synchrotrons, electrons go whizzing around a storage ring a kilometer or more in circumference. As their paths bend, the electrons throw off photons that are formed into beams.
In contrast, FELs fire electrons from a linear accelerator into an undulator, in which magnets of alternating polarity push and pull the electrons along a sinuous path. As the electrons round each bend, they produce photons. Interactions between the electrons and the accumulating photons as they travel through the undulator generate coherent laser light (Science, 10 May 2002, p. 1008).