n undulator, in use at the Advanced Light Source at DOE's Lawrence Berkeley National Laboratory. Each undulator contains two 4.55-meter-long arrays of permanent magnets with alternating polarity.The arrays are supported by a superstructure capable of resisting the force of their attraction—up to 42 tons (the weight of a 38,000 kg mass). As an electron beam passes through a vacuum chamber between the arrays, the magnets cause the beam to curve back and forth and thus to produce synchrotron radiation. Undulators produce light brighter than that from other types of synchrotron radiation sources, with the added characteristics of partial coherence and linear polarization. In this photograph, a strobe light emulates the electron beam.