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Fig. 9-1 Principle of monochromatic radiation with a quasi-periodic undulator and a crystal monochromator A crystal monochromator selectively diffracts a fundamental wave and its harmonics. A quasi-periodic undulator can generate noninteger harmonic waves, so that a purely monochromatic radiation required for measurement can be obtained through a crystal monochromator.

Synchrotron radiation is generated by a high-energy electron beam passing through magnetic field where the electron beam is deflected by the Lorenz force. This radiation has a continuous wide spectrum ranging from far infra red to x-ray wavelengths. It is very intense and highly directional. This radiation is a very useful tool for investigating the physical and chemical properties of materials by measuring their response to the radiation, such as diffraction, absorption, scattering, and resonance. Therefore this radiation has been applied to studies in the fields of science, such as physics, chemistry, medical science, bioscience, materials science, earth sciences, etc. The interaction between the radiation and material depends on the wavelength of the radiation, so an intense monochromatic radiation at a wavelength specific to the purpose of the investigation is provided by an appropriate combination of an undulator and monochromator. A conventional undulator is composed of an alternating periodic array of north and south pole magnets. A high-energy electron beam passing through the undulator is induced to sinusoidal oscillation. This generates an intense radiation at a fundamental wavelength with the harmonics of this wavelength. A crystal monochromator diffracts a fundamental wave and its harmonics so that the undulator radiation is not monochromatic but is contaminated by higher harmonics. This condition is not convenient for radiation experiments. A newly invented quasi-periodic undulator has a quasi-periodic array of magnets to generate non-integer harmonics, so pure monochromatic radiation can be obtained through a crystal monochromator. The quasi-periodic undulator is expected to lead to a drastic improvement in measurement accuracy and heat load reduction. This should promote new advances in science.

Reference S. Sasaki et al., Conceptual Design of Quasi-Periodic Undulator, Rev. Sci. Instrum., 66(2) 1953 (1995).

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