December 2007 Archives

Ultrashort X-ray photon pulses are powerful tools for time-resolved studies of molecular and atomic dynamics. Free electron lasers remain the most promising source. However, in the future, developing much more compact sources will become significant in widening the field of application. A group led by Professor D. A. Jaroszynski (University of Strathclyde, UK) has recently reported the first successful combination of a laser-plasma wakefield accelerator, producing 55-75 MeV electron bunches, with an undulator to generate visible synchrotron radiation. Here, the key would be the laser wakefield accelerator, which produces electron beams with energies from tens of MeV to more than 1 GeV within a few cm, with pulse durations of several fs. Further improvements, particularly in the energy of electrons, could contribute to the generation of X-ray photons with ultrashort pulse-width as well as extremely high peak power. For details on laser-plasma wakefield acceleration, see, for example, "Accelerator physics: Electrons hang ten on laser wake", T. Katsouleas, Nature, 431, 515-516 (2004). For more information on the present experiments, see the paper, "A compact synchrotron radiation source driven by a laser-plasma wakefield accelerator", H.-P. Schlenvoigt et al., Nature Physics, advanced online publication, DOI: 10.1038/nphys811

The lattice dynamics of materials under high strain is of great interest in materials science. Japanese scientists led by Professors S. Adachi (KEK, Tsukuba) and S. Koshihara (Tokyo Tech Institute, Tokyo) have recently succeeded in observing the irreversible deformation process of CdS single crystal by single-shot time-resolved Laue diffraction. The time-resolution here is 100 psec, which is a single-bunch X-ray pulse-width, available at the Photon Factory-Advanced Ring (6.5 GeV). The data was obtained with various time delays in the order of nsec. As the observed pattern exhibits six-fold symmetry of the wurtzite structure at 10 ns, corresponding to a shock pressure of 3.92 GPa, i.e., above the threshold pressure of phase transition to a rocksalt structure, they suggest a transient wurtzite structure. For more information, see the paper, "Shock-induced lattice deformation of CdS single crystal by nanosecond time-resolved Laue diffraction", K. Ichiyanagi et al., Appl. Phys. Lett. 91, 231918 (2007).