BACH, the beamline for advanced dichroic and scattering experiments at ELETTRA

A beamline for advanced dichroism (BACH), to perform light polarization dep endent experiments in the 35-1600 eV photon energy range is under construct ion at the ELETTRA Synchrotron Radiation Source in Trieste, Italy. The radi ation source, based on two APPLE-II helical undulators, is designed for hig h photon flux and high resolving powers. The photons dispersion system is b ased on a Padmore variable angle spherical grating monochromator with a typ ical resolving power of 20 000-6000, 20 000-6000, and 15 000-5000 in the en ergy ranges 35-200 eV, 200-500 eV, and 500-1600 eV, respectively. Two separ ate branches after the monochromator allow setting two independent experime ntal chambers. The photon flux in the experimental chamber(s), calculated a t the best resolutions achievable and with the aperture of the slits set at 10 mum, is expected to be above 10(11) photons's with linearly or circular ly polarized light. In addition, a fourth grating operates in the 400-1600 eV range to provide a higher flux, 10(12) photons's with smaller resolving power (10 000-2000), allowing fluorescence and x-ray scattering experiments . The refocusing section(s), based on plane elliptical mirrors in a Kirkpat rick-Baez scheme, will provide on the sample, a nearly free-aberration spot (s), whose dimensions are expected to be 200x10 mum(2) (horizontalxvertical ). In the following, the general layout of the beamline is reported and the characteristics of the optical elements, as well as the optical performanc es (resolving powers and efficiencies of the monochromator, flux, and spot dimensions) are described in detail. (C) 2001 American Institute of Physics .