R. Follath

STARS - a two-stage high-gain harmonic generation FEL demonstrator

BESSY is proposing a demonstration facility, called STARS, for a two-stage high-gain harmonic generation free electron laser (HGHG FEL). STARS is planned for lasing in the wavelength range 40 to 70 nm, requiring a beam energy of 325 MeV and peak current of 500 A. The facility consists of a normal conducting gun, three superconducting TESLA-type acceleration modules modified for CW operation, a single stage bunch compressor and finally a two-stage HGHG cascaded FEL. This paper describes the faciliy layout and the rationale behind the operation parameters.

Absolute energy calibration for plane grating monochromators

Abstract This report describes a new method for monochromator energy calibration, which is based on the freedom to change the fix focus constant c ff in the plane grating monochromator at BESSY. If the monochromator is not perfectly aligned, spectra taken at different c ff -values show energy shifts (of 1–2%). These can be attributed to offsets in the grating angles. Measuring spectra at different c ff -values, one can determine these offsets by employing a fitting procedure. If these offsets are inserted, c ff variation measurements lead to relative energy scale changes of less than a few 10 −4 .

Comparative monochromator studies for a soft x-ray microfocus beamline for BESSY-II

Abstract We present a comparative study for a microfocus beamline at the U41 undulator, the most brilliant source at BESSY-II. High flux and an illuminated sample spot size between 1 and 10 μm are the primary design goals to bridge the spatial resolution gap attainable with imaging photoelectron emission microscopy and microspectroscopy applications. The basic layout of the beamline consists of a monochromatisation and a subsequent refocusing stage. For the former, a focused spherical grating monochromator (FSGM) design and a collimated plane grating monochromator (PGM) were optimised. Both monochromator make use of the same refocusing stage: For limited spatial resolution but large working distance between the last mirror chamber and focal spot, a horizontal deflecting toroid will give a spot of 10×20 μm 2 . The second system, a Kirkpatrick–Baez arrangement of bent plane-elliptical mirrors is expected to reach a 1×1 μm 2 spot. The plane grating monochromator was finally chosen for its high flux and versatility. Ray trace calculations indicate a photon flux of ca. 3×10 13 photons/s at a spectral resolution E /Δ E above 2300.

A soft x-ray fluorescence spectrometer at BESSY II

A new spectrometer for resonant inelastic x-ray scattering experiments has been designed and is currently built at the synchrotron light facility BESSY II. The spectrometer with a total length of 3 m covers the photon energy range from 50 eV to 1000 eV with emphasis on the lower photon energy range. It is designed as plane grating spectrometer with two paraboloidal mirrors for collimation and focusing and a plane grating in-between. Two rotational degrees of freedom allow for a variation of the incidence and deflection angles at the grating. The geometrical acceptance of the spectrometer approaches a solid angle of 35 mrad × 40 mrad2 at low photon energies with a resolution better than 10 meV at 100 eV. A rotatable delay line detector is used to detect the fluorescence light in the line focus.

Engineering aspects for the conception of the BESSY II beamlines

Abstract The very good results achieved at the first five undulator beamlines commissioned at BESSY II showed that thoroughly developed aspects for a general conception for the realization of new undulator beamlines for BESSY II have paid off. Basic ideas like an optical design with the possibility to employ optical elements with highest precision and simple drives combined with a very precise position control as well as an indispensable metrology laboratory of highest quality have been the key for success.