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ID2 Optics

last modified 11-07-2009 23:37

Optics hutch at ID2: technical details

Monochromator

The maximum power density on the monochromator is about 20 W/mm². A cryogenically cooled Si-111 double-crystal monochromator withstands this power load without severe broadening of the rocking curve. A closed liquid-nitrogen loop provides automatic operation of the cooling system. To reduce background radiation from the storage ring, the beam is vertically displaced by 30 mm. The Si(111) crystal can select energies between λ = 0.155 nm and λ = 0.073 nm. At λ = 0.1 nm (12.4 keV) the bandwidth is of the order of ΔE/E =2.10^-4.

Mirror

An uncooled Rh-coated double-focusing toroidal mirror follows the monochromator. The mirror has a fixed shape and can only be used with wavelengths longer than 0.073 nm. The design of the beamline includes the possibility of installing a horizontally reflecting mirror. In both directions the maximum reflection angles are 12 mrad. Because of the small beam-divergence the focusing parameters can be kept fixed for all experiments. The mirror characteristics were chosen to minimise the beam size at the detector position when it is moved to the end of the SAXS station (65 m). The following figure shows the calculated beam size along the experimental area in comparison with observed values.

Alternatively, the beam is focused by a cylindrically bend mirror in the vertical direction and compound refractive lenses in the horizontal direction. This hybrid optics provide a beam size of 0.06 mm (V) x 0.45 mm (H).

Beam characteristics

optical elements:	double monochromator	toroidal (or cylindrical) mirror
distance from source:	29.6 m	32.9 m (34.5 m)
type:	Si(111), cryogenically cooled 30 mm vert. displacement	Rh coated, length 350 mm vert. refl. angle 7.9 mrad
beam size at sample posit.:	unfocused	focused
(at 55 m , 12.5 keV HxV, FWHM) in mm² :	(real mirror) 2.6 x 1.3	0.4 x 0.2 (0.45 x 0.06)

spectral range :	8- 17 keV (0.073 - 0.15 nm)
resolution in Δ E/E :	2.0 10^-4 at 12.5 keV

Beam size along the experiments hutch of the high-brilliance beamline ID2:

full line: including slope errors (σ_δS - sagittal RMS slope error, σ_δM - meridional RMS slope error),
dashed line: ideal mirror without slope errors,
dots: observed beam sizes in the high brilliance mode of the ESRF storage ring.