The brightest X-rays beams at the ESRF are produced by in-vacuum undulators that have their magnets inside the vacuum vessel of the storage ring. In these undulators, the distance between the magnets and the electron beam can be as small as 2.5 mm (5.0 mm gap) as compared to 5.5 mm for conventional undulators. That boosts the Lorentz force on the electrons, which increases the brightness of the emitted X-rays. The gain in brightness is a factor of 3 at 15 keV and 9 at 60 keV. The emitted power in the central cone varies, depending on the undulator period, between 350-650 watt and the power density, 30 m from the source, is between 70-165 watt/mm2. At these levels silicon-based monochromators need to be cooled with liquid nitrogen. We will shortly describe the test of a new monochromator that was installed on beamline ID09 at the end of 2001. It is a so-called channel-cut monochromator, machined from a monolithic block of silicon, see Figure 126.

Fig. 126: The channel-cut monochromator on ID09. The Bragg angle of the second crystal is adjusted by a stepper motor that pushes (or pulls) against a blade attached to the second crystal.

This geometry was chosen for its stability and ease of alignment. The overall length is 140 mm and the (perpendicular) distance between the two diffracting surfaces is 4.0 mm. The diffracting planes are oriented along (111) and the monochromator can be tuned between 4 to 50 keV. The monochromator is the first optical element in the beamline and it receives the direct beam from the in-vacuum undulator U17 (magnetic period 17 mm). The fundamental energy of this 2.0 m-long undulator is 15.0 keV; the deflection parameter K is 0.835; the power in the central cone is 350 watt and the power density is 70 W/mm2 at the monochromator position 31.4 m from the source. The spectrum of the U17 is dominated by its first harmonic and that keeps the heatload relatively low. The absorbed heat is extracted by two liquid nitrogen-cooled copper blocks that are clamped onto the sides of the first diffracting surface. The nitrogen is held at 80 K at a pressure of 3 bars. In the test, the two undulators U17 and U46 were used in tandem with the U17 as a (gap) variable heat source. The monochromator was set to 8.0 keV and the gap of the U46 was lowered to 16.9 mm where its fifth harmonic emits at 8.0 keV. The intensity of the monochromatic beam was measured by a PIN diode with a 2.0 mm-thick Al-sheet in front of it. The detected beam was therefore dominated by the 333 and 444 reflections that are more sensitive to thermal deformation. The deformation was determined by rocking the second crystal and recording the FWHM-width as a function of power. The power dependence of the rocking curve is shown in Figure 127.

Fig. 127: The efficiency of the cooling system is determined from the broadening of the rocking-curve of the second crystal. At the local minimum at 410 W, the thermal coefficient of silicon is zero.

The rocking width of Si (333) at 24.0 keV is 3.3 µrad, which is smaller than the observed cold-limit of 6.3 µrad. This discrepancy is attributed to strain from the clamping of the Cu-absorbers (5.3µrad). The rocking width increases to a local maximum at 280 watt, which is followed by a local minimum at 410 watt. At higher powers, the rocking width increases dramatically. The fitted curve was calculated by finite-element-analysis (FEA). The rather surprising local minimum in the FWHM-width around 410 watt is explained in the following way. As the heatload is raised from 0 watt, the crystal temperature starts to increase above 80 K. At 410 watt, the temperature reaches 125 K in the centre of the X-ray footprint. At that temperature, the thermal expansion-coefficient of silicon is zero. Therefore small temperature gradients associated with the flow of heat do not perturb the planarity of the atomic planes in the centre of the beam. The conclusion from this work is that indirect cooling is sufficient below 450 watt. Above this level, which is reached with two in-vacuum undulators in tandem, one is forced to cool the silicon directly, i.e. having liquid nitrogen flowing inside the crystal. The measured spectrum of the U17 is shown in Figure 128. Note that the flux at 15 keV attains the very high value of 8 x 1013 ph/s.

Fig. 128: The spectrum of the U17 undulator at 6.0 mm gap measured with the channel-cut monochromator.

Principal Publication and Authors
L. Zhang (a), W.-K. Lee (b), M. Wulff (a), L. Eybert (a), to be published.
(a) ESRF
(b) APS, Argonne (USA)