Performance of a liquid-nitrogen-cooled, thin silicon crystal monochromator on a high-power, focused wiggler synchrotron beam

Abstract

An experiment was performed on beamline BL 3 at the European Synchrotron Radiation Facility to test the diffraction performance of a novel internally liquid-nitrogen-cooled, thin silicon crystal monochromator exposed to high heat loads. The beam parameters were chosen to closely match the conditions expected, in terms of absorbed power and beam profile, at the Advanced Photon Source (APS) for the closed gap undulator at 7 GeV and 100 mA. The cooled crystal was oriented at 11.4° in the symmetric Bragg geometry to diffract 30 keV x-rays from the Si(333) planes. The source was a 44-pole wiggler with the insertion device gap set at 25.0 mm corresponding to a deflection parameter, K, of 4.2. A tunable toroidal mirror was used to focus the wiggler beam onto the crystal. Double-crystal rocking curves were measured at several power values using different attenuators. The maximum total power absorbed by the 0.6-mm-thick crystal was 154 W at a storage ring current of 136 mA. The peak power density at normal incidence was about 420 W/mm2 corresponding to an absorbed peak power density on the crystal face of 83 W/mm2. No thermal-induced broadening of the rocking curve was observed above the average measured mounting/fabrication strain of 2 arcsec. Rocking curves were also measured as a function of coolant flow rate and pressure. No systematic broadening occurred due to flow-induced vibrations up to 6 ℓ/min. It has been demonstrated that thin silicon crystals directly cooled with liquid nitrogen can handle high power density synchrotron beams comparable to what is expected for the APS undulators with no appreciable thermal deformation.