In optical and precision engineering, crystal monochromator, as an important splitter in synchrotron radiation beam line, is widely used in Shanghai Synchrotron Radiation Facility（SSRF）. It is the key equipment in the beam line. The monochromatic wavelength, energy resolution and spot size required by each experimental station are realized or determined by the monochromator. It is one of the most important equipment in the application of synchrotron radiation light.

The "water-cooled arc vector focusing monochromator" project team of Xi 'an Institute of Optical Machinery, which undertook the development task, successfully developed China's first water-cooled arc vector focusing monochromator with independent intellectual property rights.

The research on the structure of water - cooled monochromator is inseparable from the precision chiller with high precision temperature control. The public cooling system at the project site does not meet the monochromator's cooling requirements. Many accelerator projects in China, such as the Shanghai Synchrotron Radiation Source, the Beijing Electron-positron Collider and the Beijing Advanced Light Source under construction, use a large number of monochromators and need to be equipped with precision chiller with different technical requirements.

The precision demand of monochromator for temperature, pressure and flow of chiller is expounded here separately.

The first is the circulating water temperature, temperature is mainly the temperature control accuracy, temperature control accuracy generally requires ±0.01 K or ±0.0.2K. Most of the chiller can not meet the temperature control requirements, or the stability of temperature control is not enough, some of the temperature drift and time drift of temperature measurement can not meet the requirements.

The figure below is the 24 hours temperature test record of precision chiller of Coolium instruments.

The second is circulation pressure. Monochromator contains a large number of precision optical components, which are sensitive to the circulating pressure of cooling water. Firstly, the pressure is required to be high, and secondly, the pressure is required to be stable. Frequent pressure fluctuation causes certain interference to optical devices. For the monochromator, it is necessary to avoid both the impact of starting pressure and the fluctuation of pressure during operation. These two problem must be solved well to meet the use of the monochromator.

The third is the flow stability of cooling water. Flow fluctuation, generally not more than ±0.03L/min. The flow measurement, circulation pump selection and pipeline layout of precision chiller all need professional design consideration in order to meet the requirements of flow fluctuation.

During the development of the water-cooled arc vector focusing monochromator in the X-ray diffraction beam line (BL14B1) and small Angle scattering beam line (BL16B1) of SSRF, two water-cooled structures, the bottom cooling type and the curved top groove type, are simulated and analyzed, which can effectively reduce the temperature rise of crystal surface and reduce the deformation. The precision chiller with temperature control accuracy of ±0.01K developed by Coolium Instruments is an important water cooling component in this study.