Variable-line-spacing Grating Monochromators Design And Key Techniques

From accelerators and storage rings for high energy physics to the free-electron laser source, the development of synchrotron radiation leads to greatly improved performance of the light source. Thus, a higher spectral resolution and higher transmission efficiency beamline, which acts as a bridge between the light source and the experimental station, is needed to ultimately exploit the advantages of the light source. Monochromator is the key part of a beamline. In soft x-ray and extreme ultraviolet wavelength range, grating monochromators are commonly used. There are three main kinds of grating monochromator in this energy range, which includes the Dragon type spherical grating monochromator, SX-700plane grating monochromator and variable-line-spacing plane grating monochromator. Because of less optical components and simple component surface shape, the variable-line-spacing plane grating monochromator is easy to achieve both high spectral resolution and high transmission efficiency. It has been extensively used in recent years at different synchrotron radiation laboratories.This paper discusses the principle and the design of the variable-line-spacing plane grating monochromator, and key techniques to achieve high spectral resolution. It is described in three parts.First is the optical design of the surface physics beamline in Hefei Light Source. The surface physics beamline, which utilizes radiation from an in-vacuum undulator, covers the energy range from20eV to600eV. Undulator radiation is horizontally deflected and vertically focused by a cylinder mirror to the entrance slit, whose focusing demagnification is5, and monochromatized by a variable-line-spacing plane grating, and then horizontally and vertically focused onto the experiment station’s sample by a toroidal mirror. The total length of the beamline is19300mm. The resolving power (E/△E) is10000@29eV. Spot size at the sample is0.2(h) x0.1(V) mm2with flux of5x1010photons/s@29eV.Second is the optical design of the beamline in Dalian Coherent Light Source (DCLS). This is a fourth generation synchrotron radiation source, based on free-electron lasers, which is currently under construction in the northeast of China. The light source has unique features as the turntable radiation frequency, wide spectral range, high brightness and peak power, very short pulse time structure, et al. It is a user facility based on the principle of single-pass, high-gain harmonic generation (HGHG). It covers the EUV regime of50-150nm with pulse energy exceeds100μJ.Compare to the beamline of the synchrotron radiation, the beamline of free-electron-laser facility has new challenges, like the influence of the high peak power onto the mirror coating, and relationship between the pulse length and the spectral resolution, et al. There are four branch beamlines planed in DCLS which share beam time. The total length of the beamlines is about50m. Owing to the variations of saturation length for different wavelengths, the light source point will change at different wavelengths from6m-12m. Optical components reflective efficiency of the beamlines is from maximum of85%to45%. A key diagnostic tool in DCLS is the on-line source spectral characteristic recording during the source development, and for the definition of the experimental conditions. For this purpose, an online grazing incidence spectrometer with a toroidal mirror and a variable-line-spacing plane grating is designed. Resolving power of this spectrometer is better than12000for most of the wavelength covered. In the FERMI@Elettra facility, two orthogonal linear stages is used to scan wavelength, one is used to bring the desired wavelength on the detector center, and the other is used to change the grating-detector distance for focusing purpose. In our design, a circular stage is chosen to fit the focal curve and to realize the wavelength scanning. This scanning mechanics is simpler and stable.The third part is the key techniques of achieving the high spectral resolution of the monochromator. Based on the offline commissioning of the surface physics beamline, some key techniques techniques, including the high-resolution linear guide, rotation angle measurement system, online monitoring system to the opening of the slits, testing of the optical component, are developed and tested. Some of these techniques are innovative, like the online monitoring system to the opening of the slits, and the optical component clamping techniques, et al.A sine drive is typically used in spectrometers to convert a linear displacement to axis rotation while wavelength scanning. One high-precision linear guide, which has150mm travel with30nm resolution, is designed to meet the need of the spectral resolution. At the same time, a UHV compatible precision rotation angle measurement system is developed in this beamline to measure the rotation angle of the mirror and grating. The angle resolution of this system is0.05arcsecThe slope error of the mirrors in surface physics beamline is examined by long trace profile. The test result shows that slope error of the optical components in monochromator meets the needs. Groove density of the variable-line-spacing plane gratings is examined by a self-built2D grating groove density measurement system. Groove density accuracy△N/N of the400lines/mm is about3-7×10-5, and1-3×10-5for the1200lines/mm.The opening of the slits in surface physics beamline need to be very small to achieve the required spectral resolution, and minimum opening is about13μm. Therefore, we have designed one online interference fringe monitoring system to realize online monitoring of the slit opening. The accuracy of this system is about5μm.Monochromator is a kernel part of the beamline. Its off-line test and alignment, which includes parameters measurement of the off-axis rotation (mirror roattion), the parallelism between the grating axis and the plane mirror axis, are related directly to the spectral resolution. At the end, the deviation WG of the reflected beam from the central spot of the grating is less than0.07mm, which makes energy shift of10-4eV, and the unparallelism of the grating axis and the plane mirror axes is within5arcsec. These errors have no significant influences on the spectral resolution and fulfill the engineering requirement of the optical system.