The ALBA high-stability monochromator for VUV and soft X-rays

Abstract

LOREA is ALBA’s beamline devoted to the investigation of solids electronic structure by means of Angle Resolved Photo-Emission Spectroscopy (ARPES). The beamline operates in the photon energy range 10-1000 eV with tuneable linear and circular polarizations produced by an APPLE II helical undulator. Thanks to its energy range and the high photon flux, LOREA is suitable for high resolution VUV ARPES investigations in the 10-200 eV range, while it is feasible to extend ARPES measurements to the 200-600 eV energy range (soft X-ray ARPES). Core level photoemission, resonant photoemission and X-ray absorption spectroscopies will be accessible in the whole energy range. The energy selection is obtained by an Hettrick-Underwood monochromator without entrance slit. The optical arrangement, with 3 spherical mirrors (SM) and 4 plane varied line-spaced (VLS) gratings, is able to cover the entire energy range of the beamline. The monochromator includes the motions to select and do the fine adjustments of the mirrors (pitch and height), and to scan the energy and select among the different gratings (grating pitch and grating horizontal translation). The monochromator has been designed by the ALBA team of engineers, and has been fully assembled and commissioned at the facility. Besides the required range and resolution performances, it has been designed to achieve high stability and reproducibility, and optimal performance of the optical surfaces under different heat loads and conditions. The cooling circuits of mirrors and gratings are mechanically decoupled from the optical elements. In the case of the gratings, heat load is removed by flexible copper straps connected from optics to rigid water lines, through temperature controller devices based on Peltier elements. The use of Peltier element allows stabilizing the temperature of the gratings to room temperature also under quickly varying heat loads. The gradients within the grating are well below the one degree, and the thermal equilibrium with the surrounding mechanics contributes the long-term stability of the system. The water circuit and the Peltiers, rest in an independent platform inside the vacuum chamber, that allows them drift freely with no effect on the position of optical elements. In the case of the mirrors, the water tubes and cooling pads are not pressed against the mirrors, but just in contact through a 0.1 mm thick pellicle of eutectic InGa. This allows for a very efficient heat transfer using a minimum contact surface sufficient to evacuate up to 60 W, and without any deformation of the mirrors. The mechanics are also designed so that no flexible loops are required, which contributes to a better vibration stability of the system. Mirrors and gratings can be removed from the monochromator inside their holders and with the cooling scheme installed on it. This is a mandatory goal of design, as it is necessary for a careful installation and control of surface deformations at the optics laboratory. In this contribution we describe the main features of the monochromator that allow reaching the target performances, especially those concerning the cooling scheme. And also, we provide details about the positioning mechanics of the optical elements, the energy scanning mechanism and the vacuum system. The monochromator has been already mounted and installed and it is already in operation. The first results of the He photoionization spectra shows an energy resolution better than 10meV at 60eV, with a strong ionization signal and very low noise.