Abstract
We propose a method to control and to adjust in a closed-loop a bendable x-ray mirror using displacement-measuring devices. For this purpose, the usage of capacitive and interferometric sensors is investigated and compared. We installed the sensors in a bender setup and used them to continuously measure the position and shape of the mirror in the lab. The sensors are vacuum-compatible such that the same concept can also be applied in final conditions. The measurement is used to keep the calibration of the system and to create a closed-loop control compensating for external influences: in a demonstration measurement, using a 950 mm long bendable mirror, the mirror sagitta is kept stable inside a range of 10 nm Peak-To-Valley (P-V).
Highlights
The European X-Ray Free Electron Laser facility (European XFEL), under construction in Hamburg, Germany [1], is raising the requirement level of accuracy and reproducibility for reflective x-ray optics
We built a measurement system using three displacement-measuring sensors in an x-ray mechanical bender for European XFEL with the goal to control the mirror bending on the nanometer level
We compared two types of sensors: single-point capacitive sensors and interferometric optical sensors. Both systems can be calibrated using a Fizeau interferometer as a reference. Both are suitable for an ultrahigh-vacuum environment, according to the specifications and materials used by the manufacturers: the specific sensors chosen for European XFEL will be further tested by the Vacuum group
Summary
The European X-Ray Free Electron Laser facility (European XFEL), under construction in Hamburg, Germany [1], is raising the requirement level of accuracy and reproducibility for reflective x-ray optics. A certain change of the mirror radius of curvature is relatively more important in case of long distances, producing a displacement of the focus position or a corresponding change of the beam transversal size at a given point. Important factors to reach the desired control precision are the possibility to bend the surface with enough resolution and the presence of a reliable feedback signal to compensate backlash errors and thermal drifts Such systems are quite common in astronomical science [10] but still not fully developed in x-ray photon science. A simpler system, using only piezo actuators and without feedback control, was developed and characterized on a 950 mm length [12] Another proposed method was to control a bendable mirror having a direct measurement of the surface using a Fizeau interferometer [13,14], but it is a quite expensive solution with limitations in measuring long mirrors. The feedback signal would be quite impossible to obtain from the beam itself in the case of the European XFEL because of the high power involved
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