A Fast Switching Mirror Unit at FLASH

At the Free Electron Laser Hamburg the laser beam is diverted towards 5 different test sites by massive silicon mirrors, which are mounted into vacuum vessels. One of these vessels is operated in permanent switching mode. The pursued switching frequency is 5 Hz. The initial motion concept is characterized by the motion of the entire vessel perpendicular to the laser beam by a linear drive. The initial vessel with a steel body allowed a switching at 2.5 Hz at the demanded precision. By substituting the steel body with one made of titanium, the frequency could be increased to 3.125 Hz. Further increase should be possible with a new motion concept, leaving the vessel stationary, which is not yet implemented. Another concept which is currently examined is the exclusive motion of the mirror inside the vacuum with piezo motors. Although the motors meet the requirements concerning the accuracy and driving speed, a steady motion under ultra-high vacuum conditions is not yet possible since the cooling of the motors is still insufficient.