The toroidal mirror for single-pulse experiments on ID09B

摘要

ID09 is a dual-purpose beamline dedicated to time-resolved and high-pressure experiments. The time-resolved experiments use a high-speed chopper to isolate single pulses of x-rays. The chopper is installed near the sample (focal spot) and the shortest opening time depends on the height of the tunnel in the chopper, i.e. the sharpness of the vertical focus. In the 16-bunch mode, the opening window of the chopper has to be smaller than 0.352 μs in order to isolate single pulses of x-rays. This requires reducing the height of the tunnel to 0.143 mm. To ensure a reasonable transmission though the tunnel, we have designed a very precise toroidal mirror that focuses the beam 22.4 m downstream with a magnification M = 0.677. The 1.0 m long silicon mirror is curved by gravity into a nearly perfect toroid with a meridional radius of 9.9 km. The curvature is fine-tuned by a stepper motor that pushes via a spring from below the mirror. The overall figure error from the gravity sag and the corrective force is less than 0.3 μrad. The polishing error is 0.7 μrad (rms) averaged over the central 450 mm of the 1000 mm long mirror. The measured size of the polychromatic focus is 0.100 mm x 0.070 mm (h x v) in agreement with the prediction from the ESRF long trace profiler data. The small focal spot, which integrates the full central cone of the U17 undulator, is the result of very high optical quality, curvature fine-tuning, strain-free mount, vibration free cooling and careful alignment.