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Figure 8
After removing the constant pitch of −0.6 µrad from the tangential-plane height-error curves plotted in Fig. 5[link] and zooming in to the central ±70 mm of the mirror, the thermoelastic distortion of the mirror for cool-down from 295 to 77 K (cold) and a range of undulator K values can be more easily compared. Because of the thermal tuning of the mirror system, the mirror temperature is entirely in the negative CTE regime for silicon up to K = 2.75 and is therefore concave. At K = 2.75 the maximum temperature crosses 125 K, which can be seen by the small convex bump at x ≃ 40 mm. At K = 3 the mirror temperature has increased and the central portion of the mirror begins to flatten. The curves are given for the linear horizontal polarization mode of the undulator.

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