Abstract
This paper presents a novel cantilevered liquid-nitrogen-cooled silicon mirror design for the first optic in a new soft X-ray beamline that is being developed aspart of the Advanced Light Source Upgrade (ALS-U) (Lawrence Berkeley National Laboratory, USA). The beamline is optimized for photon energies between 400 and 1400 eV with full polarization control. Calculations indicate that, without correction, this design will achieve a Strehl ratio greater than 0.85 for the entire energy and polarization ranges of the beamline. With a correction achieved by moving the focus 7.5 mm upstream, the minimum Strehl ratio is 0.99. This design is currently the baseline plan for all new ALS-U insertion device beamlines.
Highlights
A project to upgrade the Advanced Light Source (ALS) is currently underway
In this paper we have presented the novel cantilevered liquidnitrogen-cooled silicon mirror design that is being developed as the baseline M1 for the Advanced Light Source Upgrade (ALS-U)
Without correction, this design will achieve a Strehl ratio greater than 0.85 for the entire energy and polarization ranges of the beamline
Summary
A project to upgrade the Advanced Light Source (ALS) is currently underway. This project (known as the ALS-Upgrade or ALS-U) includes a new soft X-ray ‘FLEXON’ beamline (FLuctuation and EXcitation of Orders in the Nanoscale) optimized for photon energies between 400 and 1400 eV with full polarization control. In this paper we present the design of a novel liquidnitrogen-cooled silicon mirror (Fig. 1); this end-cooled cantilever design addresses the fundamental challenges of thermal strain, mounting stiffness, unknown coolant line forces and thermal control, as described in the previous paragraph. In the second part we first describe our method for calculating the r.m.s. height error, phase error and Strehl ratio from the finite-element results, and present wavefront propagation simulations using the deformed mirror shape Based on these calculations we predict that with a fixed pitch adjustment – but without any higher-order (for example circular) correction – our design will achieve a Strehl ratio greater than 0.85 for the entire operating range of the beamline for two polarization modes. With a correction achieved by adjusting the focal length by 7.5 mm, the minimum Strehl ratio is calculated to be 0.988
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