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Soft x-ray performance of flight-like critical-angle transmission gratings for Arcus-Probe

Presentation #103.80 in the session Missions and Instruments.

Published onJul 01, 2023
Soft x-ray performance of flight-like critical-angle transmission gratings for Arcus-Probe

High resolving power soft x-ray spectroscopy has been confirmed by the recent Astro2020 Decadal Survey as a high-priority strategic measurement technique with R = λ/Δλ as high has 7500 for some science cases. Examples are the characterization of highly ionized gases in galaxy halos and within and around galaxy clusters, accretion onto supermassive black holes, coronal mass ejections and coronal heating. While far exceeding current capabilities, the Arcus grating Explorer mission concept, with a minimum R of 2500 (expected R = 3800) and effective area on the order of 300 cm2, was not selected in the latest MidEx round. Following the recent NASA Astrophysics Probe call, we are currently designing a Probe-class mission with significantly larger effective area and similar or higher resolving power. Called Arcus-Probe, it relies on light-weight, high-efficiency, blazed and alignment insensitive critical-angle transmission (CAT) gratings for dispersion. The mission calls for hundreds of ~30x30 mm2 gratings with a hierarchy of integrated support structures. The most recent CAT gratings have been fabricated from 200 mm silicon-on-insulator wafers using commercial tools from the semiconductor and MEMS industries. We report x-ray results from quasi-fully illuminated, co-aligned CAT gratings showing record-high R for co-aligned gratings up to 1.3x104 in 18th and 21st orders at Al-K (~ 1.5 keV), and diffraction efficiency of blazed orders in agreement with pencil beam synchrotron measurements and model predictions at O-K. Tilt of the deep-etched, freestanding grating bars relative to the grating surface is measured and successfully compensated through angular alignment during bonding of the Si gratings to metal frames. We will also give updates on enhanced diffraction efficiency for deeper gratings and grating fabrication process improvements.

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