Flight-like critical-angle transmission grating x-ray performance for Arcus

Abstract

High resolving power soft x-ray spectroscopy has been confirmed by the recent Astro2020 Decadal Survey as a high-priority strategic goal with R =λ/Δλ as high as 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. Below the level of an expensive strategic mission, but far exceeding current capabilities, falls the Arcus Grating Explorer mission concept, with a minimum R of 2500 (expected R = 3500) and effective area up to ∼ 300 cm2 in the 12-50 Å bandpass. Arcus relies on light-weight, high-efficiency, blazed and alignment-insensitive critical-angle transmission (CAT) gratings for dispersion. The mission calls for hundreds of ∼ 30×30 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, volume production compatible tools from the semiconductor and MEMS industries. We report x-ray results from quasifully illuminated, co-aligned CAT gratings showing record-high R ∼ 1.3×104 in 18th order at Al-Kα wavelengths, 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 also report on updates to the Arcus resolving power error budget, and on post-fabrication thinning of grating bars, which could lead to increased diffraction efficiency.