Arcus is a high-resolution soft x-ray spectroscopy mid-size Explorer mission selected for a NASA Phase A concept study. It is designed to explore structure formation through measurements of hot baryon distributions, feedback from black holes, and the formation and evolution of stars, disks, and exoplanet atmospheres. The design provides unprecedented sensitivity in the 1.2-5 nm wavelength band with effective area up to 350 cm2 and spectral resolving power R > 2500. The Arcus technology is based on a highly modular design that features 12 m-focal length silicon pore optics (SPO) developed for the European Athena mission, and critical-angle transmission (CAT) x-ray diffraction gratings and x-ray CCDs developed at MIT. CAT gratings are blazed transmission gratings that have been under technology development for over ten years. We describe technology demonstrations of increasing complexity, including mounting of gratings to frames, alignment, environmental testing, integration into arrays, and performance under x-ray illumination with SPOs, using methods proposed for the manufacture of the Arcus spectrometers. CAT gratings have demonstrated efficiency > 30%. Measurements of the 14th order Mg-Kα1,2 doublet from a co-aligned array of four CAT gratings illuminated by two co-aligned SPOs matches ray trace predictions and exceeds Arcus resolving power requirements. More than 700 CAT gratings will be produced using high-volume semiconductor industry tools and special techniques developed at MIT
Arcus, a mission proposed as a Medium Size Explorer for high-resolution x-ray spectroscopy, requires unprecedented sensitivities: high resolving power (λ/Δλ >; 2500) and large collecting area (~ 350 cm2). The core instruments on Arcus are Critical-Angle Transmission (CAT) grating spectrometers consisting of hundreds of co-aligned diffraction gratings. The gratings require thorough quality control along the entire manufacturing process: from bare silicon wafers to CAT grating petal assembly. Period variation, grating bar tilt angles, misalignment, and grating film buckling are potential errors of interest which could degrade the performance of the x-ray grating spectrometer. We present progress towards development of metrology techniques to measure and manage aforementioned errors during the entire alignment and integration processes: starting right after fabrication of CAT grating membranes to their assembly into large arrays. A scanning laser reflection tool (SLRT) was developed to measure period variations, alignment, and area percentage of pinched grating bars. An array of four CAT gratings was successfully aligned to satisfy Arcus alignment allocations for a grating window alignment test (GWAT). No discernible signal was found from an effort to measure a ‘half’ diffraction order to characterize stiction between grating bars. A metrology protocol was developed to measure grating bar tilt angle variations and average bar tilt angles relative to the grating surface normal, based on small-angle x-ray scattering (SAXS, Cu-Kα) and an optical surface normal measurement (OSNM) setup. A grating holder was designed with integrated slits to relate independent measurements from two different setups using visible and x-ray beams. Bar tilt variations of 1 degree and average bar tilt angles of ~0.3 degree were observed for seven different CAT grating samples. Bar tilt angle variations induced from buckled grating films were also measured. We discuss implications for a more demanding CAT grating spectrometer for the proposed Lynx X-ray Surveyor mission to be presented to the next Astrophysics Decadal Survey.
Soft x-ray spectroscopy with high resolving power (R = λ/Δλ) and large effective area (A) addresses numerous unanswered science questions about the physical laws that lead to the structure of our universe. In the soft x-ray band R > 1000 can currently only be achieved with diffraction grating-based spectroscopy. Criticalangle transmission (CAT) gratings combine the advantages of blazed reflection gratings (high efficiency, use of higher diffraction orders) with those of conventional transmission gratings (relaxed alignment tolerances and temperature requirements, transparent at higher energies, low mass), resulting in minimal mission resource requirements, while greatly improving figures of merit. Diffraction efficiency > 33% and R > 10, 000 have been demonstrated for CAT gratings. Last year the technology has been certified at Technology Readiness Level 4 based on a probe class mission concept. The Explorer-scale (A > 450 cm2 , R > 2500) grating spectroscopy Arcus mission can be built with today's CAT grating technology and has been selected in the current Explorer round for a Phase A concept study. Its figure of merit for the detection of weak absorption lines will be an order of magnitude larger than current instruments on Chandra and XMM-Newton. Further CAT grating technology development and improvements in the angular resolution of x-ray optics can provide another order of magnitude improvement in performance, as is envisioned for the X-ray Surveyor/Lynx mission concept currently under development for input into the 2020 Decadal Survey. For Arcus we have tested CAT gratings in a spectrometer setup in combination with silicon pore optics (SPO) and obtained resolving power results that exceed Arcus requirements before and after environmental testing of the gratings. We have recently fabricated the largest (32 mm x 32 mm) CAT gratings to date, and plan to increase grating size further. We mounted two of these large gratings to frames and aligned them in the roll direction using a laser-based technique. Simultaneous x-ray illumination of both gratings with an SPO module demonstrated that we can exceed Arcus grating-to-grating alignment requirements without x rays.
We report progress toward developing a scanning laser reflection (LR) tool for alignment and period measurement of critical-angle transmission (CAT) gratings. It operates on a similar measurement principle as a tool built in 1994 which characterized period variations of grating facets for the Chandra X-ray Observatory. A specularly reflected beam and a first-order diffracted beam were used to record local period variations, surface slope variations, and grating line orientation. In this work, a normal-incidence beam was added to measure slope variations (instead of the angled-incidence beam). Since normal incidence reflection is not coupled with surface height change, it enables measurement of slope variations more accurately and, along with the angled-incidence beam, helps to reconstruct the surface figure (or tilt) map. The measurement capability of in-grating period variations was demonstrated by measuring test reflection grating (RG) samples that show only intrinsic period variations of the interference lithography process. Experimental demonstration for angular alignment of CAT gratings is also presented along with a custom-designed grating alignment assembly (GAA) testbed. All three angles were aligned to satisfy requirements for the proposed Arcus mission. The final measurement of roll misalignment agrees with the roll measurements performed at the PANTER x-ray test facility.