Arcus, a Medium Explorer (MIDEX) mission, was selected by NASA for a Phase A study in August 2017. The observatory provides high-resolution soft X-ray spectroscopy in the 12-50Å bandpass with unprecedented sensitivity: effective areas of >450 cm<sup>2</sup> and spectral resolution >2500. The Arcus key science goals are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, groups, and clusters, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback and (3) to explore how stars, circumstellar disks and exoplanet atmospheres form and evolve. Arcus relies upon the same 12m focal length grazing-incidence silicon pore X-ray optics (SPO) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings, and the results are imaged with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. Mission operations are straightforward, as most observations will be long (~100 ksec), uninterrupted, and pre-planned, although there will be capabilities to observe sources such as tidal disruption events or supernovae with a ~3 day turnaround. Following the 2nd year of operation, Arcus will transition to a proposal-driven guest observatory facility.
Arcus will be proposed to the NASA Explorer program as a free-flying satellite mission that will enable high-resolution soft X-ray spectroscopy (8-50) with unprecedented sensitivity – effective areas of >500 sq cm and spectral resolution >2500. The Arcus key science goals are (1) to determine how baryons cycle in and out of galaxies by measuring the effects of structure formation imprinted upon the hot gas that is predicted to lie in extended halos around galaxies, groups, and clusters, (2) to determine how black holes influence their surroundings by tracing the propagation of out-flowing mass, energy and momentum from the vicinity of the black hole out to large scales and (3) to understand how accretion forms and evolves stars and circumstellar disks by observing hot infalling and outflowing gas in these systems. Arcus relies upon grazing-incidence silicon pore X-ray optics with the same 12m focal length (achieved using an extendable optical bench) that will be used for the ESA Athena mission. The focused X-rays from these optics will then be diffracted by high-efficiency off-plane reflection gratings that have already been demonstrated on sub-orbital rocket flights, imaging the results with flight-proven CCD detectors and electronics. The power and telemetry requirements on the spacecraft are modest. The majority of mission operations will not be complex, as most observations will be long (~100 ksec), uninterrupted, and pre-planned, although there will be limited capabilities to observe targets of opportunity, such as tidal disruption events or supernovae with a 3-5 day turnaround. After the end of prime science, we plan to allow guest observations to maximize the science return of Arcus to the community.
Since the inception of the laser-divestment process, emphasis has focused on the treatment of reasonably durable materials. Marble, limestone, sandstone, and bronze are foremost among these. In most situations the objective of laser divestment is the removal of superficial corrosion or chemical-decomposition products. To a lesser extent laser ablation is also used to treat diverse surface problems for a spectrum of other historic and artistic substrates such as paper, vellum, ivory, paint, and plaster. Although materials of this sort are not particularly strong, their optical, thermodynamical, and mechanical properties are sufficiently propitious to enable successful laser treatment (with the exercise of precise control). There is another, quite different, cleaning problem encountered in the maintenance of museum collections. This is often referred to as "dusting" (in contrast to "divestment" or "conservation"). Vacuuming, wiping, blowing, and feather dusting are used most often to improve the cosmetic appearance of museum objects after dust and aerosols have accumulated on exposed surfaces. However, many collections include extremely friable pieces composed of feathers, fir, hair, plant fibers, or mummified skin. Conventional dusting may be impossible in such instances. From experimental observations and theoretical analyses we speculate that at very low fluxes laser-induced acoustic and electrostatic forces are responsible for the ejection of debris. Laboratory experiments demonstrated that laser dusting was effective on feathers and textiles, The practical viability of laser dusting was demonstrated by laser-cleaning two very large sand sculptures by San Diego artist C.R. Faust. In contrast, all conventional cleaning techniques damaged the surface by dislodging sand grains.
We have developed a novel process for releasing MEMS and nano-scale devices formed in SiO<sub>2</sub> on a Si substrate. Current approaches for releasing MEMS made of SiO<sub>2</sub> use wet chemical etches (e.g. EDTA or KOH) or gas phase chemical etches such as xenon difluoride. These approaches are inherently messy and difficult to control. We have shown that it is possible to release patterned SiO<sub>2</sub> structures using a direct write laser assisted chemical etching technique. The developed process removes Si only from the immediate area leaving behind the SiO<sub>2</sub> device. The technique allows the surrounding larger area of the Si wafer to be conserved for use in packaging or integration with electronics. Further, as the release is accomplished in the gas phase, we see none of the problems of "stiction" associated with a liquid etch release process. In fact, we have found this method to be so gentle that we have been able to release devices made from SiO<sub>2</sub> films on the order of hundreds of nanometers.
Lasers can induce subtle and not so subtle changes in material structure. We have found that certain pigments can undergo chemical and crystallographic changes and concomitant color shifts. Minerals and the related pigments may experience a loss of hydroxyl groups or other chemical reordering. The organic component of skeletal, keratinaceous, and cellulosic materials can be pyrolized, ablated, or etched. Polymers can discolor, undergo structural weakening, or be volatilized. A few of these processes have been investigated with regards to changes on ivory and bone, selected pigments and the removal of dye-based pen ink from porous substrates.
A study to evaluate three processes used for the release of standard devices produced by MCNC using the MUMPS process was undertaken by Jet Propulsion Laboratory with the collaboration of The Aerospace Corporation, and Polytec PI. The processes used were developed at various laboratories and are commonly the final step in the production of micro- electro-mechanical systems prior to packaging. It is at this stage of the process when the devices become extremely delicate and are subject to yield losses due to handling errors or the phenomenon of stiction. The effects of post processing with HF on gain boundaries and subsequent thermal processing producing native oxide growth during packaging will require further investigation.
GaP lens arrays have been routinely produced in large formats (substrate dimensions up to 1.5 by 1.0 cm) with high yield and uniformly good finish. Diffraction-limited performance for collimation of single-mode diode lasers has been demonstrated. Laser-diode bars and coherent 2-D surface-emitting arrays have also been collimated with low transmission losses (98%) for 2.5 micrometers > (lambda) > 0.8 micrometers . Microlenses up to 300 micrometers in diameter with f/#s as low as 0.7 have been enabled by a new mass transport fabrication technique using sealed quarts ampoules rather than a flowing tube furnace. In this modification only small pieces of phosphorus are required (no phosphine or hydrogen); consequently, little safety burden is incurred, and initial expenses are reduced. The mesa-step-spacing was increased from 10 micrometers to 15 micrometers , and, by additional chemistry control, 30 micrometers spacings have been demonstrated. Also, time-at-temperature for mass-transport smoothing has been shortened to as little as 8 h. Mass-transport chemical mechanisms and material incompatibilities are discussed. Smoothing in the fused-quartz ampoules is shown to be self- terminated by wafer oxidation, probably caused by oxygen from thermal equilibrium dissociation of the silicon dioxide ampoule. This mass transport technique lends itself to a wide variety of novel lens fabrication strategies and clearly extends the potential applications.
The use of high power laser diodes in applications such as
pumping of solid state lasers requires devices which are highly
reliable. We report the results of a series of experiments in
which the effects on device reliability of several key processing
steps are investigated. Electron Beam Induced Current (EBIC) is
used to nondestructively characterize the Dark Line Defects
(DLD5) throughout the lifetest and provides information regarding
the source and propagation of the DLDs.