GEOspace X-ray imager (GEO-X) is a small satellite mission aiming at visualization of the Earth’s magnetosphere by X-rays and revealing dynamic couplings between solar wind and the magnetosphere. In-situ spacecraft have revealed various phenomena in the magnetosphere. X-ray astronomy satellite observations recently discovered soft X-ray emissions originating from the magnetosphere. We are developing GEO-X by integrating innovative technologies of a wide field of view (FOV) X-ray instrument and a small satellite for deep space exploration. The satellite combines a Cubesat and a hybrid kick motor, which can produce a large delta v to increase the altitude of the orbit to about 30 to 60 RE from a relatively low-altitude (e.g., geo transfer orbit) piggyback launch. GEO-X carries a wide FOV (5 × 5 deg) and a good spatial resolution (10 arcmin) X-ray (0.3 to 2 keV) imaging spectrometer using a micro-machined X-ray telescope and a CMOS detector system combined with an optical blocking filter. We aim to launch the satellite around the solar maximum of solar cycle 25.
We have been developing an ultra-lightweight Wolter type-I X-ray telescope fabricated with micro electro mechanical systems (MEMS) technologies for GEO-X (GEOspace X-ray Imager) mission.
GEO-X will aim global imaging of the Earth's magnetosphere using X-rays.
The telescope is our original micropore optics which is light in weight (~5 g), compact with a short focal length (~250 mm), and has a wide field-of-view (~5 deg x 5 deg).
In this talk we show developed assembly processes to meet the requirements of the GEO-X mission and the telescope's X-ray imaging performance as an engineering model with this method.
GEO-X is a small satellite mission in near moon orbit to visualize the Earth’s magnetosphere. Since the Earth is a bright x-ray source, its x-rays have a potentially effect on the GEO-X observations. Fluxes of the unexpected x-rays, stray lights, and of the GEO-X signal can be estimated. In order to estimate the stray light effect on the GEO-X FOV, we carried out a ray-tracing simulation and calculated the signal-to-noise ratio for elongations from the Earth. The S/N ratio shows a range depending on signal flux. The signal estimated by MHD simulations is smaller than estimation based on the typical observation. When we apply the small signal flux, the S/N ratio is <10 at 7 deg elongation of the GEO-X FOV from the Earth in an orbital altitude of 60RE. In order to improve the S/N ratio, there are two ways, installing a collimator in front of the optics and adjusting the observation position to obtain a large elongation. The ray-tracing simulation reveals that the collimator with 30 µm pore width and 300 µm thickness can improve the S/N ratio. The S/N ratio with the collimator can achieve >10 when the elongation is 7 deg in the orbital altitude of 60 RE. A sample collimator was fabricated by a Si dry etching. Difference of the pore width from the designed value was occurred. Since the difference can lead to extra photon loss, a trade-off study between fabrication precision and observation position is important.
We have been developing an ultra-lightweight Wolter type-I x-ray telescope fabricated with MEMS technologies for GEO-X (geospace x-ray imager) which is an 18U CubeSat (∼20 kg) to perform soft x-ray imaging spectroscopy of the entire Earth’s magnetosphere from Earth orbit near the moon. The telescope is our original micropore optics which possesses lightness (∼15 g), a short focal length (∼250 mm), and a wide field of view (∼5 ◦ × ∼5 ◦ ). The MEMS x-ray telescope is made of 4-inch Si (111) wafers. The Si wafer is firstly processed by deep reactive ion etching such that they have numerous curvilinear micropores (20-µm width) whose sidewalls are utilized as X-ray reflective mirrors. High-temperature hydrogen annealing and chemical mechanical polishing processes are then applied to make those sidewalls smooth and flat enough to reflect X-rays. After that, the wafer is plastic-deformed into a spherical shape and Pt-coated by plasma atomic layer deposition (ALD) process to focus x-rays with high reflectivity. Finally, we assemble two optics bent with different curvatures (1000- and 333-mm radius) into the Wolter type-I telescope. Optimizing the annealing and polishing processes, we found that the optic achieves an angular resolution of ∼5.4 arcmins in HPW. This is comparable with the requirement for GEO-X (∼5 arcmins in HPD at single reflection). Our optic was also successfully Pt-coated by a plasma-enhanced ALD process to enhance x-ray reflectivity. Moreover, we fabricated an STM telescope and confirmed its environmental tolerances by conducting an acoustic test with the H-IIA rocket qualification test level and a radiation tolerance test with a 100 MeV proton beam for 30 krad equivalent to a 3-year duration in the GEO-X orbit.
We are developing a novel Bragg reflection x-ray polarimeter using hot plastic deformation of silicon wafers. A Bragg reflection polarimeter has the advantage of simple principle and large modulation factor but suffers from the disadvantage of a narrow detectable energy band and difficulty to focus an incident beam. We overcome these disadvantages by bending a silicon wafer at high temperature. The bent Bragg reflection polarimeter have a wide energy band using different angles on the wafer and enable focusing. We have succeeded in measuring x-ray polarization with this method for the first time using a sample optic made from a 4-inch silicon (100) wafer.
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