Measuring Earth's energy budget from space is an essential ingredient for understanding and predicting Earth's climate. We have demonstrated the use of vertically aligned carbon nanotubes (VACNTs) as photon absorbers in broadband radiometers own on the Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) 3U CubeSat. VACNT forests are some of the blackest materials known and have an extremely at spectral response over a wide wavelength range. The radiation measurements are made at both shortwave, solar-reflected wavelengths and in the thermal infrared. RAVAN also includes two gallium phase-change cells that are used to monitor the stability of RAVAN's radiometer sensors. RAVAN was launched November 11, 2016, into a nearly 600-km sun-synchronous orbit and collected data over the course of 20 months, successfully demonstrating its two key technologies. A 3-axis controlled CubeSat bus allows for routine solar and deep-space attitude maneuvers, which are essential for calibrating Earth irradiance measurements. Funded by the NASA Earth Science Technology Office, RAVAN is a pathfinder to demonstrate technologies for the measurement of Earth's radiation budget that have the potential to lower costs and enable new measurement concepts. In this paper we report specifically on the VACNT growth, post-growth modification, and pre-launch testing. We also describe the novel door mechanism that houses the gallium black bodies.
Detecting energetic particles is a useful approach in studying space plasmas. Of specific interest are energetic neutral atoms (ENA) because their trajectories are unaffected by electric or magnetic fields. Imaging the ENA flux allows for the mapping of remote plasmas. In order to detect such particles, solid-state detectors are advantageous due to their lightweight and low power. However in the sensing environment the photon flux is usually several orders of magnitude higher than the ENA flux. Thus, in order to detect the energetic particles the photon flux must be blocked. Therefore, thin metal or carbon film filters that allow the transmission of ENAs while attenuating the photon signal are used. Here we report tests of low-density mats of carbon nanotubes (CNTs) as a filter medium. For a given mass per unit area (the parameter which sets the particle transmission energy threshold), CNTs are expected to absorb photons significantly better than thin films. The CNTs were grown by a water assisted chemical vapor deposition technique and pulled from their substrate to generate a CNT sheet covering an aperture. In order to test the performance of the CNT sheet as a filter, the transmissions of light and alpha particles were measured. We were able to achieve filter performance that resulted in alpha particle energy loss of only 5 keV with an optical density of 0.5.
We describe the fabrication of an Orotron driven by a sheet beam of electrons. The sheet beam is generated by a carbon
nanotube field emission electron gun, which is less than 2 mm in total thickness. The orotron cavity is 2 cm long and 1
cm wide, and houses a microfabricated Smith-Purcell grating which generates the THz radiation. The sheet beam is 5
μm thick and 6 mm wide, and it travels within 15 μm of the top surface of the Smith-Purcell grating for the length of the
cavity. The Orotron is discretely tunable, which means that there are a number of cavity resonances that can be driven
by changing the energy of the beam such that for the period of the Smith-Purcell grating the cavity is driven on one of
the resonances. For this work, a target frequency of 0.5 THz, corresponding to a beam energy of 3 keV, was used.
Observations of the Earth are extremely challenging; its large angular extent floods scientific instruments with high flux
within and adjacent to the desired field of view. This bright light diffracts from instrument structures, rattles around and
invariably contaminates measurements. Astrophysical observations also are impacted by stray light that obscures very
dim objects and degrades signal to noise in spectroscopic measurements. Stray light is controlled by utilizing low
reflectance structural surface treatments and by using baffles and stops to limit this background noise. In 2007 GSFC
researchers discovered that Multiwalled Carbon Nanotubes (MWCNTs) are exceptionally good absorbers, with potential
to provide order-of-magnitude improvement over current surface treatments and a resulting factor of 10,000 reduction in
stray light when applied to an entire optical train. Development of this technology will provide numerous benefits
including: a.) simplification of instrument stray light controls to achieve equivalent performance, b.) increasing
observational efficiencies by recovering currently unusable scenes in high contrast regions, and c.) enabling low-noise
observations that are beyond current capabilities. Our objective was to develop and apply MWCNTs to instrument
components to realize these benefits. We have addressed the technical challenges to advance the technology by tuning
the MWCNT geometry using a variety of methods to provide a factor of 10 improvement over current surface treatments
used in space flight hardware. Techniques are being developed to apply the optimized geometry to typical instrument
components such as spiders, baffles and tubes. Application of the nanostructures to alternate materials (or by contact
transfer) is also being investigated. In addition, candidate geometries have been tested and optimized for robustness to
survive integration, testing, launch and operations associated with space flight hardware. The benefits of this technology
extend to space science where observations of extremely dim objects require suppression of stray light.
We describe progress towards an Oroton-based sub-millimeter-wave source with a design frequency of 500 GHz. Key
features of the devices are a microfabricated, carbon nanotube field-emission-based electron gun which creates a sheet-beam
at the required current density without the need for beam compression, and a microfabricated Smith-Purcell
grating, and a uniform Z-direction magnetic field confinement.