Andrew H. Monica

Multiwalled carbon nanotubes for stray light suppression in space flight instruments

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.

Carbon nanotube electron ionization source for portable mass spectrometry.

Cold cathode carbon nanotubes (CNTs) are used in a low-voltage quadrupole ion trap mass spectrometer and shown to be a viable low-power alternative to filament sources for portable mass spectrometry instrumentation. No heating is necessary, and the power consumption depends only on the switching characteristics of the electronics. The CNT electron sources are mounted directly in the ring electrode, and their performance is compared directly with a filament source also mounted in the ring electron. Up to a 5 × 10(-4) Torr CO(2) environment, reflecting conditions expected during operation in a Mars atmosphere, the CNT emitters may provide up to 1 μA of current over more than 200 h.

A micro-fabricated sheet-beam Orotron THz source

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.

Carbon-nanotube field-emitter driven compact frequency-scanning THz source

We describe a scanning Orotron Terahertz radiation (THz) source. The operational principle is as follows: a sheet beam of electrons passes near a corrugated metal surface (Smith-Purcell grating) contained in a resonant cavity. The periodic forces on the electrons drive the cavity on its resonances in the THz regime. We describe theoretical predictions for the sheet beam parameters required and the likely performance of the device. We also describe experimental progress towards sheet-beam generation using field-emitted electrons from a carbon-nanotube array. We describe the carbon nanotube growth process and demonstrate sheet-beam current densities which exceed the predicted turn-on current density of the Orotron cavity.

Orotron-based sub-millimeter-wave source

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.

Photon filter for energetic neutral atom detectors from carbon nanotubes

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.

Influence of argon on field emission from CVD-grown in-plane single-walled carbon nanotube meshes

The influence of argon (Ar) on the growth of in-plane single-walled carbon nanotube (SWCNTs) meshes using chemical vapor deposition (CVD) was investigated in this study. The SWCNT samples were synthesized using a three-zone furnace (750°C-900°C-750°C) with a methane/hydrogen/argon mixture (total flow of 60 sccm). We verified that the use of argon influences the diameter distribution and the field emission properties of the resulting SWCNTs. The threshold voltage for electron emission was significantly decreased with higher argon concentration due to higher layer conductivity of the samples.

A lateral carbon nanotube based field emission triode

We are investigating techniques for fabricating lateral CNT-based field emission devices for the purpose of creating diodes, triodes, and ultimately integrated circuits. Our goal is to create CNT-based field emission devices that offer both integration and production advantages over the more typical vertical emission sources, such as those used in display technologies. Since field-emitted current is a very strong function of the applied electric field, these devices will undoubtedly find potential use in applications requiring large gain.

Miniature Orotrons Utilizing Carbon Nanotube Cathodes

Summary form only given. The goal of producing THz radiation from miniature electron beam devices has refocused interest in orotrons. Here we consider planar orotrons driven by electron beams produced by carbon nanotube cathodes. The efficiency of these devices improves with increasing current density. However, with increasing current density, self-fields become more important. Here, the theory of self-fields in a planar orotron is developed. We find that the parameters of the grating, which provides the slow wave fields that interact with the beam, also affect the self-fields, which give rise to the slow space charge wave. Thus, optimization of the grating parameters requires consideration of their impact on the dispersive properties off the slow space charge wave. Thus, the field emitter arrays meet both the geometric and current-density requirements predicted for the orotron. miniature electron beam devices