Todd King

The Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatile Evolution Mission

The Neutral Gas and Ion Mass Spectrometer (NGIMS) of the Mars Atmosphere and Volatile Evolution Mission (MAVEN) is designed to measure the composition, structure, and variability of the upper atmosphere of Mars. The NGIMS complements two other instrument packages on the MAVEN spacecraft designed to characterize the neutral upper atmosphere and ionosphere of Mars and the solar wind input to this region of the atmosphere. The combined measurement set is designed to quantify atmosphere escape rates and provide input to models of the evolution of the martian atmosphere. The NGIMS is designed to measure both surface reactive and inert neutral species and ambient ions along the spacecraft track over the 125–500 km altitude region utilizing a dual ion source and a quadrupole analyzer.

Mechanical property measurement of InP-based MEMS for optical communications

We investigate mechanical properties of indium phosphide (InP) for optical micro-electro-mechanical systems (MEMS) applications. A material system and fabrication process for InP-based beam-type electrostatic actuators is presented. Strain gradient, intrinsic stress, Young’s modulus, and hardness are evaluated by beam profile measurements, nanoindentation, beam bending, and electrostatic testing methods. We measured an average strain gradient of δe0/δt = 4.37 × 10 −5 m −1 , with an average intrinsic stress of σ0 =− 5. 4M Pa for [0 1 1] beams. The intrinsic stress results from arsenic contamination during molecular beam epitaxy and (MBE) can be minimized by careful MBE growth and through the use of stress compensating layers. Nanoindentation of (1 0 0) InP resulted in E = 106. 5G Pa and H = 6.2 GPa, while beam bending of [0 1 1] doubly clamped beams resulted in E = 80.4 GPa and σ0 =− 5.6 MPa. We discuss the discrepancy in Young’s modulus between the two measurements. In addition, we present a method for simultaneously measuring Young’s modulus and residual stress using beam bending. Electrostatic actuation in excess of 20 V is demonstrated without breakdown.

Enhanced magnetocaloric effects in R3(Ga1−xFex)5O12 (R=Gd, Dy, Ho; 0<x<1) nanocomposites

A series of R 3 (Ga 1-x Fe x ) 5 O 12 (R=Gd, Dy, Ho; 0<x<1) compounds for potential magnetic refrigerants were synthesized by chemical routes and characterized by X-ray diffraction, and SQUID magnetometry. Dy and Ho were chosen since they respectively possess increasing orbital contributions to the total angular magnetic moment of the atom over the zero value for Gd. X-ray data showed that garnet structures were obtained and that improvements over the Gd 3 (Ga 0.5 Fe 0.5 ) 5 O 12 compound, which was reported in 1992 as possessing enhanced magnetocaloric effects, may be achieved by equilibrating at 1473K for 15 h, rather than at 1173K for 15h as was done in the earlier studies. Magnetometry measurements showed that when Gd was substituted either by Dy or Ho, the material was superparamagnetic, possessing fine magnetic clusters resulting in enhanced magnetocaloric effects (ΔS m ) with respect to the basic paramagnetic garnet (i.e., x = 0). In addition, with variation in x, the optimal ΔS m was measured for the x = 0.5 compound, similar to that found for the Gd-containing garnet nanocomposites. The optimal ΔS m values for the Ho- and Dy-containing compounds, respectively, were found to be about the same or smaller than that for the optimal Gd-containing nanocomposite Gd 3 (Ga 0.5 Fe 0.5 ) 5 O 12 , despite the increased total angular moment. We interpret these results as indicating a reduction in the interaction strength between the rare-earth elements and the Fe as the Gd is replaced by Dy or Ho, and that Dy reduces this interaction strength faster than does Ho.

Programmable microshutter arrays for the JWST NIRSpec: optical performance

Two-dimensional microshutter arrays (MSAs) are being developed at the NASA Goddard Space Flight Center for the James Webb Space Telescope (JWST) for use as a programmable aperture mask for object selection for the Near Infrared Multiobject Spectrograph (NIRSpec). The MSAs are designed to provide high transmission efficiency for the selected objects and high on to off contrast ratio at the /spl sim/35 K operating temperature of JWST. The arrays of shutters are produced from silicon nitride membranes on a 100/spl times/200 /spl mu/m pitch. Individual shutters consist of a shutter blade of silicon nitride suspended from the shutter frame by a nitride torsion flexure. The shutters are normally closed. All shutters in the array are opened by the scanning magnetic field, and are held open by an electrostatic potential applied between the open shutters and the shutter support grid electrodes. To close the required shutters for a specific configuration, the potential between the shutter to be deselected and the support frame is set to zero, allowing the shutter to close. In this way, full random access addressing is achieved. We have produced such shutters and have demonstrated mechanical actuation and selection. Optical tests of open and closed shutters have demonstrated the required contrast for the JWST application. The MSA is a pioneering technology that provides the most capable possible multiobject spectrograph for JWST. It provides high contrast selection, high transmission efficiency, and can meet the environmental requirements for JWST.

A compact, high-performance continuous magnetic refrigerator for space missions

Abstract We present test results of the first adiabatic demagnetization refrigerator (ADR) that produce true continuous cooling at sub-kelvin temperatures. This system uses multiple stages that operate in sequence to cascade heat from a “continuous” stage up to a heat sink. Continuous operation avoids the usual constraints of long hold times and short recycle times that lead to the generally large mass of single-shot ADRs, and allows us to achieve much higher cooling power per unit mass. Our design goal is 10 μW of cooling at 50 mK while rejecting heat to a 6–10 K heat sink. The total cold mass is estimated to be less than 10 kg, including magnetic shielding of each stage. These parameters envelop the requirements for currently planned astronomy missions. The relatively high temperature heat rejection capability allows it to operate with a mechanical cryocooler as part of a cryogen-free, low temperature cooling system. This has the advantages of long mission life and reduced complexity and cost. At present, we have assembled a three-stage ADR that operates with a superfluid helium bath. Additional work is underway to develop magnetocaloric materials that can extend its heat rejection capability up to 10 K. Design, operation and performance of the ADR are discussed.

Performance of a carbon nanotube field emission electron gun

A cold cathode field emission electron gun (e-gun) based on a patterned carbon nanotube (CNT) film has been fabricated for use in a miniaturized reflectron time-of-flight mass spectrometer (RTOF MS), with future applications in other charged particle spectrometers, and performance of the CNT e-gun has been evaluated. A thermionic electron gun has also been fabricated and evaluated in parallel and its performance is used as a benchmark in the evaluation of our CNT e-gun. Implications for future improvements and integration into the RTOF MS are discussed.

Simulation of a Miniature, Low-Power Time-of-Flight Mass Spectrometer for In Situ Analysis of Planetary Atmospheres

We are implementing nano- and micro-technologies to develop a miniaturized electron impact ionization mass spectrometer for planetary science. Microfabrication technology is used to fabricate the ion and electron optics, and a carbon nanotube (CNT) cathode is used to generate the ionizing electron beam. Future NASA planetary science missions demand miniaturized, low power mass spectrometers that exhibit high resolution and sensitivity to search for evidence of past and present habitability on the surface and in the atmosphere of priority targets such as Mars, Titan, Enceladus, Venus, Europa, and short-period comets. Toward this objective, we are developing a miniature, high resolution reflectron time-of-flight mass spectrometer (Mini TOF-MS) that features a low-power CNT field emission electron impact ionization source and microfabricated ion optics and reflectron mass analyzer in a parallel-plate geometry that is scalable. Charged particle electrodynamic modeling (SIMION 8.0.4) is employed to guide the iterative design of electron and ion optic components and to characterize the overall performance of the Mini TOF-MS device via simulation. Miniature (< 1000 cm3) TOF-MS designs (ion source, mass analyzer, detector only) demonstrate simulated mass resolutions > 600 at sensitivity levels on the order of 10-3 cps/molecule N2/cc while consuming 1.3 W of power and are comparable to current spaceflight mass spectrometers. Higher performance designs have also been simulated and indicate mass resolutions ~1000, though at the expense of sensitivity and instrument volume.

A miniature MEMS and NEMS enabled time-of-flight mass spectrometer for investigations in planetary science

Solar system exploration and the anticipated discovery of biomarker molecules is driving the development of a new miniature time-of-flight (TOF) mass spectrometer (MS). Space flight science investigations become more feasible through instrument miniaturization, which reduces size, mass, and power consumption. However, miniaturization of space flight mass spectrometers is increasingly difficult using current component technology. Micro electro mechanical systems (MEMS) and nano electro mechanical systems (NEMS) technologies offer the potential of reducing size by orders of magnitude, providing significant system requirement benefits as well. Historically, TOF mass spectrometry has been limited to large separation distances as ion mass analysis depends upon the ion flight path. Increased TOF MS system miniaturization may be realized employing newly available high speed computing electronics, coupled with MEMS and NEMS components. Recent efforts at NASA Goddard Space Flight Center in the development of a miniaturized TOF mass spectrometer with integral MEMS and NEMS components are presented. A systems overview, design and prototype, MEMS silicon ion lenses, a carbon nanotube electron gun, ionization methods, as well as performance data and relevant applications are discussed.

Microshutter array system for James Webb Space Telescope

We have developed microshutter array systems at NASA Goddard Space Flight Center for use as multi-object aperture arrays for a Near-Infrared Spectrometer (NIRSpec) instrument. The instrument will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope, after the Hubble Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light from objected galaxies in space with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size close to 100x200 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with minimized stress concentration. In order to enhance optical contrast, light shields are made on each shutter to prevent light leak. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining and packaged utilizing a novel single-sided indium flip-chip bonding technology. The MSA flight system consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays. The system will be placed in the JWST optical path at the focal plane of NIRSpec detectors. MSAs that we fabricated passed a series of qualification tests for flight capabilities. We are in the process of making final flight-qualified MSA systems for the JWST mission.

Microshutters arrays for the JWST near-infrared spectrometer

The Near Infrared Spectrograph (NIRSpec) for the James Webb Space Telescope (JWST) is a multi-object spectrograph operating in the 0.6-5.0 μm spectral range. One of the primary scientific objectives of this instrument is to measure the number and density evolution of galaxies following the epoch of initial formation. NIRSpec is designed to allow simultaneous observation of a large number of sources, vastly increasing the capability of JWST to carry out its objectives. A critical element of the instrument is the programmable field selector, the Microshutter Array. The system consists of four 175 x 384 close packed arrays of individually operable shutters, each element subtending 0.2” x 0.4”on the sky. This device allows simultaneous selection of over 200 candidates for study over the 3.6’ x 3.6’ field of the NIRSpec, dramatically increasing its efficiency for a wide range of investigations. Here, we describe the development, production, and test of this critical element of the NIRSpec.