FEATURE

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Goddard’s New Innovators

Building a Better Electron Gun

Spacecraft instruments have followed a technology-development trajectory similar to computers. Through advances in technology, they have become smaller and lighter. But assuring a berth on future NASA missions will require scientists and engineers to shrink them even more.

Stephanie Getty, a materials engineer in the Goddard Materials Engineering Branch who was hired in 2004 specifically to apply nanotechnology solutions to instrument designs, has found a promising approach that could assure Goddard’s leadership in the field of mass spectrometry, a technique scientists use to determine the composition and abundance of atoms in a molecular substance.

She is now working with scientists in Goddard’s Atmospheric Experiments Laboratory and engineers in Goddard’s Materials Engineering and Detector Systems Branches to develop a miniaturized “electron gun” for a next-generation, time-of-flight mass spectrometer. In electron-impact-ionization mass spectrometry, the electron gun is the heart of the instrument. It produces and focuses an electron beam that ionizes gas molecules so that the spectrometer can measure their masses and ultimately determine the molecules’ chemical makeup.

The team’s ultimate goal is to eventually createa mass spectrometer that would be about as large as a CD case and consume less power than a clock radio (about 1 watt). In contrast, the Goddard-developed mass spectrometer that flew on the Huygens probe was roughly the size and weight of a bowling ball and consumed as much power as a small light bulb, which at the time was considered a significant engineering feat.

Carbon Nanotubes

Getty’s proposed electron gun, however, would be substantially smaller. Developed in part with Goddard Internal Research and Development funding, her technology is made of carbon nanotubes (CNTs), a cylindrical form of graphite.

Grown in a laboratory, carbon nanotubes are just a few tens of nanometers in diameter and demonstrate extraordinary mechanical and electrical properties. For mass spectrometry, their use is especially ideal. By simply applying an electrical field to the nanotubes, electrons are released, which can be focused to form an electron beam — the sole purpose of an electron gun. Compared with more traditional electron-gun technologies, carbon nanotubes also are more efficient because they consume less power and mass and can operate at ambient temperatures.

What makes her design different from other carbon-nanotube designs is the fact that she produces the CNTs not as films, but as patterned arrays of towers on a 5-micron by 5-micron grid grown on a substrate, each tower containing 1,000 individual nanotubes. Although films produce electrons, Getty says testing showed that her patterned towers were more effective at operating at low voltages. They also consumed less power.

Since that demonstration, Getty has developed a box-shaped prototype that measures just one centimeter in width, height, and depth. Over the next year, she plans to improve its packaging, among other improvements, and integrate it with the prototype mass spectrometer now under development.

Currently, Getty places her technology at a readiness level of about three or four. However, in the emerging field of nanotechnology, where most research remains at the proof-of-concept level, her readiness level is actually quite high, she says. “The unique part about this is the application. We’re moving past proof-of-concept.”

Extreme Electronics

Designing and developing a complex integrated circuit that operates at room temperature is difficult enough. Now imagine trying to craft one that can withstand the harsh temperatures found in space. It’s not easy.

“Right now, we don’t fully understand how each component or piece reacts to super-cold temperatures or how it will fit together with other components as a system when exposed to extreme environments,” says La Vida Cooper, an electronics engineer. She has won Internal Research and Development (IRAD) funding to learn how they behave. The aim is to give Goddard electronics engineers the models they need to more efficiently design application-specific integrated circuits (ASICs) that carry out a particular task even when exposed to harsh temperatures.

The pay-off for NASA could be significant. It could assist the Agency in its goal to further miniaturize spacecraft and instruments.

Today, spacecraft electronics are typically placed inside a protective, thermally controlled enclosure that takes up more mass, says Mike Johnson, an assistant chief for technology for Goddard’s Electrical Engineering Division. “What we’d like to do is get rid of that enclosure and develop electronics that can operate under extremely cold temperatures,” he says.

Paucity of Standards

At this stage, however, getting rid of that box isn’t a viable option. Currently, industry standards are based on components operating over a fairly limited temperature range — not the extremes found in space, Johnson says. Therefore, it takes engineers longer to design the complicated circuits because they can’t accurately predict how the individual transistors — the tiny components that make up an integrated circuit — will behave undersignificantly colder conditions. The lack of models also hinders simulation testing, an important step in ASIC development.

Johnson says models are available for the full military temperature range of -67°F to 257°F (-55°C to 125°C), but they fall far short of the -364° F (-220°C) needed to develop NASA systems. “The models don’t exist,” he says. “This project will derive models for CMOS circuit-fabrication processes. It complements a related Exploration Technology Development Program effort focused on silicon-germanium circuits.”

The First Step

With her IRAD award, Cooper has developed an automatic test set-up that exposes commercially available transistors to a range of temperatures down to -432.67°F (-258.14°C). Initial results so far show that the single transistors she’s tested can operate at extremely cold temperatures and power up after 72 hours of no operation. In addition to testing single transistors, she’s also planning to test a full suite of circuits, including those used for analog, digital, and mixed-signal electronics.

Once the data are collected, Cooper will be able to extract important parameters and create design and simulation models optimized for cold environments. Using these models Goddard’s ASICs Group will be able to efficiently design and test integrated circuits that operate reliably at low temperatures. These circuits can be used in telescope readouts and instrument front-ends.

“If our engineers had this information from the start, they could really shorten the design time,” Cooper says. “Principal investigators could reference higher TRLs in their instrument concepts. We also could save on bench and cryo-testing. This really would help in a lot of ways.”