A team of MIT scientists has created the first completely digitally manufactured plasma sensors — also known as retarding potential analyzers (RPAs) — for orbiting spacecraft. The sensors are used by satellites to determine the chemical composition and ion energy distribution of the atmosphere.
The 3D-printed, laser-cut hardware performed on par with expensive, state-of-the-art semiconductor plasma sensors that were manufactured in a cleanroom. By contrast, the 3D-printed sensors can be produced for tens of dollars in a matter of days.
Their low cost and speedy production make the sensors ideal for CubeSats, and they’re often used for communication and environmental monitoring in Earth’s upper atmosphere.
The researchers developed RPAs using a glass-ceramic material more durable than traditional sensor materials (e.g., silicon and thin-film coatings). Using the glass-ceramic in a fabrication process that was developed for 3D printing with plastics enabled the team to create sensors with complex shapes that could withstand the wide temperature swings a spacecraft would encounter.
“Additive manufacturing can make a big difference in the future of space hardware,” said Luis Fernando Velásquez-García, Principal Scientist, MIT’s Microsystems Technology Laboratories. “Some people think that when you 3D-print something, you have to concede less performance. But we’ve shown that is not always the case. Sometimes there is nothing to trade off.”
An RPA was first used in a space mission way back in 1959. How it works is the sensors detect the energy in ions, or charged particles, that are floating in plasma. Aboard an orbiting spacecraft, they measure energy and conduct chemical analyses that can help scientists predict the weather or monitor climate change.
The sensors contain a series of electrically charged meshes dotted with tiny holes. As plasma passes through the holes, electrons and other particles are removed until only ions remain. These ions then create an electric current measured and analyzed by the sensor.
The housing structure that aligns the meshes must be electrically insulating while also being able to withstand sudden, drastic temperature swings. The team used Vitrolite — a printable, glass-ceramic material. Vitrolite, pioneered in the early 20th century, was often used in colorful tiles that became a common sight in art deco buildings; it can withstand temperatures as high as 800 °C without breaking down. Conversly, polymers used in semiconductor RPAs start to melt at 400 °C.
“When you make this sensor in the cleanroom, you don’t have the same degree of freedom to define materials and structures and how they interact together,” said Velásquez-García. “What made this possible is the latest developments in additive manufacturing.”
The team used vat polymerization, a decades-old process for additive manufacturing with polymers or resins. With vat polymerization, a 3D structure is built one layer at a time by submerging it repeatedly into a vat of liquid material — Vitrolite, in this instance.
UV light is used to cure the material after each layer is added before the platform is submerged again. Each layer is only 100 microns thick — about the diameter of a human hair — which enables the creation of smooth, pore-free, complex ceramic shapes.
Since the sensors were cheap and easy to produce, the team prototyped four designs. One design was especially effective at capturing and measuring a wide range of plasmas, a la a satellite in orbit, while another was well-suited for sensing extremely dense and cold plasmas.
Going forward, the team wants to enhance the fabrication process. Reducing the thickness of layers or pixel size in glass-ceramic vat polymerization could create complex hardware that is even more precise. Also, fully additively manufacturing the sensors would make them compatible with in-space manufacturing. The team also hopes to explore the use of artificial intelligence to optimize sensor design for specific use cases.