Inexpensive and quick to produce, these digitally fabricated plasma sensors could help scientists predict the weather or study climate change
MIT scientists have created the first fully digitally fabricated plasma sensors for orbiting spacecraft. These plasma sensors, also known as Retarding Potential Analyzers (RPA), are used by satellites to determine the chemical composition and ion energy distribution of the atmosphere.
3D-printed and laser-cut hardware worked as well as state-of-the-art solid-state plasma sensors that are fabricated in a clean room, making them expensive and requiring weeks of complex fabrication. In contrast, 3D printed sensors can be produced for tens of dollars in days.
Due to their low cost and quick production, the sensors are ideal for CubeSats. These inexpensive, low-power, and lightweight satellites are often used for communication and environmental monitoring in Earth’s upper atmosphere.
The researchers developed RPAs using a glass-ceramic material that is more durable than traditional sensor materials such as silicon and thin-film coatings. By using glass-ceramics in a manufacturing process developed for 3D printing with plastics, it was possible to create intricately shaped sensors capable of withstanding the large temperature variations that a spacecraft would encounter in lower Earth orbit.
“Additive manufacturing can make a big difference to the future of space hardware. Some people think that when you 3D print something, you have to concede less performance. But we’ve shown that’s not always the case. Sometimes there’s nothing to compromise,” says Luis Fernando Velásquez-García, senior scientist at MIT’s Microsystems Technology Laboratories (MTL) and lead author of a paper introducing plasma sensors.
Alongside Velásquez-García on the article, lead author and MTL postdoc Javier Izquierdo-Reyes; graduate student Zoey Bigelow; and postdoc Nicholas K. Lubinsky. The research is published in Additive manufacturing.
An RPA was first used on a space mission in 1959. The sensors detect the energy of ions, or charged particles, floating around in plasma, which is a superheated mixture of molecules found in Earth’s upper atmosphere. Aboard an orbiting spacecraft like a CubeSat, the versatile instruments measure energy and perform chemical analyzes that can help scientists forecast the weather or monitor climate change.
The sensors contain a series of electrically charged meshes dotted with tiny holes. As the plasma passes through the holes, electrons and other particles are removed until only ions remain. These ions create an electric current that the sensor measures and analyzes.
The key to the success of an RPA is the housing structure that aligns the stitches. It must be electrically insulating while being able to withstand sudden and drastic temperature variations. The researchers used a printable glass-ceramic material that exhibits these properties, known as Vitrolite.
Launched in the early 20th century, Vitrolite was often used in the colored tiles that became a common sight in art deco buildings.
The durable material can also withstand temperatures as high as 800 degrees Celsius without breaking down, while the polymers used in semiconductor RPAs begin to melt at 400 degrees Celsius.
“When you make this sensor in the clean room, you don’t have the same degree of freedom to define materials and structures and how they interact. What made this possible were the latest developments in additive manufacturing,” says Velásquez-García.
The 3D printing process for ceramics typically involves ceramic powder being struck with a laser to fuse it into shapes, but this process often leaves the material coarse and creates weak spots due to the high heat of the lasers.
Instead, the MIT researchers used vat polymerization, a process introduced decades ago for additive manufacturing with polymers or resins. With vat polymerization, a 3D structure is built one layer at a time by repeatedly immersing it in a vat of liquid material, in this case Vitrolite. Ultraviolet light is used to harden the material after each layer is added, then the platform is submerged in the tank again. Each layer is only 100 microns thick (about the diameter of a human hair), allowing the creation of smooth, poreless and intricate ceramic shapes.
In digital manufacturing, the objects described in a design file can be very complex. This precision allowed the researchers to create laser-cut meshes with unique shapes so that the holes lined up perfectly when placed inside the RPA case. This allows more ions to pass through, leading to higher resolution measurements.
Because the sensors were cheap to produce and could be made so quickly, the team prototyped four unique designs.
While one design was particularly good at capturing and measuring a wide range of plasmas, such as those a satellite would encounter in orbit, another was well suited for detecting extremely dense and cold plasmas, which are usually only measurable at using ultra-precise semiconductor devices.
This high precision could enable 3D printing of sensors for applications in fusion energy research or supersonic flight. The rapid prototyping process could even further spur innovation in satellite and spacecraft design, Velásquez-García adds.
“If you want to innovate, you have to be able to fail and take the risk. Additive manufacturing is a very different way of making space hardware. I can make space hardware and if it fails, it doesn’t matter. important because I can make a new version very quickly and cheaply, and really iterate on the design. It’s a great sandbox for researchers,” he says.
While Velásquez-García is happy with these sensors, in the future he wants to improve the manufacturing process. Reducing layer thickness or pixel size in glass-ceramic tank polymerization could create even more precise complex material. In addition, fully additive manufacturing of the sensors would make them compatible with manufacturing in space. He also wants to explore the use of artificial intelligence to optimize sensor design for specific use cases, such as dramatically reducing their mass while ensuring they remain structurally sound.
This work was funded, in part, by MIT, the MIT-Tecnológico de Monterrey Nanotechnology Program, the MIT Portugal Program, and the Portuguese Foundation for Science and Technology.