WPI Research Publication

FALL 2012

WPI Research is the research magazine of Worcester Polytechnic Institute. It contains news and features about graduate research in the arts and sciences, business, and engineering, along with notes about new grants, books, and faculty achievements.

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TOOLS OF THE TRADE Pulling Knowledge Out of Thin Air The stainless steel cylinder in the basement of Higgins Laboratories is big enough for two adults to climb inside. Surrounded by industrial-strength pumps and blowers, it resembles a high-tech furnace. But it is actually the best tool researchers at WPI have for simulating outer space right here on Earth. "It's a unique facility," says John Blandino, associate professor of mechanical and aerospace engineering, who came to WPI a decade ago from NASA's Jet Propulsion Laboratory in California. "It opens up a whole range of possibilities." As Blandino explains how the chamber works, Zachary Taillefer, a PhD student in mechanical engineering, flips a switch to start a mechanical pump that will begin the process of removing the air. The machine whirs and the numbers on the readout fall from 790 torr (a little over one atmosphere of pressure) to 8 torr within just a few minutes. Then a second pump kicks in, and the pressure within the chamber falls even lower. Once Taillefer is sure there are no leaks, he can turn on the third pump, called a cryopump, which uses temperatures close to absolute zero to freeze out most of the remaining gas molecules. "We get down to about 10-7 torr, or less than a billionth of an atmosphere," Blandino says. While it may not look much like the far reaches of space, the vacuum chamber is a vital tool for Blandino's current research, which seeks to improve the durability of the cathodes used in electric propulsion systems designed for spacecraft. [40] By Amy Crawford In contrast to explosive chemical rockets, electric pro- pulsion systems use streams of electrically charged particles to create thrust. They have higher exhaust velocities than chemical rockets, and can therefore deliver a heavier payload for a given mass of propellant. But they generate far less thrust, and they only work in a near vacuum, so they can't be used to propel spacecraft into Earth orbit. They are typi- cally used for delicate operations, such as keeping satellites in geosynchronous orbit, and for deep space missions that require thrusters to function for a long time. "It takes you longer to get there," Blandino says, "but you use a lot less fuel along the way." The most commonly used elec- tric thrusters rely on hollow cathodes to generate electrons, which in a vacuum help ionize gaseous fuel into plasma. Cathodes also neutralize the stream of positive ions that the thruster emits, so the spacecraft does not become too negatively charged. But NASA has found that after years of use — the mission timeframe for spacecraft such as Dawn, which is currently exploring the asteroid belt — these cathodes wear out under the bombard- ment of countless energetic ions, a phenomenon scientists call "sputtering." "Sputtering happens when an ion hits the surface with enough energy to knock out some of the material," Blan- dino explains. "So over time, the ions will erode part of the cathodes completely away and they'll fail." Inside the vacuum chamber, Blandino and Taillefer have set up a cathode on loan from NASA's Glenn wpi.edu/+research

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