Joshua Rovey’s laboratory has a sense of busyness. Sunlight streams in through several large windows. Shelves against one wall contain books and binders; others are filled with supplies for the machinery scattered throughout the room. Work benches fill the spaces not occupied by large metal cylinders—vacuum chambers, required for the experiments Rovey and his research assistants run. Sitting nearest to me is a steel-and-glass device that will generate Röntgen radiation—x-rays. “We’re not actually at the x-ray generation phase yet, so you’re safe, you know, sitting here in the room,” Rovey laughs. This is the Aerospace Plasma Laboratory at Missouri Science and Technology, where the assistant professor of Mechanical and Aerospace Engineering has been working since 2008.
“Plasmas have application to a wide variety of fields and problems,” he explains. “Right now, in this room, we’re breathing air, right? If you were to heat this air up, get it hot enough, eventually the electrons will start popping off the atoms. Now you have free charges, you’ve got negative electrons, positive ions, this conductive fluid: it’s called a plasma.” He gestures to the fluorescent lights overhead. “Plasma is not some archaic thing you’ve never seen before; it’s actually in the lights right here.” Rovey’s laboratory focuses on three particular applications of plasma: aircraft flow control, medical applications, and aerospace propulsion.
In each field, plasma opens up new ways of accomplishing old tasks. Aircraft can be controlled by adjustments in plasma, rather than mechanical flaps; x-rays can use the fluid instead of relying on traditional techniques. Standard x-ray machines function through a thermionic cathode, a heated tungsten wire that ejects electrons. These electrons are accelerated to high speeds and then strike a target—a large tungsten plate—releasing Bremsstrahlung or “braking” radiation. This Röntgen radiation is absorbed only by certain materials, such as lead or bone. The material to be x-rayed is placed between the radiation source and a camera, and only the x-rays that pass through the material trigger the chemical reaction that creates a picture.
“One of the drawbacks with the conventional x-ray machine is what is called a space-charge limit,” Rovey points out. Once a certain number of free electrons have been generated and are clustered around the wire, the cloud of particles becomes so negatively charged that it repels any newly created electrons back into the (comparatively positive) wire. “To pull off more current, you have to increase the voltage. Instead of 70,000 volts, maybe you have to go up to 100,000 volts to get more electrons to come off,” the researcher explains. Increasing voltage leads to increased heat and danger, as well as lower efficiency: greater voltage also indicates larger size, to draw off the heat and reduce risk. More electrons are produced with less energy, which in turn produces less heat that needs to be managed. With thermal restraints that are less strict than of conventional x-ray machines, a plasma x-ray generator may be much smaller than the traditional cart: “We want to take the x-ray machine and shrink it down into something you could basically hold in your hand,” Rovey says.
He gestures to the vacuum chamber beside him. “In our experiment, we have a very high voltage at the bottom of this small vacuum chamber that might be on the order of 70,000 to 100,000 volts. When the voltage gets high enough, we’ll develop a spark in a small tube at the bottom. That spark produces a plasma at very high voltage, and that high voltage will also eject electrons out of the plasma. Instead of having a thermionic cathode that gets hot and emits electrons, we have a plasma that is producing the electrons that accelerate, hit the target, and make the Röntgen x-rays.”
Currently, work in the lab is focused on the properties of the plasma and the electron beam it generates, but phase two of the project will bring generation and measurement of actual x-rays. Also underway are studies on space propulsion, Rovey’s favorite topic. “I always wanted to do aerospace engineering, and rocketry was always interesting to me. I’d never heard of plasma and space propulsion, but one of the very first classes I took in college was called Freshman Engineering. The instructor’s background was in space propulsion, and he eventually became my advisor,” Rovey recalls. “I worked on plasma propulsion systems as an undergraduate in his laboratory, and fell in love with it. I knew that’s what I wanted to learn more about.”
Through his research, Rovey also hopes to revolutionize space propulsion. Current techniques rely heavily on chemical reactions, creating the dramatic “fire and smoke” launch images. “All that fire and smoke is due to a chemical reaction, just like in the engine of your car,” he notes. “Turns out, that’s not a very fuel-efficient method.” Electric propulsion offers the same amount of energy for less mass, which leads to a more affordable space launch; as Rovey points out, “mass equals dollars: the less mass you have to put into space, the less expensive it’s going to be.”
To reduce cost, both cars and spacecraft are turning to new sources of energy: instead of chemical power, many are now tapping into electric propulsion. Problems in aerospace engineering may be solved by solar power, dynamos gathering electricity from radioactive decay, or even on-board nuclear generators—but Rovey is looking for propulsion techniques that derive from plasma. The fluid can provide a lot of “bang for your buck,” he insists, especially in systems like the one he is preparing to test in the advanced plasma laboratory.
“Imagine a coffee can,” he suggests. A device producing electrons sends them into an inert gas filling the can. Electrons moving around the can run into gas atoms and knock electrons from that atom: free (negative) electrons and positive ions are now inside of that can. “If we’re sticking with the coffee can analogy, at the lid we have a grid, just a piece of metal with holes in it. We bias this grid with some very negative voltage, so that positive charges want to go towards it. The positive ions inside the can go towards this negative grid, and shoot through it. The negativity of the voltage determines how fast the ions are going to get shot out, so if we make it very negative, we have really fast ions coming out of the grid.”
“That’s how we would propel a spacecraft: shooting ions out this way, so that the spacecraft wants to move in the opposite direction. That’s the transfer of momentum we need to impart a force onto the spacecraft,” Rovey explains. Plasma offers more momentum for less mass, making it ideal for space propulsion. Tests on plasma propulsion are just beginning in Rovey’s laboratory, and the work is equipment-intensive: “We need an environment without any air to test our space systems,” he notes. The lab currently has two vacuum tanks, which pump a large percentage of the air out of the tank to simulate conditions in outer space.
Though physical experiments are just beginning, Rovey has been running tests for some time already. “The UMRB Grant was used to get a plasma modeling code, to run some simulations to better understand how we can apply high-density plasmas to propulsion,” he explains. When applying for an Air Force grant last year, he was able to present an existing modeling capability and clearer plans for intended experiments—an edge that made Rovey the recipient of a three- to five-year grant from the Young Investigators Research Program. He has received similar funding for the x-ray project. “[The UMRB grant] was instrumental, it was very helpful,” he says, nodding. “Starting out as a new assistant professor, you’re trying to get funding to start your research program. The UMRB grant was just last year, and it’s led to two funded projects so far.”
Work is not limited to the aerospace plasma laboratory: Rovey’s interests also emerge in his teaching and cooperation with students and staff on the Rolla campus. “I have a pretty diverse teaching load,” he admits, with the undergraduate propulsion course as well as several graduate-level fluid dynamics courses. “Plasma is pretty important subject matter,” he suggests, “not just for aerospace students, but for mechanical engineering, nuclear engineering, electrical engineering, and physics students. Plasmas have application to a wide variety of fields and problems, so I think students could really benefit from learning more about them.” Engineering students can choose to apply plasma to Rovey’s favored field—aerospace propulsion—in their senior design projects. “The students divide into different groups,” Rovey says. “One group might work on guidance and control of the satellite, one might work on propulsion, one would work on communications, and so they would divide up work on different aspects of the satellite.” Seniors in the space design team have competed in the Air Force’s University Nanosatellite Program under the direction of Dr. Henry Pernicka for the last five years, earning third place on their very first entry. Rovey’s research and classes assist these students as they search for propulsion systems for their designs.
Working with faculty throughout the University of Missouri system, Rovey is already beginning to troubleshoot, planning not just for tests in a vacuum chamber, but for years of operation in outer space. This level of collaboration and dedication starts in the Aerospace Plasma Laboratory itself, where a polite crowd of research assistants is already gathering near the glass entry of the lab, waiting for this interview to conclude before diving into the real work of the day: creating an x-ray machine that fits in a doctor’s pocket or finding a more efficient way to travel to the stars.