Small spark gaps; big X-rays
Pulsed power technology has many potential applications and University of Missouri’s College of Engineering researchers are hotly pursuing some of them in the College’s pulsed power labs.
Funded by Sandia National Laboratories in Albuquerque, N.M., a National Nuclear Security Administration Lab, an investigation by Scott Kovaleski, an assistant professor in electrical and computer engineering, involves the study of the precision timing of the laser-triggered spark gap switches that power a Sandia accelerator known as the Z machine. The accelerator’s operational goal is to produce the holy grail of energy: high-yield nuclear fusion.
Kovaleski’s research employs a Marx capacitor bank similar to those that drive the Z machine. He and John Gahl, an electrical and computer engineering professor and interim chair of the Chemical Engineering Department, are fine-tuning its laser triggering system—varying pulsed power and laser parameters—with the intent of reducing the inherent temporal wiggle that is a natural phenomenon in high-voltage pulse formation.
“High-voltage pulse forming involves compressing stored electrical energy in space and time for delivery to a load, using the techniques of pulsed power engineering,” said Kovaleski, describing the gradual build-up of power in the Z machine.
The Z machine’s successful production of inertial confinement fusion relies on the synchronous firing of 32 laser-triggered switches within nanoseconds of one another. The resulting pulse produces 100s of terawatts—one terawatt is the equivalent of a million megawatts—of X-ray power that will set the high-energy reaction into motion.
Electrical pulsed energy travels through switches and down water- and vacuum-filled transmission lines to deliver 26 million amps of current to an array of wires the size of a spool of thread. The wires are turned to plasma—compressed on an axis in what is called a magnetic pinch—generating nearly 300 terawatts of X-ray power, making the Z machine the largest producer of X-rays in the world.
Recent promising technological advancements in electrical circuitry have provided Sandia scientists with renewed enthusiasm that controlled high-yield nuclear fusion is on the horizon. The new system, known as a linear transformer driver (LTD), uses lower voltage, but is capable of firing in regular intervals in brief, powerful bursts.
“The switch studies we are doing are generally applicable to both LTD- pulsed power drivers and Marx bank drivers,” said Kovaleski. “The needs for precision triggering actually increase for LTD systems, making our work even more relevant. We are continuing switching studies on the Marx at high voltage, and are also doing the same studies on a smaller test stand at lower LTD-type voltages.”
In January, Kovaleski received $100,000 in additional funding from Sandia to continue his precision work with laser triggered-spark gaps.
The University of Missouri College of Engineering has enjoyed a long and productive relationship with Sandia. Besides funding the research of Mizzou Engineering professors, a collaboration among 19 entities—national labs, industries and universities, including MU—aims to entice top-rated undergraduate students into pursuing graduate study and careers in the critical disciplines of pulsed power, high-energy density science, and radiation effects. Internships expose promising students to the interesting and challenging work at Sandia.
Thrust into the wild blue yonder
In addition to his pulsed power research project for Sandia National Labs, Scott Kovaleski, an assistant professor in electrical and computer engineering, invented and is conducting research with ferroelectric plasma thrusters (FEPT) for space propulsion of small satellites weighing less than 25 pounds or less.
The size of three quarters stacked together, the thruster developed by Kovaleski’s research group contains a lithium niobate crystal. “By applying hundreds of volts of radio frequency power to the crystal and making use of the magic of ferroelectricity, plasma is generated,” he explained. “As the plasma ions are pushed away by electric fields from the crystal and radio frequency voltage, an equal and opposite reaction force pushes on the device creating thrust.”
Small satellites can be used to repair larger satellites and may also be used to spy on them or to destroy them. Additionally the so-called micro-satellites can work together and if one fails the others can work independently, reducing mission risk. They also represent a great cost savings when launched compared to larger spacecraft.
Size matters when the craft being powered is modified with the prefix micro. “Our propulsion source may possibly be the most compact and lightest weight electrostatic thruster developed so far,” said Kovaleski. The fact that the FEPT and its power supply are a simple, self-contained unit is also an advantage over other types of propulsion.
In the course of testing the thruster, Kovaleski’s research group made the exciting discovery that the thruster will work equally well in the earth’s atmosphere, as it will in outer space. That finding opens a window to the possibility of near space propulsion of very large blimp-like airships similar to but much larger than those seen above sporting events. But these airships are hard to navigate in the rarified atmosphere 40 miles above the earth’s surface.
“It’s very windy in near space and the air is so thin that propellers won’t function,” said Kovaleski. “The thrusters would allow the air ship to hover and survey the same area over a long period of time.”
Two of Kovaleski’s undergraduate research assistants, Nelson DeSouza and Brett Scott, are working to build a small-scale airship to model the thruster. They expect to complete their work on it this semester.
“Pulsed power and plasma science are technologies on the cusp of showing up in many new applications, including biomedical processes like disinfection and blood coagulation, but a limiting factor is compactness,” said Kovaleski. “That’s the beauty in the simplicity and compactness of ferroelectric devices like the plasma thruster we have developed.”