The Sound of Fusion

Imploding plasma

How a big bang in the basement contributes to a future of fusion energy

When the COBRA fires, the sound is startling. 

The COBRA (Cornell Beam Research Accelerator) pulsed-powered generator delivers one million amperes of current in a 100-nanosecond BANG that reverberates through the Laboratory of Plasma Studies (LPS) in the basement of Grumman Hall.

The massive electric discharge is routed into a vacuum chamber where it annihilates the substance contained there, a puff of gas or an array of wires, breaking it down into ions and electrons, a state of matter known as a plasma. In an instant, the plasma is squeezed, or pinched, by a similarly massive magnetic field in a process called a Z-pinch for the axis orientation of the squeeze.

“Plasmas tend to be unstable. They don't want to go where you want to push them,” said David Hammer, the J. Carlton Ward, Jr. Professor of Nuclear Energy Engineering who leads the Multi-University Center of Excellence for Pulsed-Power-Driven High-Energy-Density Science. “It's like squeezing a balloon, and the balloon goes out between your fingers. That's what a plasma does as well. So, developing a configuration of plasma that can be squeezed by a magnetic field is very important to the field of magneto inertial fusion.”

LPS researchers are not working at a scale that would produce fusion energy here below populated classrooms, labs and offices in Grumman Hall, though hearing the COBRA fire might make it seem like they are. Similar experiments are conducted at the Sandia National Laboratories in Albuquerque, New Mexico, at a much greater scale using proper fusion fuel. The work in LPS is aimed at developing diagnostic tools and testing processes that can later be scaled up at the national labs.

A variety of instruments are used to measure what’s happening inside the vacuum chamber, but none produce results quite as spectacular as the cameras. During one pulse, which lasts less than a millionth of a second, a bank of synchronized cameras take a set of pictures, each 10 nanoseconds apart, showing a rapidly growing instability in the plasma.

“You see the plasma going unstable as the pictures show the plasma coming down to a very tight pinch,” Hammer said. “These measurements are fundamental to knowing what causes the plasma to do the motions that it does. Understanding the motions that we see with those cameras is a fundamental aspect of what we're working on.”

Jay Angel in LPS
Ph.D. student Jay Angel prepares the vacuum chamber for the next experiment. Photo by Eric Laine.

Jay Angel, a fourth year Ph.D. student, is developing a novel diagnostic to make measurements of the magnetic fields being produced in plasma expanding in a vacuum using a technique called Zeeman spectroscopy. Diagnostic techniques he develops can be used to show whether a particular method of experimentation will produce favorable results in labs working at greater scales than LPS. “Every diagnostic will work within its own limits,” Angel explained, “What I'm doing is finding the limits.” 

Angel worked on biomedical imaging techniques at Boston University College of Engineering where he earned his master’s degree on laser speckle rheology, which is shining a laser on blood as it coagulates to get real time information about clotting coefficients—basically how fast your blood clots. He appreciated working on a project with a goal that provided some societal benefit, which is a feature that drew him to fusion research as well.

“About a month after I applied to Cornell, Dave Hammer reached out to me,” Angel said. “As soon as he said ‘fusion’ I was like, wow, that seems like possibly one of the best fits for the goal of improvement for humanity. It was hard to even imagine another research topic that could beat it.”

Angel’s work in LPS takes advantage of his experience with various imaging techniques, many of them using lasers. Analyzing the images captured by magnetically squeezing plasma sometimes produces more questions than answers. “There's a lot of things we still don't understand that happen on our machines,” Angel said, “and if you scale it up, it’s even more complicated.”

But he’s focused on the larger ambitions of fusion energy. “Whether my diagnostic works or not is not that important in the big picture,” he said. “I'm still spending every day working on plasma physics, looking at how things work, determining how a diagnostic could apply in various scenarios. We’re working towards fusion.”

Chiatai Chen, fourth year Ph.D. student in LPS, agrees. “My work is trying to contribute to the understanding of some of the physics required to make nuclear fusion happen,” he said. Chen is studying whether it's possible to improve the confinement of a very hot and dense plasma with what’s called a magnetic mirror. 

While Angel sends COBRA’s electrical energy through a puff of gas to generate plasma, Chen drives the current through an array of wires which implode in the process. A magnetic mirror is formed simultaneously by the twisted electrodes to confine the plasma. Confining the plasma with this magnetic mirror can allow it to reach a higher temperature and density, and high density and temperatures are what you need to achieve fusion, Chen explained. 

Wire array
A look inside the vacuum chamber prepared for a COBRA shot. Eight 17-micron diameter aluminum wires are threaded through stainless steel tubes twisted to act like a magnetic coil. Photo by William Potter (LPS).

“Here in LPS at the scale we're working at,” Chen said, “the temperatures and densities are still too low for fusion. So the motivation behind this project is to demonstrate a method for containing and compressing the plasma that could be scaled up to a larger system at the national labs that might actually produce fusion energy.”

Chen speaks from experience, having spent more than two years as a research scientist at Lawrence Livermore National Laboratory after earning a B.A. in physics from the University of California at Berkeley.

“I was working on things completely unrelated to plasma,” Chen said, “but even though I wasn't working on it personally, I was exposed to it.” At Livermore, the plasma experiments were similar, but instead of driving an electric current through a wire array to create a plasma, they were doing inertial confinement fusion using a very powerful laser.

“That’s where I got interested in high energy density plasma physics, and that's why I decided to change my research direction,” Chen said. “I was hoping to get an opportunity to participate in research work that one day will lead us to fusion energy. And that's why I came to Cornell LPS.” 

The student researchers in LPS are supported by a staff of technicians who maintain the complex and mostly decades-old equipment, often machining their own parts as needed, and who help set the stage for the experiments. “COBRA experiments are a team effort,” Angel said. “The lab just wouldn't operate without them.”

“It can take three or four days, up to a week, to set up all the diagnostics including the optics, the mirrors, the lens, making sure the lasers are going in the right direction, and the cameras are looking at the right angles,” Chen explained. “The experiment itself happens in less than a second—boom—and the cameras capture it.”

Chiatai Chen in LPS
Ph.D. student Chiatai Chen and LPS equipment technician Dan Hawkes (background) await the results of a COBRA shot. Photo by Eric Laine.

The excitement in LPS is palpable to those who visit, whether it’s coming from the epic discharges of the million-amp COBRA, or from the possibilities contained in the research.  “Having a limitless source of energy, if we can make nuclear fusion a reality, is something that really excites me,” said Chen. 

That’s the mission of LPS: to understand the fundamental physics underlying plasma with the ultimate goal of discovering a way to generate a controlled fusion system that produces nuclear reactions safe to harness for energy production. The work is supported by the United States Department of Energy and the National Nuclear Security Administration’s Stewardship Sciences Academic Programs.

Top: This set of four images is the extreme ultraviolet self-emission of an imploding aluminum wire array situated between a dynamic magnetic mirror formed by a pair of twisted electrodes. Starting from the bottom left in clockwise direction, each image was taken 10 nanoseconds after the preceding one. Imaging by Chiatai Chen.

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