New instrument could help scientists tailor plasma to produce more fusion heat

Written by
Raphael Rosen
April 30, 2024

Creating heat from fusion reactions requires carefully manipulating the properties of plasma, the electrically charged fourth state of matter that makes up 99% of the visible universe. Now, scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have finished building a new plasma measurement instrument, or diagnostic, that could aid the effort to boost the heat of fusion reactions in facilities known as tokamaks and potentially improve the power output of future fusion power plants.

Known as ALPACA, the diagnostic observes light emitted by a halo of neutral atoms surrounding the plasma inside DIII-D, a doughnut-shaped device known as a tokamak operated for the DOE by General Atomics in San Diego. By studying this light, scientists can glean information about the neutral atoms’ density that could help them keep the plasma hot and increase the amount of power generated by fusion reactions. Scientists around the world are trying to harness on Earth the fusion reactions that power the stars to generate electricity without producing greenhouse gases or long-lived radioactive waste.   

ALPACA helps scientists study a process known as fueling. During this process, clouds of neutral atoms of varying densities around the plasma break apart into electrons and ions and enter the plasma. “We’re interested in fueling because neutral atom density can increase plasma particle density, and plasma density affects the number of fusion reactions,” said Laszlo Horvath, a PPPL physicist stationed at DIII-D who helped coordinate ALPACA’s assembly and installation. “If we can increase the plasma’s density, then we can have more fusion reactions, which generate more fusion power. That’s exactly what we want to have in future fusion power plants.”

The hydrogen atoms involved in this type of fueling come from three sources. The first is the original puffs of hydrogen gas that scientists used to initiate the plasma. The second is the combining of electrons and nuclei in the cooler regions of the chamber to form whole atoms. The third is the leaking of hydrogen atoms from the material that makes up the inner chamber surfaces, where they are sometimes trapped during tokamak operations.

Akin to a pinhole camera, the nearly 2-foot long ALPACA collects plasma light that has a specific property known as the Lyman-alpha wavelength. Researchers can calculate the neutral atoms’ density by measuring the light’s brightness. Previously, scientists have inferred the density from measurements taken by other instruments, but the data has been hard to interpret. ALPACA is one of the first diagnostics designed specifically to collect plasma light at the Lyman-alpha frequency, so its data is much clearer.

A schematic drawing of ALPACA

A schematic drawing of ALPACA, a new plasma measurement instrument that observes the light of neutral atoms to determine their density. (Photo courtesy of David Mauzey / PPPL)

Scientists want to increase their understanding of fueling so they can control it. With control over fueling, scientists could make the fusion reactions in tokamaks more efficient and increase the amount of heat they produce. The increased heat is important because the hotter the plasma, the more electricity a tokamak-based power plant could generate. This project is another example of PPPL’s world-class expertise in engineering and plasma diagnostics.

ALPACA is one of a pair of diagnostics. Its twin is called “LLAMA,” which stands for “Lyman-alpha measurement apparatus.” The two diagnostics complement each other in that while LLAMA observes the inner and outer regions of the lower part of the tokamak, ALPACA observes the inner and outer regions of the upper part.

“We need both devices because although we know that neutral atoms surround the plasma, the number of neutral atoms varies from place to place, so we don’t know exactly where they accumulate,” said Alessandro Bortolon, PPPL principal research physicist who heads the PPPL collaboration with the DIII-D National Fusion Facility. “Because of that, and because we can’t extrapolate from single measurements, we have to measure in multiple locations.”

Like all diagnostics, ALPACA serves a crucial purpose. “When we are running experiments on machines like DIII-D, we need to understand what is going on inside the device, especially if we want to boost its performance,” Horvath said. “But because the plasma is at 100 million degrees Celsius, we can’t just use an oven thermometer or anything conventional. They would just melt. Diagnostics give us knowledge about what would otherwise be a black box.”

VIDEO: What is a Diagnostic?


ALPACA’s design incorporated 3D printing, a technique that allowed the integration of a hollow chamber inside the main structural backbone for cooling conduits. “There would be no way to machine this part in any other way,” said David Mauzey, a senior at San Diego State University and technical associate at PPPL. Mauzey also led the mechanical engineering aspects of the ALPACA project. “This is the first big project for which I’ve handled the majority of the mechanical engineering,” Mauzey said. “There were challenges — figuring out the positioning of the optical components, for instance — but the process was fun.”

ALPACA was designed and built solely by PPPL, though the full system, consisting of ALPACA and LLAMA, will be operated collaboratively by PPPL and the Massachusetts Institute of Technology. Significant contributions were also made by Alexander Nagy, deputy head of PPPL’s DIII-D off-site research, and Florian Laggner, assistant professor of nuclear engineering at North Carolina State University. 

ALPACA is currently being tested. Once DIII-D resumes operations in April after a period of maintenance, ALPACA will start taking actual measurements. 

This work was funded by the DOE’s Office of Science (Fusion Energy Sciences).

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