The new instrument could help scientists tailor plasma to produce more fusion heat

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 US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have finished building a new plasma measurement, or diagnostic, instrument that could aid that effort, helping to increase heat from fusion reactions in facilities known as tokamaks and potentially improve the output power of future fusion power plants.

Known as ALPACA, the diagnostic looks at 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 gain information about the density of neutral atoms that could help them keep the plasma hot and increase the amount of energy generated by fusion reactions. Scientists around the world are trying to harness the fusion reactions that power stars to generate electricity on Earth without producing greenhouse gases or long-lived radioactive waste.

ALPACA helps scientists study a process known as feeding. During this process, clouds of neutral atoms of varying densities around the plasma are broken into electrons and ions and enter the plasma.

“We are interested in feeding because the density of neutral atoms can increase the density of plasma particles, and plasma density affects the number of fusion reactions,” said Laszlo Horvath, a PPPL physicist located at DIII-D who helped coordinate the assembly and installation of ALPACA.

“If we can increase the plasma density, then we can have more fusion reactions, which generate more fusion power. This is exactly what we want to have in future fusion power plants.”

The hydrogen atoms involved in this type of fuel come from three sources. The first is the original puffs of hydrogen gas that scientists used to start the plasma. The second is the combination of electrons and nuclei in the coldest regions of the chamber to form whole atoms. The third is the leakage of hydrogen atoms from the material that makes up the surfaces of the inner chamber, where they sometimes become trapped during tokamak operations.

Similar to a pinhole camera, the nearly two-foot-long ALPACA collects plasma light that has a specific property known as the Lyman-alpha wavelength. Researchers can calculate the density of neutral atoms by measuring the brightness of the light.

Scientists had previously deduced the density from measurements taken by other instruments, but the data have been difficult to interpret. ALPACA is one of the first diagnostics specifically designed to collect plasma light at the Lyman-alpha frequency, so its data is much clearer.

Scientists want to increase their understanding of the fuel so they can control it. By controlling the feed, scientists could make the fusion reactions in tokamaks more efficient and increase the amount of heat they produce.

The increase in 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 plasma engineering and diagnostics.

ALPACA is actually one of a couple of diagnoses. Its twin is called “LLAMA”, which stands for “Lyman-alpha measuring device”. The two diagnostics complement each other because 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 , a PPPL principal research physicist who directs the PPPL Collaboration with General Atomics DIII-D National Fusion Facility in San Diego.

“Because of this, and because we can’t extrapolate from individual measurements, we have to measure at multiple locations.”

Like all diagnoses, ALPACA serves a crucial purpose. “When we do experiments on machines like DIII-D, we need to understand what’s going on inside the device, especially if we want to increase 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 just melted. The diagnostics give us insight into what would otherwise be a black box.”

ALPACA’s design incorporated 3D printing, a technique that allowed the integration of a hollow chamber within the main structural backbone for cooling ducts. “There would be no way to machine this part any other way,” said David Mauzey, a senior at San Diego State University and a PPPL technical associate. Mauzey also led the mechanical engineering aspects of the ALPACA project.

“This is the first major project for which I have handled the majority of mechanical engineering,” Mauzey said. “There were challenges figuring out the positioning of the optical components, for example, but the process was fun.”

ALPACA was designed and built solely by PPPL, although the complete system, consisting of ALPACA and LLAMA, will be operated by PPPL and the Massachusetts Institute of Technology in collaboration. Also making important contributions were Alexander Nagy, PPPL’s ​​Deputy Chief of DIII-D Offsite Research, and Florian Laggner, Assistant Professor of Nuclear Engineering at North Carolina State University.

ALPACA is currently being tested. Once DIII-D resumes operations this month after a maintenance period, ALPACA will begin taking real action.