ALPACA Diagnostic Tool: New Plasma Measurement Instrument Helps Boost Heat of Fusion Reactions and Improve Power Output of Power Plants

Fusion energy powers the Sun and other stars, filling the cosmos with heat and light. Here on Earth, advances in nuclear fusion technology promise a future of carbon-free energy. According to the International Atomic Energy Agency (IAEA), 90 nuclear fusion reactors are operating globally.

(Photo: Wikimedia Commons/ Princeton Plasma Physics Laboratory)

To generate heat from fusion reactions, experts need to manipulate plasma properties, the electrically charged state of matter that makes up 99% of the visible universe.

Producing More Fusion Heat

A team of scientists at the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) has finished developing a new plasma diagnostic tool that can boost the heat of fusion reactions in tokamaks. It also has the potential to enhance the power output of future nuclear fusion power plants.

Known as ALPACA, the measurement instrument works by observing the light released by a halo of neutral atoms that surround the plasma inside DIII-D. This facility is a doughnut-shaped device operated by General Atomics in San Diego.

By investigating this light, experts can gain information about the density of neutral atoms, which could help keep the plasma hot. It can also support scientists' goal of generating electricity without releasing greenhouse gases or radioactive waste.

How Does ALPACA Work?

ALPACA aids experts in studying a process called fueling. During this procedure, clouds of neutral atoms of different densities around the plasma disintegrate into electrons and ions and enter the plasma.

According to PPPL physicist Laszlo Horvath, his team is interested in fueling because neutral atom density can increase plasma particle density, affecting the number of fusion reactions. If plasma's density can be increased, scientists can have more fusion reactions, producing more fusion power.

There are three sources of hydrogen atoms involved in this type of fueling. The first is the original hydrogen gas used by the experts to initiate the plasma. The second is the combination of electrons and nuclei in the cooler portions of the chamber to make whole atoms. The third is the leaking of hydrogen atoms coming from the materials that make the surfaces of the inner chamber.

Like a pinhole camera, the two-foot-long (61 centimeters) ALPACA gathers plasma light with a specific Lyman-alpha wavelength. By measuring the brightness of this light, researchers can measure the density of the neutral atom.

ALPACA serves an essential purpose, like all measuring instruments. When experts run experiments on machines such as DIII-D, they need to understand what is happening inside the device, especially if they want to boost its performance. However, since the plasma is as hot as 100 million degrees Celsius, scientists cannot just use an oven thermometer or any conventional instrument. Diagnostic tools can give them the necessary knowledge without melting during the process.

The design of the ALPACA also involves 3D printing, which enables the integration of a hollow chamber inside the main structural backbone for cooling channels. This diagnostic tool is being tested, but once DIII-D resumes operations, ALPACA will begin taking actual measurements.