Scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have discovered critical new details about fusion facilities that use lasers to compress the fuel that produces fusion energy. The new data could help improve the design of future laser installations that harness the fusion process that drives the sun and stars.
Fusion combines light elements in the form of plasma – the hot, charged state of matter composed of free electrons and atomic nuclei – which generates huge amounts of energy. Scientists seek to replicate fusion on Earth for a virtually inexhaustible supply of energy to generate electricity.
Major experimental facilities include tokamaks, the magnetic fusion devices studied by PPPL; stellarators, the magnetic fusion machines that PPPL is also studying and which have recently become widespread throughout the world; and laser devices used in so-called inertial confinement experiments.
Researchers explored the impact of adding tungsten metal, which is used to make cutting tools and lamp filaments, to the outer layer of plasma fuel pellets in inertial confinement research. They found that tungsten increased the performance of implosions that cause melting reactions in pellets. The tungsten helps block heat that would prematurely raise the temperature in the center of the pad.
The research team confirmed the results by performing measurements using krypton gas, which is sometimes used in fluorescent lamps. When added to the fuel, the gas emitted high-energy light known as X-rays which was captured by an instrument called a high-resolution X-ray spectrometer. X-rays gave clues to what was going on inside the capsule.
“I was thrilled to see that we could make these unprecedented measurements using the technique we’ve developed over the past few years. This information helps us assess pellet implosion and helps researchers calibrate their computer simulations“said physicist PPPL Lan Gao, lead author of the paper reporting the results in Physical examination letters. “Better simulations and general theoretical understanding can help researchers design better future experiments.”
The scientists performed the experiments at the National Ignition Facility (NIF), a DOE user facility at Lawrence Livermore National Laboratory. The installation shines 192 lasers on a gold cylinder, or hohlraum, which is one centimeter high and houses the fuel. The laser beams heat the hohlraum, which emits x-rays evenly onto the fuel pellet inside.
“It’s like an X-ray bath,” said PPPL physicist Brian Kraus, who contributed to the research. “That’s why it’s good to use a hohlraum. You can shine lasers directly on the fuel pellet, but it’s hard to get even coverage.”
The researchers want to understand how the pellet is compressed so that they can design future installations to make heating more efficient. But getting information about the inside of the pellet is difficult. “Because the material is very dense, almost nothing can come out of it,” Kraus said. “We want to measure the inside, but it’s hard to find anything that can go through the fuel pellet shell.”
“The results presented in Lan’s paper are of great importance for inertial fusion and have provided a novel method for characterizing hot plasmas,” said Phil Efthimion, head of the Plasma Science & Technology department at PPPL and responsible for collaboration with the NIF.
The researchers used a high-resolution X-ray spectrometer designed by PPPL to collect and measure the radiated X-rays in greater detail than had been measured before. By analyzing the evolution of X-rays every 25 trillion seconds, the team was able to follow the evolution of the plasma over time.
“Based on this information, we were able to estimate the pellet core size and density more accurately than before, which helped us determine the efficiency of the melting process,” Gao said. “We have provided direct evidence that the addition of tungsten increases both the density and the temperature and therefore the pressure in the compressed pellet. Accordingly, fusion yield increases.
“We look forward to collaborating with theoretical, computational and experimental teams to further this research,” she said.
Lan Gao et al, Hotspot evolution measured by high-resolution X-ray spectroscopy at the National Ignition Facility, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.128.185002
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