Scientists from the U.S. Department of Energy's Princeton Plasma Physics Laboratory have discovered crucial new information about fusion facilities that use lasers for compressing the fuel that creates fusion energy. These new details could lead to improved designs for future laser facilities that harness fusion energy that powers the sun and stars.
Fusion is the combination of light elements into plasma, a hot, charged state composed of free electrons or atomic nuclei that generates large amounts of energy. Scientists are trying to recreate fusion on Earth to provide an inexhaustible supply for electricity generation.
Tokamaks are the magnet fusion devices that PPPL studies. Stellarators, which are the magnetic fusion machines that PPPL also studies, have recently become more common around the globe. Laser devices used in inertial confinement experimentation.
Researchers looked at the effects of adding tungsten to plasma fuel pellets' outer layer in inertial confinement research. This is used to make cutting tools, lamp filaments, and other related items. The researchers found that tungsten improves the performance of implosions which cause fusion reactions in pellets. The tungsten blocks heat from rising prematurely at the center of the pellet.
The researchers confirmed their findings using krypton gas (often used in fluorescent lamps). The gas emits high-energy light, which is known as Xrays. This was captured using an instrument called a high resolution X-ray spectrumrometer. The X-rays provided clues as to what was going on inside the capsule.
"I was thrilled to see that we were able to make these extraordinary measurements with the technique we've been working on for the past few years. This information allows us to evaluate the pellet's implosion, and it helps researchers calibrate computer simulations," Lan Gao (PPPL physicist), who led the paper that reported the results in Physical Review Letters. Researchers can design better experiments by using better simulations and theoretical understanding.
The experiments were performed at the National Ignition Facility, a DOE user facility located at Lawrence Livermore National Laboratory. The facility shines 192 beams of lasers onto a one-centimeter high gold cylinder (or hohlraum) that encases the fuel. The laser beams heat hohlraum and emit X-rays evenly onto fuel pellets within.
Brian Kraus, PPPL physicist, said that it's "like an X-ray bathtub." He also contributed to the research. It's why it's so good to use a Hohlraum. Although you could shine lasers directly on the fuel pellet, it is difficult to achieve even coverage.
To improve the efficiency of heating, researchers want to know how the pellet is compressed. It is not easy to find information about the interior of the pellet. Kraus stated that the pellet's interior is difficult to get information about because it is dense. "We would like to measure the inside of the fuel pellet, but it is difficult to find anything that can penetrate the shell."
Phil Efthimion of PPPL, who heads the Plasma Science & Tech Department and is the leader of the NIF collaboration, stated that "The results presented by Lan's paper are very important to inertial fusion"
To collect and analyze the radiated Xrays, the researchers used a PPPL-designed high resolution X-ray spectrometer. The team was able track the changes in plasma over time by analysing how the Xrays change every 25 trillionths of a second.
Gao stated that this information allowed them to estimate the size of the pellet core and its density more accurately than ever before. This helped determine the efficiency of the fusion. "We have direct evidence that adding tungsten to the pellet increases its density, temperature, and pressure. The result is a higher fusion yield.
She said, "We look forward to working with theoretical, computational and experimental teams to further this research."
