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January 27, 2022 10:00 am

'Burning' Hydrogen Plasma In the World's Largest Laser Sets Fusion Records

The secret behind a record-breaking nuclear fusion experiment that spit out 10 quadrillion watts of power in a split second has been revealed: a "self-heating" -- or "burning" -- plasma of neutron-heavy hydrogen inside the fuel capsule used in the experiment, according to researchers. Live Science reports: Last year, scientists at the Lawrence Livermore National Laboratory in Northern California announced the record release of 1.3 megajoules of energy for 100 trillionths of a second at the National Ignition Facility (NIF), Live Science reported at the time. In two new research papers, NIF scientists show the achievement was due to the precision engineering of the tiny cavity and fuel capsule at the heart of the world's most powerful laser system, where the fusion took place. Although the fuel capsule was only about a millimeter (0.04 inch) across, and the fusion reaction lasted only the briefest sliver of time, its output was equal to about 10% of all the energy from sunlight that hits Earth every instant, the researchers reported. The researchers said the reaction blasted out that much energy because the process of fusion itself heated the remaining fuel into a plasma hot enough to enable further fusion reactions. "A burning plasma is when heating from the fusion reactions becomes the dominant source of heating in the plasma, more than required to initiate or jump-start the fusion," Annie Kritcher, a physicist at the Lawrence Livermore National Laboratory (LLNL), told Live Science in an email. Kritcher is the lead author of a study published Jan. 26 in Nature Physics describing how the NIF was optimized to achieve the burning plasma, and the co-author of another study published in Nature the same day that details the first burning plasma experiments at NIF in 2020 and early 2021. The two new studies describe burning plasma experiments conducted in the months before the 10 quadrillion watt reaction; those earlier experiments culminated in the production of 170 kilojoules of energy from a pellet of just 200 micrograms (0.000007 ounces) of hydrogen fuel -- around three times the energy output of any earlier experiments. It was achieved by carefully shaping both the fuel capsule -- a tiny spherical shell of polycarbonate diamond that enclosed the pellet -- and the cavity that contained it -- a small cylinder of depleted (not very radioactive) uranium lined with gold, known as a hohlraum. The new designs allowed the NIF lasers that heated the pellet to operate more efficiently within the hohlraum, and the hot shell of the capsule to rapidly expand outward while the fuel pellet "imploded" -- with the result that the fuel fused at such a high temperature that it heated other parts of the pellet into a plasma.

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