Scientists at Lawrence Livermore National Laboratory’s National Ignition Facility (LLNL NIF) said that, for the first time, physicists have engineered and tested a laser system in which fusion itself — as opposed to external heating mechanisms — provided most of the heat needed for a fusion reaction. The accomplishment will enable scientists to achieve higher levels of fusion performance on the road to attaining energy from nuclear fusion and self-sustaining fusion energy. As one of the most energetic laser systems in the world, the system located at LLNL NIF has played an instrumental role in enabling scientists to achieve a burning plasma state after decades of fusion research. When the energy from self-heating is dominant over the energy that was injected to initiate the fusion reactions, the plasma enters a burning plasma state. In this state, the fusion reactions themselves become the primary source of heating in the plasma. The burning plasma state is a critical step toward self-sustaining fusion energy. “Fusion requires that we get the fuel incredibly hot in order for it to burn — like a regular fire, but for fusion we need about a hundred million degrees [Fahrenheit],” physicist Alex Zylstra said. “For decades we’ve been able to cause fusion reactions to occur in experiments by putting a lot of heating into the fuel, but this isn’t good enough to produce net energy from fusion. Now, for the first time, fusion reactions occurring in the fuel provided most of the heating.” In experiments taking place in November 2020 and February 2021, scientists directed the NIF’s 192 laser beams toward a target containing a capsule about 2 mm in diameter. The capsule was filled with fusion fuel consisting of a plasma of deuterium and tritium (two forms of hydrogen). The scientists used a laser-generated, radiation-filled cavity called a hohlraum to spherically implode the capsule containing the hydrogen fuel in a central hot spot where the fusion reactions occurred. This caused the implosion process to compress and heat the fuel. The burning plasma state was achieved by increasing the spatial scale of the capsule. By increasing the scale while maintaining high levels of plasma pressure, the team delivered more of the initial laser energy directly to the fusion plasma and jump-started the burn process. The team found that keeping the driver pressure up longer (i.e., keeping a longer laser pulse), relative to the time it took for the target to implode, was important for maintaining a high plasma pressure. “Without this pressure, and enough energy coupled to the hot dense plasma, we would not reach the extreme conditions required for significant fusion,” Annie Kritcher, lead designer for the experiments, said. A cryogenic target used for experiments producing burning-plasma conditions. Courtesy of Jason Laurea/Lawrence Livermore National Laboratory. In the experiments, fusion produced about 10× as much energy as went into heating the fuel, but less than 10% of the total amount of laser energy. The laser was used for only about 10 billionths of a second in each experiment, with fusion production lasting 100 trillionths of a second. The fusion experiments provided the basis for a milestone achievement: the NIF laboratory’s realization of 1.35 megajoules (MJ). The team said that this achievement validated the work done years ago to establish the power and energy specifications for NIF. The NIF laser facility can deliver up to 1.9 MJ of energy in pulses with up to 500 terawatts in peak power. Improvements to implosion symmetry control involved moving energy between the laser beams and by designing advanced hohlraum geometry that will allow for larger implosions to be driven at the NIF’s current laser energy and power capabilities. “Following this work, the team further improved hohlraum efficiency in both platforms, increasing hot-spot pressure, which resulted in higher performance and the record 1.35-MJ HYBRID-E experiment,” Kritcher said. An illustration of the two inertial confinement fusion (ICF) designs reaching the burning plasma regime, as published in a recent article in Nature. The HYBRID-E cylinder (left) effectively leveraged cross-beam energy transfer to control implosion symmetry as the capsule containing fusion fuel grew larger relative to the size of the enclosing radiation cavity, or hohlraum. The I-Raum-shape hohlraum (right) adds ‘pockets’ to displace the wall (and the material blow-off that obstructs laser beam propagation) away from the capsule, controlling implosion symmetry through a combination of geometry and cross-beam energy transfer. Courtesy of A.L. Kritcher et al. While the burning plasma state produced only a small amount of energy — about the equivalent of nine 9-V batteries — the experiments at the NIF are a breakthrough in the quest to harness fusion energy. “Fusion experiments over decades have produced fusion reactions using large amounts of ‘external’ heating to get the plasma hot,” Zylstra said. “Now, for the first time, we have a system where the fusion itself is providing most of the heating. This is a key milestone on the way to even higher levels of fusion performance.” Kritcher said that the new platform will serve as the “basecamp” for ongoing work that will focus on understanding the sensitivity of the new regime, improving the robustness of the platform, and further increasing the energy and pressure of the fusion hot spot. Unlike burning fossil fuels or the fission process of nuclear power plants, fusion offers the prospect of abundant energy without pollution, radioactive waste, or greenhouse gases. The research was published in two papers, Nature (www.doi.org/10.1038/s41586-021-04281-w) and Nature Physics (www.doi.org/10.1038/s41567-021-01485-9).