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| Fig. 1:The preamplifiers increase the energy of the laser beams as they make their way to the target chamber. (Source: Wikipedia Commons) |
Scientists have long dreamed of harnessing the power of nuclear fusion. Our own sun runs on the fusion of hydrogen atoms to form helium. If mankind is somehow able to manage a self-sustaining nuclear fusion reaction, it could provide limitless clean energy for the world. The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is precisely working on this exact task, and while the possibility of a net-positive nuclear fusion reaction is still in the remote future, scientists at NIF are making amazing discoveries about the way matter behaves at extremely high temperatures and pressures.
The energy released from the fusion of two light nuclei originates from the interplay of two opposing forces, the nuclear force which makes protons and neutrons stick, and the Coulomb force which causes protons to repel each other. The strong nuclear force is what holds the nuclei together, despite the strong electric repulsion between the positively charged protons. This nuclear force is stronger than the Coulomb force for atomic nuclei (for elements smaller than iron and nickel), so the fusion of lighter nuclei releases the extra energy from the net attraction of these particles.
The newly fused nucleus is lighter than the sum of the masses of its components, and since mass is energy by Einstein's famous equation, the lost mass is released as energy. For example, in the fusion of two hydrogen nuclei to form helium, 0.7% of the mass is converted into energy.[1] It is this released energy that powers the stars and scientists hope to one day be able to harness. It takes a considerable amount of initial energy, however, to force the nuclei to fuse.
The National Ignition Facility, or NIF, is a stadium-sized laser-based inertial confinement fusion research device that contains a 192-beam, 1.8-megajoule, 500-terawatt, ultraviolet laser system together with a 10-meter-diameter chamber and room for diagnostics. [2] At NIF, the energy of 192 giant laser beams are focused on a BB-sized target filled with hydrogen fuel, with the aim to fuse the hydrogen nuclei and release many times more energy than it took to initiate the fusion reaction.
The beams from NIF begin with nanojoule sized pulses, which are divided first into 48 beams and passed through amplifiers. These beams are divided further into a total of 192 beams, each about 40 cm square, that then pass through neodymium glass amplifiers that ultimately bring the beam to 4 MJ of 1051 nm laser light. The infrared light at 1051 nm is then converted into ultraviolet light by passing through a single crystal of potassium dihydrogen phosphate (KDP). The KDP cystal sheets convert the 1051 nm light first to 526 nm, then to 351 nm by a second KDP crystal. The ultraviolet light enters the 10 m diameter target chamber where the beams are focused onto the inside walls of a hohlraum, a small metal cylinder that houses the pellet of deuterium and tritium fuel. The laser light produces x-rays inside the hohlraum that heat and compress the DT fuel to temperatures of over 20 to 40 million degrees, enough to initiate nuclear fusion. [3]
As of October 7, 2013, NIF reported that, for the first time, the fuel capsule gave off more energy than was absorbed by the fuel, a crucial milestone towards commercialization of fusion energy. The 14 kJ output of the reaction, however, is significantly less than the total amount of energy used to run the laser - 1.8 MJ. Only 10-20% of the laser energy initially input is actually absorbed by the capsule. The rest of the energy ends up as scattered light, heat, and x-rays that do not contribute to the fusion reaction. [4]
© Darren Handoko. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] Bethe, Hans A. "The Hydrogen Bomb," Bull. Atomic Scientists 6, No. 4, 99 (April, 1950)
[2] G. H. Miller, E. I. Moses, and C. R. Wuest, "The National Ignition Facility," Opt. Eng. 43, 2841 (2004).
[3] E. I. Moses et al., "The National Ignition Facility: Ushering a New Age for High Energy Density Science." Phys. Plasmas 16 041006 (2009).
[4] P. Rincon, "Nuclear Fusion Milestone Passed at US Lab," BBC News, 7 Oct 13.
