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Lasers for Thermonuclear Fusion
Published in Hitendra K. Malik, Laser-Matter Interaction for Radiation and Energy, 2021
Presently, a number of laser facilities around the world are approaching the required conditions of ignition by inertial confinement. Scientists were able to design the latest and larger machines, which could attain the required condition for ignition energies. National Ignition Facility (NIF) located at the Lawrence Livermore National Laboratory (LLNL) in California is the largest ICF experimental facility and has been operational since 2008, whereas the ignition campaign started in 2010. This was a major step forward. The NIF was designed after decades long experience and experimental results. This was the biggest laser system ever built. NIF failed initially to attain ignition condition many times, but it produced about one-third of the energy levels required as of 2015 and set a milestone for the first commercialization of fusion. It was possible to generate more energy (∼2 MJ) from a fuel capsule than the input energy applied to it. NIF also has reached many other records like production of numbers of neutrons and alpha particles. Now, there is a need for phenomenally large scientific instruments for experimentation. The 10-beam LLNL Nova laser and the 20-beam neodymium doped glass Shiva laser (its predecessor) have entered the realm of ‘big science’.
Ultrashort Pulses
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
Laser-induced nuclear reactions are made possible by the extreme power densities available at the focus of a high-power ultrafast amplifier chain. Inertial confinement fusion uses an intense pulse to create a shock wave which compresses a capsule of nuclear fuel to a sufficient density that fusion is initiated in the core of the material, and a thermonuclear burn wave travels outwards through the fuel releasing energy. The geometry of the fuel and the laser focusing arrangement is critical, and both conical [247] and spherical [248] geometries have been used. In the fast ignitor concept, the fuel is compressed by a pre-pulse and the thermonuclear burn wave is initiated at the outside of the fuel by a second shorter ‘ignition’ pulse. Nuclear fission is possible by using intense laser pulses to remove neutrons from atomic nuclei to create radioactive isotopes, and fission of uranium has been demonstrated using this approach. The short-lived isotopes which can be produced in this way may have applications in medical therapy.
High Power Laser Systems Applications to ICF
Published in M.B. Hooper, Laser-Plasma Interactions 4, 2020
Inertial confinement fusion (ICF) is an approach to controlled fusion that depends on rapid heating and compression of fusion fuel contained in a spherical target of several millimeters diameter to ultra-high densities (1000 times liquid DT density ≃ 200 gm/cm3). At these densities, and at temperatures around 5 keV, the fusion reaction rate is high enough to allow efficient burning before the plasma disassembles. Ablation of the outer layers of the fuel pellet creates the ultra-high pressures (≥50 Mbar) required to compress the fuel to ultra-high densities. A potential advantage of the ICF approach is the decoupling of the energy source (particle beam or laser) from the reactor vessel, significantly reducing the reactor operating costs and minimizing the amount of material subjected to intense neutron bombardment and activation compared with a magnetic confinement reactor.
Simulation of the Post-Shot Radiation Environment in the National Ignition Facility
Published in Fusion Science and Technology, 2018
Hesham Khater, Sandra Brereton, Lucile Dauffy, Jim Hall, Luisa Hansen, Soon Kim, Bertram Pohl, Shiva Sitaraman, Jerome Verbeke, Mitchell Young
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) is the world’s largest and most energetic laser system for inertial confinement fusion (ICF). The NIF is a 192-beam Nd-glass laser facility capable of producing up to 1.8 MJ and 500 TW of ultraviolet light for use in ICF and high-energy density experiments.1 The NIF is designed to perform shots with varying fusion yield (up to 20 MJ or 7.1 × 1018 neutrons per shot) and a maximum annual yield of 1200 MJ (Ref. 2). The main laser systems are installed in two laser bays. Each laser bay delivers 96 beams into one of two switchyards (SYs). Sets of turning mirrors in each SY redirect the beams into the target bay (TB). Additional sets of optics in the TB align and focus the beams onto a fusion target in the center of the target chamber (TC). The 192 beams enter the TC through 48 indirect-drive beam ports, passing through 48 final optics assemblies (FOAs), where each beam is converted from infrared (1ω) to ultraviolet (3ω) light. Laser beams are then focused to enter a small metal can (hohlraum) held at the center of the TC creating thermal X-rays that heat the surface of the fusion capsule contained within. The NIF has a variety of diagnostics to track shock velocities and measure X-ray and neutron emissions from the target during shots.3,4 A layout of the NIF is shown in Fig. 1.
Prevention of Residual Gas Condensation on the Laser Entry Hole Windows on Cryogenic NIF Targets Using a Protective Warm Film
Published in Fusion Science and Technology, 2018
Suhas Bhandarkar, Jim Fair, Ben Haid, Evan Mapoles, Jeff Atherton, Cliff Thomas, John Moody, Jeremy Kroll, Abbas Nikroo
Experiments at the National Ignition Facility (NIF) seek to understand the parameter space for achieving inertial confinement fusion1 (ICF). The basic pathway involves generating the high pressures and temperatures required for this event through an implosion using a powerful laser as the energy source. Indirect-drive targets use a hohlraum to convert the light energy into X-rays which then impinge on a specially designed sphere causing it to ablate and compress the payload to very high densities.2,3 The fuel, commonly a mixture of hydrogen isotopes, is supplied in the form of a smooth and uniform solid layer with the equilibrated vapor on the inside.3 To meet these requirements, the process used at NIF to form the ice layer is a sequence of several steps intended to ensure that a monocrystalline solid is formed from a single seed with high surface smoothness.4–7 Furthermore, the process requires millikelvin level control of the temperature at 18 to 19 K, so the so-called target that houses the ablator and the hohlraum is actually a complex assembly of precision-machined components with multiple, strategically located sensors and heaters.8,9 For the same reason, the nominally 500-nm membranes that seal the laser entry holes (LEHs) on either axial end are Al coated to prohibit any infrared (IR) light from reaching the capsule.10 High-quality layers can require multiple starts of the slow crystallization process and hence, a cumulative timeframe of multiple days.
Zinc Oxide–Coated Poly(HIPE) Annular Liners to Advance Laser Indirect Drive Inertial Confinement Fusion
Published in Fusion Science and Technology, 2018
Paul Fitzsimmons, Fred Elsner, Reny Paguio, Abbas Nikroo, Cliff Thomas, Kevin Baker, Haibo Huang, Mike Schoff, David Kaczala, Hannah Reynolds, Sean Felker, Mike Farrell, Brian J. Watson
Inertial confinement fusion (ICF) is a potential next-generation energy source that promises to ignite a thermonuclear fusion reaction with a net gain of energy by transferring laser energy into a frozen deuterium-tritium (DT) fuel.1,2 One approach being investigated to achieve ICF is laser indirect drive (LID) in which the fuel capsule is housed in the center of a cylindrical casing, commonly referred to as a hohlraum (often made of a high-Z material such as gold or depleted uranium; other geometries also considered in literature).3–5 The hohlraum is usually filled with low-pressure helium gas contained with thin laser entrance hole (LEH) windows on both ends.4 The lasers propagating through the LEH windows are aimed at the inner hohlraum walls, which in turn release thermal X-rays (~300 eV) that ablate the fuel-containing capsule and compress the fuel with a convergence ratio of 25:40 (Refs. 4 and 6). This compression process occurs over multiple shocks that adiabatically compress the fuel to create a hot spot for nuclear burn that propagates throughout the fuel-containing capsule (the duration of the entire process is on the order of 10 to 20 ns).3,4,6 The work in this paper focuses on mitigating two critical problems facing LID: propagation of high-Z ions ablated from the hohlraum wall (wall motion) and premature heating of the cryogenic fuel by m-band X-rays (preheat).