Lawrence Livermore National Laboratory



Ablator: The plastic, beryllium, or high-density carbon (diamond) outer layer of the fuel capsule.

Ablation: Laser ablation is a process in which the molecular bonds of a material are dissolved by a laser. During an ignition experiment, laser beams strike the inside walls of the hohlraum and generate x rays that ablate the outer shell of the fuel capsule and cause the capsule to implode like a spherical rocket at velocities greater than 400 kilometers per second.

Adiabat: The internal capsule energy, or resistance of the fusion fuel to compression during an implosion. Technically, adiabat refers to the ratio of fuel pressure at peak implosion velocity to the “Fermi degenerate” pressure, which is caused by the inability of identical fermions (subatomic particles such as electrons, protons, and neutrons) to occupy the same quantum state simultaneously. The higher the adiabat, the lower the compression.

Alpha Heating: When the alpha particles (helium nuclei) produced by a fusion reaction expand and heat the cold fuel surrounding the central hot spot, they trigger a process known as alpha heating. If the density and temperature are high enough and last long enough, the fuel will ignite and generate a self-sustaining thermonuclear reaction that spreads through the fuel and releases a large amount of energy (see Thermonuclear Burn).

Alpha Particle: The nucleus of a helium atom, consisting of two protons and two neutrons. A deuterium-tritium fusion reaction, in which the nuclei of these two hydrogen isotopes fuse, produces an alpha particle and a free neutron.

Areal Density: The combined thickness and density of the imploding frozen fusion fuel shell. The areal density of the fuel, known as ρR (rho-R), and the temperature and shape of the implosion at peak compression, are two critical experimental factors for achieving ignition. Areal density is controlled by varying the entropy of the fuel that is measured through the fuel adiabat. The lower the adiabat, the higher is the areal density of the compressed fuel. Ignition requires large enough areal densities (low adiabat) and hot enough hot spots, in addition to high-velocity implosions. NIF ignition experiments have demonstrated a steady increase in areal density.

Astrophysics: The branch of physics that deals with the universe, including the physical properties (luminosity, density, temperature, and chemical composition) and the interactions of astronomical objects such as stars, galaxies, and the interstellar medium. It also includes studies of the nuclear reactions that power stars. NIF experiments are recreating many astrophysical phenomena in the laboratory for the first time.

Backlighter: A diagnostic device placed close to a NIF target and illuminated by one or more laser beams. The backlighter generates x rays or other types of radiation which pass through the target and create radiographic images of the implosion.

Blowoff: The rocket-like expulsion of hot material from the surface of an ignition fuel capsule, which causes it to implode, or compress rapidly, just as the expulsion of hot gas from a jet engine propels an aircraft.

Breakeven: In fusion, scientific breakeven occurs when the fusion power is equal to the laser input power (see Ignition). Commercial breakeven is when sufficient fusion power can be converted into electric power to cover the costs of the fusion power plant at economically competitive rates (see Inertial Fusion Energy).

Bundle: An array of eight laser beams stacked four high and two across, the basic building block of the NIF main laser system. Six bundles make up a cluster.

Burn: See Thermonuclear Burn.

Calorimeter: A device for measuring the energy of a laser pulse by measuring the temperature rise of an absorber.

Cluster: A grouping of six bundles of NIF beamlines, or 48 beamlines. There are two clusters in each of NIF's two main laser bays.

Commissioning: Certification that a system meets required specifications (see Operational Qualification and Performance Qualification).

Convergence Ratio: The initial radius of the fuel divided by the radius of the imploded fuel; a measure of the fuel’s compression.

Convergent Ablator Experiment: An experiment in which radiographs of an imploding capsule are analyzed to provide time-resolved measurements of the in-flight shape, velocity, remaining mass, and peak density of a NIF implosion.

Depleted Uranium (DU): The byproduct of enriched uranium, mainly composed of uranium-238, with less than one-third as much U-235 and U-234. The external radiation dose from DU is about 60 percent that of natural uranium.

Diagnostics: Ultra-fast and sensitive detectors, oscilloscopes, interferometers, streak cameras, and other instruments surrounding the NIF Target Chamber that are designed to capture and record the details and nuances of every shot. NIF currently has more than 100 optical, nuclear, and x-ray diagnostics.

Direct-Drive Ignition: An approach to inertial confinement fusion in which laser light is directly focused on a fusion fuel capsule, uniformly illuminating the capsule from all directions and resulting in compression heating from the ablation of the target surface (see Ignition). Compare with Indirect-Drive Ignition.

Disposable Debris Shield (DDS): A thin, 1-millimeter-thick sheet of inexpensive borosilicate glass that protects the main debris shield from target debris.

Downscatter Ratio (DSR): The ratio between the number of high-energy neutrons and lower-energy neutrons that have been scattered through interactions with the hydrogen isotopes in the fuel, an indication of fuel density (compression) and distribution of the cold fuel surrounding the hot spot. The higher the DSR (the larger the number of scattered neutrons), the greater the fuel density and opportunity for ignition.

D-T Fuel: A mixture of the hydrogen isotopes deuterium and tritium. When sufficiently heated and compressed in a fuel capsule, the nuclei of the atoms fuse, producing an alpha particle and a free neutron.

Deuterium: An isotope of hydrogen, with one neutron and one proton in the nucleus.

Electron: A subatomic particle with negative electric charge that lies outside the nucleus of an atom.

Energy: The capacity of a physical system to do work; the units of energy are joules.

Enrichment: See Uranium Enrichment.

Final Optics Assembly (FOA): An assembly that includes the target chamber vacuum window, final optics cell, diffractive optics plate, debris shield, and some laser diagnostics. The FOAs focus and smooth NIF's beams and convert their frequency from infrared to ultraviolet light (see Wavelength) for more efficient interaction with the target (see Integrated Optics Module).

Final Optics Damage Inspection System (FODI): A system used to monitor the condition of the final optics in NIF beamlines, a key capability in the operation of the facility.

Fissile Material: Material that is fissionable, or capable of sustaining a chain reaction of nuclear fission.

Fission: A nuclear reaction in which the nucleus of an atom splits into at least two other nuclei, releasing a relatively large amount of energy. Two or three neutrons are usually released during this type of transformation. Fission is usually restricted to heavier nuclei such as isotopes of uranium, plutonium, and thorium.

Frequency: The number of complete oscillations of an object in a given period of time (see Wavelength).

Fuel Capsule: A hollow spherical capsule about the size of a BB containing a frozen layer of deuterium-tritium fusion fuel. Fuel capsules typically can have outer shells made of plastic, beryllium, or high-density carbon (diamond).

Fluence: The amount of laser energy passing through a given area, typically expressed in joules per square centimeter (see Irradiance).

Fusion: A nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of enormous amounts of energy.

High Energy Density (HED) Science: The field of physics concerned with the properties and interactions of materials with energy densities exceeding 1011 joules per cubic meter, or equivalently, pressures exceeding one megabar (one million Earth atmospheres). NIF has become the leading international center for advancing high energy density physics.

High-Level Waste: Material resulting from the reprocessing of spent nuclear fuel and other highly radioactive material that must be permanently isolated. One of the advantages of a fusion power plant is the absence of high-level and long-lived waste.

Hohlraum: German for "hollow space," a hohlraum is a hollow metal cylinder about the size of a pencil eraser surrounding a fusion fuel capsule. The hohlraum converts energy from laser light into x-ray radiation that symmetrically compresses a fuel capsule (see Indirect-Drive Ignition).

Hot Spot: The highly compressed fuel at the core of a target capsule. At the peak of a NIF implosion, the compressed fuel is a hundred millionths of a meter in diameter, as much as 8 times denser than lead, and hotter than the core of the sun.

Ignition: In laser fusion, ignition is defined as energy output equal to or greater than the amount of laser energy delivered to the target. As in an ordinary (chemical) fire, ignition is the point at which the temperature and confinement of heat in the fuel (plasma in the case of fusion) are such that energy released from ongoing reactions is sufficient to maintain the temperature of the system and no external heating is needed. An ignited fusion plasma produces so much energy from fusion reactions that the plasma is fully heated by the fusion reaction products (alpha particles in the case of deuterium–tritium fusion).

Ignition Point Design: The set of specifications integrating all of the parameters—target design characteristics, diagnostics, and facility capabilities—required to build the components necessary to reach the goal of ignition.

Implode: Explode inward. The NIF fuel capsule implodes at speeds of more than 400 kilometers a second.

Inertial Confinement Fusion: The approach to fusion in which a fuel capsule is imploded very quickly, so that the inertia of the converging fuel allows fusion to occur before the particles can disperse.

Indirect-Drive Ignition: An approach to inertial confinement fusion in which the inner surface of a hohlraum containing a fuel capsule is illuminated by laser beams entering through holes at its end caps. The laser energy is converted to x rays, which heat and ablate the capsule surface, causing a rocket-like pressure on the capsule and forcing it to implode (see Ignition). Compare with Direct-Drive Ignition.

Inertial Fusion Energy: In inertial fusion energy (IFE), beams of intense laser light are focused on a target filled with hydrogen fuel. An IFE power plant would have separate areas for the driver, a factory for making the targets, a target chamber where the fusion reactions occur, and a steam turbine to generate electricity. The laser system would ignite several fusion targets per second to produce the desired power.

Integrated Optics Module (IOM): An assembly that holds and positions the Target Chamber vacuum window, final optics cell, diffractive optics plate, and debris shield for a single beam. The IOM is a line replaceable unit in the final optics assembly.

Ion: An atom or molecule that has acquired an electrical charge by the loss or gain of electrons.

Ionizing Radiation: Radiation, such as alpha, beta, gamma, and x rays, capable of displacing electrons from atoms or molecules, thereby producing ions. Exposure to high doses of ionizing radiation can produce severe skin or tissue damage, radiation sickness, and reproductive anomalies.

Irradiance: The amount of light energy incident on a given area of surface in a given amount of time, measured in watts per square meter (see Fluence).

Isotope: Different forms of atoms of the same element. Isotopes have the same number of protons in their nuclei but a different number of neutrons (the same atomic number but different atomic weights). Uranium-238 and uranium-235 are isotopes of uranium. Isotopes may be stable (not spontaneously decaying) or unstable (spontaneously decaying, emitting ionizing radiation).

Joule: A unit of energy. One joule is the energy required to lift a small apple 1 meter against the Earth’s gravity. NIF has generated energy of more than 2 megajoules (2 million joules) of ultraviolet light.

KDP Crystal: A potassium–dihydrogen–phosphate (KDP) crystal is a transparent material used in large laser systems such as NIF to change the frequency, or color, and the polarization of laser light. NIF laser light starts out as infrared but is converted to ultraviolet for better interaction with the target. NIF scientists helped to pioneer the technology of fast-growing KDP crystals.

Laser: An acronym for light amplification by stimulated emission of radiation. If the electrons in special atoms in glasses, crystals, or gases are energized into excited atomic states, they will emit light photons in response to a weak laser pulse. The photons will all be at the same wavelength and will also be “coherent,” meaning the light wave’s crests and troughs are all in lockstep. In contrast, ordinary visible light comes in multiple wavelengths and is not coherent.

Laser Fusion: A form of inertial confinement fusion in which laser beams are used to compress and heat a fuel capsule.

Light Water Reactor (LWR): A nuclear reactor that uses ordinary water as a neutron moderator, in contrast to heavy water reactors, which produce greater amounts of hazardous radioactive substances as by-products.

Line Replaceable Unit (LRU): A self-contained package containing multiple laser components that can be assembled and tested off-line in a cleanroom, then installed on the laser as a unit. NIF uses thousands of line replaceable units for lowered costs and higher efficiency.

Main Laser: The portion of the NIF beampath in which the laser light is generated, amplified, and transported to the Target Chamber (see Wavelength).

Magnetic Confinement Fusion: An approach to fusion energy in which magnetic fields confine a plasma that is heated until fusion occurs.

Master Oscillator: A device made from a ytterbium-doped optical fiber laser that generates the initial low-energy laser pulse of just a new nanojoules with a beam diameter of a few micrometers. The pulse is then broadened, split, and amplified many times before reaching the Target Chamber.

National Ignition Campaign: An effort sponsored by the National Nuclear Security Administration that began in 2006 and ended on September 30, 2012. Among its goals were to develop a platform for ignition and high energy density applications (including target and diagnostic fabrication) and to transition NIF to routine operations as the world’s preeminent high energy density science user facility.

National Nuclear Security Administration (NNSA): Part of the U.S. Department of Energy, NNSA is responsible for the management and security of the nation’s nuclear weapons, nuclear nonproliferation, and naval reactor programs. NNSA funds the National Ignition Facility.

Neutron: A subatomic particle with no charge located in the nucleus of an atom.

NIF Early Light: An experimental campaign in July 2003 in which NIF produced 10.4 kilojoules (kJ) of 350-nanometer (ultraviolet) laser light in a single beamline, setting a world record for laser performance and exceeding the NIF design requirements. World records for producing 11.4 kJ of green light and 26 kJ of infrared light were also achieved (see Wavelength).

Nuclear Fuel Cycle: The progression of nuclear fuel through a series of stages—preparation, service during reactor operation, and either containment and storage (open fuel cycle) or reprocessing for further use (closed fuel cycle).

Nuclear Reprocessing: See Reprocessing.

Nucleus: The positively charged dense core of an atom, containing protons and neutrons.

Omega: The last letter of the Greek alphabet, omega is the symbol used by physicists to denote frequency. The optics in NIF’s main laser produce laser light in the infrared portion of the electromagnetic spectrum, which is designated one-omega (1ω). NIF’s final optics double the light’s frequency to visible green light (2ω) and finally to ultraviolet light (3ω). Ultraviolet light is more effective than infrared at compressing NIF’s fuel capsule (see Wavelength).

Operational Qualification (OQ): Certification of a system’s operational capability (see Performance Qualification).

Performance Qualification (PQ): Certification of a system’s ability of operate at full energy (see Operational Qualification).

Petawatt Laser: A laser capable of producing pulses with more than one quadrillion watts of peak power. A petawatt is 1015 watts.

Photon: A particle of light; a quantum (the smallest unit in which waves may be emitted or absorbed) of electromagnetic energy moving at the speed of light.

Plasma: Known as the “fourth state of matter,” a plasma is a substance in which many of the atoms or molecules are ionized (having lost some or all of their electrons), allowing charges to flow freely. Some 99 percent of the known universe is in the plasma state. Plasmas have unique physics compared to solids, liquids, and gases. Examples of plasmas include the sun, fluorescent light bulbs and other gas-discharge tubes, much of interplanetary, interstellar and intergalactic space, laser-produced plasmas, and plasmas produced for inertial and magnetic confinement fusion.

Power: The amount of work done per unit of time (see Energy). NIF has generated more than 500 trillion watts of power.

Proliferation: The spread of nuclear weapons; the chance that fuel destined for a nuclear power plant could be diverted for weapons purposes.

Proton: A positively charged subatomic particle usually found in the nucleus of an atom.

Quad: A group of four NIF main laser beamlines. Two quads make up a bundle, and six bundles make up a cluster.

Radiation: Energy in the form of waves or particles sent out over a distance (see Ionizing Radiation).

Radioactivity: The emission of radiation, generally alpha particles or beta particles, often accompanied by gamma rays, from the nucleus of an unstable isotope. Also, the rate at which radioactive material emits radiation.

Rayleigh-Taylor Instability: A type of hydrodynamic (fluid) instability caused by accelerating a denser material into a less-dense material, causing turbulence that interferes with an implosion.

Richtmyer–Meshkov Instability: A hydrodynamic instability that results when a shock wave impinges on an interface separating two gases with different densities.

Stockpile Stewardship Program: A science-based program run by the National Nuclear Security Administration that maintains the reliability, security, and safety of the U.S. weapons stockpile without full-scale testing. NIF is a key element of stockpile stewardship.

Subcritical: Incapable of spontaneously starting or sustaining a nuclear chain reaction.

Target: For indirect-drive experiments, the NIF target is the fuel capsule together with the encompassing hohlraum. For direct-drive experiments the target is the fuel capsule alone.

Target Chamber: A ten-meter-diameter spherical structure assembled from ten-centimeter-thick aluminum panels that were preformed and welded in place. The NIF Target Chamber is covered with 0.3 meters of concrete injected with boron to absorb neutrons. The entire structure weighs 264,000 pounds.

Thermonuclear Burn: In a fusion reaction, burn is achieved by heating the fuel to the point where nuclei have sufficient energy to fuse when they collide, typically requiring temperatures of at least 1 million kelvins (1.8 million degrees Fahrenheit). The fusion reactions in the central hot spot send alpha particles (helium nuclei) into the surrounding cool fuel; the alpha particles heat the remaining fuel and cause a self-sustaining thermonuclear reaction (see Alpha Heating). Fusion converts a small amount of the mass of the reactants into energy via E = mc2 and is the process by which most types of stars, including the sun, produce the energy to shine.

Transmute:To change or alter in form, especially to a higher form. During fusion, deuterium and tritium, both isotopes of hydrogen, transmute to helium.

Tritium: An isotope of hydrogen that has two neutrons and one proton in the nucleus. Tritium is unstable, with a half-life of 12.32 years.

Uranium: The heaviest element normally found in nature. The principal fuel material used in today’s nuclear reactors is the fissile isotope uranium-235.

Uranium Enrichment: A process by which the percent composition of uranium-235 is increased through isotope separation to make it an effective fuel in nuclear reactors. Naturally occurring uranium ore is 99 percent uranium-238. Enriched uranium is a critical component for both nuclear power generation and nuclear weapons.

Wavelength: The distance between adjacent crests or troughs of a light wave, which determines its color. The optics in NIF’s main laser produce infrared laser light with a wavelength of 1,053 nanometers (billionths of a meter), also known as one-omega (1ω). In NIF’s final optical assembly, the infrared light is converted to visible green light (527 nm, or 2ω) and then to ultraviolet light (351 nm, or 3ω) as it’s directed to the center of the Target Chamber (see Frequency).