Lawrence Livermore National Laboratory
Glossary
Ablate – To remove material from the surface of an object by vaporization, chipping, or other erosive processes. Laser ablation is a process in which the molecular bonds of a material are dissolved by a laser.
Actinides – The highly toxic, long-lived radioactive materials, such as neptunium and americium, contained in spent nuclear fuel.
Alpha Particle – The nucleus of a helium-4 atom, consisting of two protons and two neutrons. A deuterium-tritium fusion reaction, in which the nuclei of two hydrogen isotopes fuse, produces an alpha particle and a free neutron.
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.
Blowoff – The rocket-like expulsion of hot material from the surface of an object, such as an ignition target, which causes the object 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).
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.
Diagnostic Equipment – See Diagnostics.
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.
Direct-Drive Ignition – An approach to inertial confinement fusion in which laser or particle beam energy 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, one-millimeter-thick sheet of inexpensive boro-silicate glass designed to be used for one to ten laser shots before replacement. The DDS protects the main debris shield from target debris.
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 – A heavy 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.
Fertile Material – Elements such as thorium-232 and uranium-238 that generally do not undergo induced fission themselves but which are capable of being transformed into fissionable material by capture of a neutron (see Fissile Material).
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, by thermal (slow) neutrons.
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.
Flibe – LiF-BeF2, a molten salt mixture of lithium and beryllium used as a coolant in nuclear reactors.
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 DT Fuel. NIF will use capsules having outer shells made of beryllium, carbon or carbon–hydrogen polymers. For inertial fusion energy, the shell is filled with a mixture of deuterium and tritium. The fuel capsule must be cold enough for the DT fuel to freeze and form a layer of DT ice on the inner wall of the capsule.
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 Physics – 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).
High-Level Waste – Material resulting from the reprocessing of spent nuclear fuel and other highly radioactive material that must be permanently isolated.
Hohlraum – German for "hollow space," a hohlraum is a hollow metal cylinder the size of a pencil eraser surrounding a fusion fuel capsule. In radiation thermodynamics, a hohlraum is defined as "a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity." The hohlraum converts directed energy from either laser light or particle beams into x-ray radiation. The use of x-ray indirect radiation rather than laser light direct radiation provides additional options for compressing and heating the fuel capsule, or target, under the required conditions of inertial confinement fusion (see Indirect-Drive Ignition).
Ignition – In laser fusion, ignition is defined as energy output equal to or greater than the amount of laser energy incident on the target assembly (for indirect drive, the target assembly consists of the hohlraum and fuel capsule; for direct drive, it consists of the capsule). 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, diagnostic needs, and facility capabilities—required to build the components necessary to reach the goal of creating ignition.
Implode – Explode inward. The NIF fuel capsule will implode at a speed of up to 300 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 ions 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 rocketlike pressure on the capsule and forcing it to implode (see Ignition). Compare with Direct-Drive Ignition.
Inertial Fusion Energy – In inertial fusion energy (IFE), a "driver" focuses beams of accelerated ions or intense laser light 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 driver must ignite several fusion targets per second to produce the desired power level in the chamber (see Ignition).
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 and radiation sickness.
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).
Isotropic – Exhibiting properties (such as velocity of light transmission) with the same values when measured along axes in all directions.
Joule – A unit of energy. One joule is the energy required to lift a small apple one meter against the Earth's gravity.
KDP Crystal – A potassium–dihydrogen–phosphate (KDP) crystal is a transparent dielectric material used in large-aperture laser systems such as NIF to change the frequency, or color, and the polarization of the laser light. KDP crystals are used at NIF because they can be grown as a single crystal to large size.
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. Heavy water reactors 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.
Main Laser – The portion of the NIF beampath in which the nominal one-omega, or infrared, 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 which 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, which is then broadened, split and amplified many times before reaching the target chamber.
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 watt.
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 effectively ionized, allowing charges to flow freely. Some 99% 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 magnetic confinement fusion.
Plutonium – A very heavy element formed when uranium-238 absorbs neutrons. Like uranium, it has two principal isotopes that are fissile.
Point Source – A single identifiable, localized source that has a definite position, but no extension in space.
Power – The amount of work done per unit of time (see Energy). At NIF, the production of 1.8 million joules of energy in a few billionths of a second is equivalent to 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.
Reprocessing – A mechanical and chemical process that separates the components of spent nuclear fuel such as uranium, plutonium, actinides, and fission products (atomic fragments) for other uses such as fuel for nuclear reactors or nuclear weapons.
Spent Nuclear Fuel (SNF) – Nuclear fuel in a reactor that has been depleted (used up) and has lost its ability to efficiently create energy. The fission byproducts created within SNF continue to emit ionizing radiation; thus SNF must be stored in shielded basins of water or dry storage vaults until the radioactivity decreases to safe levels.
Stockpile Stewardship Program – The maintenance of the U.S. weapons stockpile through an ongoing process of surveillance, assessment, refurbishment, and recertification, without nuclear testing.
Subcritical – Incapable of spontaneously starting or sustaining a nuclear chain reaction.
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 into the plasma state to the point where nuclei have sufficient energy to fuse when they collide, typically requiring temperatures of at least one million kelvins (1.8 million degrees Fahrenheit). Thermonuclear 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, appearance, or nature, especially to a higher form.
TRISO – Tristructural isotropic fuel, a type of micro fuel particle consisting of a fuel kernel composed of uranium oxide (sometimes uranium carbide) in the center, coated with four layers of three isotropic materials.
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 effective as a fuel in nuclear reactors. Naturally occurring uranium ore is 99% 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) at the center of the target chamber (see Frequency).
