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Glossary Active galactic nuclei (AGN): Refers to the existence of energetic phenomena in the nuclei, or central regions, of galaxies that cannot be attributed clearly and directly to stars. Advanced Strategic Computing Initiative (ASCI): Now known as Advanced Simulation and Computing (ASC), this Department of Energy program was established in 1995 to develop the simulation capabilities needed to analyze and predict the performance, safety, and reliability of nuclear weapons and to certify their functionality. To realize its vision, ASC is creating simulation and prototyping capabilities based on advanced weapons codes and high-performance computing. Akeno Giant Air Shower Array (AGASA): Designed to study the origin of extremely high energy cosmic rays, AGASA covers an area of about 100 km2 and consists of 111 detectors on the ground (surface detectors) and 27 detectors under absorbers (muon detectors). Each surface detector is placed with a nearest-neighbor separation of about 1 km, and the detectors are sequentially connected with a pair of optical fibers. When an extremely high energy cosmic ray enters the atmosphere, it collides with an atomic nucleus and starts a cascade of charged particles that produce light as they zip through the atmosphere. AGASA and other similar detectors measure the light emitted in these so-called air showers. Alfvén critical current: The current at which the self magnetic field of a beam reflects the beam particles. The electron Alfvén current, in units of amperes, is IA=17,000 βγ, where β is the electron velocity normalized to the speed of light and γ is the relativistic Lorentz factor.
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Alfvén wave: A transverse wave that occurs in a region containing a magnetic field and a plasma. The ionized and therefore highly conducting plasma is said to be “frozen in” to the magnetic field and is forced to take part in its wave motion. The existence of these waves was predicted by Hannes Olof Gosta Alfvén in 1942— this work inaugurated the study of magnetohydrodynamics for which Alfvén was awarded the Nobel Prize in 1970. SeeMagnetohydrodynamics. Alpha particle: A helium-4 nucleus emitted by a larger nucleus during a type of radioactive decay known as alpha decay. An alpha particle consists of two protons and two neutrons. Auger project: The Pierre Auger Observatory project, an international effort to study the highest-energy cosmic rays. Two giant detector arrays, each covering 3,000 km2, will be constructed in the Northern and Southern Hemispheres. Each will consist of 1,600 particle detectors and an atmospheric fluorescence detector. The objective of the arrays is to measure the arrival direction, energy, and mass composition of cosmic-ray air showers above 1019 eV. Baryon: A class of subatomic particles that includes the proton and neutron. Baryons are a subclass of the class of particles known as hadrons that interact via the strong interaction. Baryons have half-integral spin. Berkeley Illinois Maryland Association Array (BIMA):A consortium consisting of the Radio Astronomy Laboratory at the University of California, Berkeley; the Laboratory for Astronomical Imaging at the University of Illinois, Urbana; and the Laboratory for Millimeter-Wave Astronomy at the University of Maryland. BIMA operates a millimeter-wave radio interferometer at Hat Creek, California, with support from the National Science Foundation. Bethe-Heitler process: As an energetic electron slows down in a material, it emits photons via bremsstrahlung. At high electron energies, these photons can be reconverted back to electron-positron pairs. The Bethe-Heitler process describes the bremsstrahlung emitted by an electron in a coulomb field. Bisnovatyi-Kogan, Zel’dovich, Sunyaev (BKZS) limit: Thermally produced electron-positron plasmas are thought to play an important role in the evolution of the cores of massive stars, neutron-star and black-hole accretion disks, pulsars, quasars, astrophysical gamma-ray bursters, and in the big bang. In the past few years, discoveries of intense, broadened 511-keV annihilation lines from several galactic black-hole candidates suggest that, in addition to transient-pair production, steady-state thermal pair plasmas exist. Since pairs annihilate on short time scales, maintaining such steady-state conditions requires the copious production of pairs in order to balance the annihilation rate. Such pair-balanced steady-state plasmas represent a new state of matter, with unique radiative and thermodynamic properties. For a plasma to be in a steady state, the heating rate must be balanced by the cooling rate, which consists of bremsstrahlung, inverse Compton scattering, and pair
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annihilation. It turns out that for a pair-balanced plasma, there exists a fundamental limit to the temperature of approximately 10 MeV for hydrogen, above which pair creation can no longer be balanced by annihilation and pair density will exponentiate rapidly, leading to a pair-dominated plasma and net cooling of the system. This limiting temperature is referred to as the BKZS limit. Black hole, accreting: A black hole is an object with such a strong gravitational field that its escape velocity exceeds the velocity of light. When a black hole attracts matter from the space around it, it is said to be an “accreting black hole.” The matter falling into the accreting black hole radiates energy mostly in the x-ray regime, due to momentum conservation. Bose-Einstein condensation: A phenomenon occurring in a macroscopic system consisting of a large number of bosons (particles with integral spin) at sufficiently low temperature, in which a significant fraction of the particles occupy a single quantum state of lowest energy (the ground state). Bose-Einstein condensation can only take place for bosons whose total number is conserved in collision. Bose-Einstein statistics: See Quantum statistics. Boussinesq convection (approximation): In his attempts to explain the motion of the light in the ether, Boussinesq (in 1903) opened a wide perspective on mechanics and thermodynamics. With a theory of heat convection in fluids and of propagation of heat in deforming or vibrating solids, he showed that density fluctuations are of minor importance in the conservation of mass. The motion of a fluid initiated by heat results mostly in an excess of buoyancy and is not due to internal waves excited by density variations. In other words, the continuity equation may be reduced to the vanishing of the divergence of the velocity field, and variations of the density can be neglected in the inertial accelerations but not in the buoyancy term. Although used before him, Boussinesq’s theoretical approach established a cardinal simplification for a special class of fluids that fundamentally differ from gases and may eliminate acoustic effects. Bremsstrahlung: The x rays emitted when a charged particle, especially an electron, is rapidly slowed down, as when it passes through the electric field around an atomic nucleus. Bulk modulus: A parameter associated with the elastic properties of isotropic solids. It is the ratio of the pressure to the fractional change in volume necessary to produce that pressure. Cepheid variable: One of an important group of yellow giant or supergiant pulsating variable stars whose period of pulsation is directly related to their absolute magnitude. The resulting period-luminosity relationship is used to determine cosmological distances. CERN: European Organization for Nuclear Research (originally the European Center for Nuclear Research), located near Geneva, Switzerland.
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Chirped pulse amplification: Allows one to avoid the strong nonlinear effects that can destroy an amplifier when attempting to build a high-power laser pulse. To overcome the nonlinearities, the input pulse to the amplifier is stretched in time so that the peak power is decreased. This “chirped” pulse is then amplified and subsequently compressed to obtain a high-power pulse with a duration nearly equal to the input pulse. Compact objects: Remnants of stars that have burned all of their nuclear fuel, forming white dwarfs, neutron stars, or black holes. The extreme gravitational fields near these objects make them valuable as physical laboratories for studying the gravitational force itself. Compton effect: The change in wavelength and direction of a photon when it collides with a particle, usually an electron; also known as Compton scattering. Some of the photon’s energy is transferred to the particle, and the photon is reradiated at a longer wavelength in the electron’s initial rest frame. Compton wavelength (λc): λc=h/mc, where h is Planck’s constant, m is the electron rest-mass, and c is the speed of light. The length scale below which a particle’s quantum-mechanical properties become evident in relativistic quantum mechanics. Cosmic rays: The term for a broad class of energetic particles that bombard Earth from space. Featuring a variety of energies and constituent particles, cosmic rays have been found at energies less than an MeV and as great as approximately 3×1020 eV. Cosmic rays consist mostly of protons, electrons, and some heavier nuclei. The term ultrahigh-energy cosmic rays (UHECRs) is used to describe cosmic rays with energies exceeding approximately 5×1019 eV. Cyclotron radiation: The electromagnetic radiation emitted by a charged particle circling in a magnetic field substantially below the speed of light. Degeneracy: Several states of an atom that differ in many of their properties but nevertheless have the same value of some particular quantity, usually the total energy. Frequently an external influence such as a magnetic or an electric field can “remove” the degeneracy, which means that the energies become slightly different. An example is the splitting of lines that occurs when atoms are placed in a magnetic field. Detector, ANTARES: An undersea neutrino detector under development in the Mediterranean Sea. It consists of an array of approximately 1,000 photomultiplier tubes in 10 vertical strings, spread over an area of about 0.1 km2 and with an active height of about 0.3 km. Detector, IceCube: A neutrino detector under development. It consists of 4,800 photomultiplier tubes buried in a 1-km3 block of clear ice deep below the South Pole Station. Detector, LIGO: The Laser Interferometer Gravitational-wave Observatory; a
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National Science Foundation-sponsored project to build and operate two 4-km laser interferometers to detect gravitational waves. Detector, VIRGO: A 3-km gravitational wave observatory under development in France. Dwarf, brown: An object that, because of its low mass (less than 0.08 solar masses), never becomes hot enough to begin hydrogen fusion in its core; hence it is not considered to be a star. Dwarf, white: A very small star that is the remnant core of a star which has completed fusion in its core. The Sun will become a white dwarf. White dwarfs are typically composed primarily of carbon, have about the radius of Earth, and do not significantly evolve further. Energy, Fermi: Synonymous with the electron chemical potential at absolute zero, the Fermi energy represents the energy level that the next electron into the system must have, to be at the lowest-possible freely available state. Energy, nuclear binding: The energy release when protons and neutrons bind together to form an atomic nucleus, or the energy required to break up that nucleus. Energy, rest-mass: A body’s mass expressed in energy terms when the body is at rest, when measured by an observer in the same frame of reference. The energy is given by the relationship E0=m0c2, where m0 is the rest mass and c is the speed of light. Equation of state: A description of a material’s properties, for example, its mass or energy density as a function of applied pressure. Faraday cup: Allows a beam of charged particles (electrons, ions) to be measured accurately. A Faraday cup collects all the particles that enter it, measuring the current with an ammeter. Faraday rotation: The rotation of the plane of polarization of electromagnetic radiation upon passing through an isotropic medium with an embedded magnetic field; also known as the Faraday effect. Fermi-Dirac statistics: See Quantum statistics. Free electron laser (FEL): The amplification of a photon beam via the electromagnetic interaction between the photon beam and a relativistic electron beam that passes through a spatially periodic magnetic field (a so-called undulator). Unlike a conventional laser in which the lasing frequency is determined by the properties of the lasing medium, the frequency of a FEL is determined by the speed (energy) of the electron beam and the period of the magnetic field and hence can be varied (or tuned) over a wide range. Gamma-ray burst (GRB): A burst of gamma rays from a cosmic source. Several hundred gamma-ray bursts are detected per year, and they range in duration from fractions of a second to several seconds. Most of them come from objects at cosmological distances.
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Greisen-Zatsepin-Kuzmin (GZK) cutoff: Constrains ultrahigh-energy cosmic rays above a certain energy to have been produced within a certain distance of where they were detected—namely, particles above 4×1019 eV should not be able to propagate in the cosmic microwave background more than about 160 light-years. Hohlraum: In radiation thermodynamics, a hohlraum is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealized cavity can be approximated in practice by making a small perforation in the walls of a hollow container of any opaque material. The radiation escaping through such a perforation will be a good approximation to blackbody radiation at the temperature of the interior of the container. Inertial confinement fusion (ICF): Uses powerful energy beams, such as lasers or particle beams, to compress and heat hydrogen fuel to fusion temperatures, and uses the inertia of the fuel to confine it long enough for fusion to occur. Instability, filamentation: The tendency of a single beam of particles or light to break into smaller beamlets or filaments. Instability, hosing: The tendency of a beam of particles or light to kink like a firehose carrying high-pressure water. Instability, magnetic hoop: The pinching or narrowing of a beam of charged particles due to its self-magnetic field, which encircles the beam like a hoop. Instability, parametric: The driving of a resonance in a system by varying some property (or parameter) of the system in a periodic way. For example, a laser of frequency w0 can go parametrically unstable to a mode (wave) at frequency w0−wp due to the presence of background plasma modulations (i.e., noise) at frequency wp. Instability, Rayleigh-Taylor: Classical hydrodynamic interface instability that occurs when a high-density fluid is supported against gravity by a lower-density fluid. Instability, Richtmyer-Meshkov: Classical hydrodynamic interface instability of a shock-driven system with a density discontinuity. Instability, Weibel: The electromagnetic instability of a plasma due to anisotropic distribution of velocities in the plasma—in the case of a wide beam flowing through a plasma, it leads to filamentation of the beam into beamlets of diameter of the order of c/ωp, where c is the speed of light and ωp is the plasma frequency. Interstellar medium (ISM): The material between the stars. Inverse Compton effect: The gain in energy of a photon when it is struck by a fast-moving electron; also known as inverse Compton scattering. The electron passes on a small proportion of its energy to the photon, and the photon’s wavelength decreases. The electron must suffer a large number of collisions before it loses an appreciable fraction of its energy due to this process. Isentropic process: Takes place without a change of entropy (a measure of the unavailability of a system’s energy to do work).
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Isotope: One of two or more atoms of the same element that have the same number of protons in their nucleus but different numbers of neutrons. Jet, astrophysical: A stream of fast-moving material flowing outward from an object such as a young star or a massive central black hole in a galaxy. Jets are detected by the radiation emitted by the fast-moving matter. Knudsen number(Kn): The ratio of the mean free path to the flow scale in a medium. Knudsen or molecular flow scale in a medium occurs for a gas when the mean free path is large compared to the dimensions of the flow. Laser Megajoule (LMJ): A large solid-state laser facility in France similar in design and scale to the U.S. National Ignition Facility. Lattice gauge theory: A formulation of a gauge or quantum field theory—used to explain fundamental interactions—in which space and time are taken to be discrete rather than continuous. At the end of lattice gauge theory calculations, it is necessary to take the continuum limit. Lattice gauge theory is used to make calculations for gauge theories with strong coupling, such as quantum chromodynamics. Local group: A group of galaxies about 3 million light-years in diameter, which contains our Galaxy, the Milky Way. Lorentz factor: γ=(1−v2/c2)1/2, where v is the particle speed and c is the speed of light, the Lorentz factor is a parameter that indicates if a particle is moving at relativistic velocities. If the Lorentz factor is much greater than unity, the particle is moving at speeds close to c. If the Lorentz factor is close to unity, the particle’s speed is nonrelativistic, much less than c. Mach number (Ma): The ratio of the speed of an object to the speed of sound. Shock waves result when an object or flow has a Mach number greater then unity. Magnetars: Neutron stars with the largest-known magnetic fields in the universe. Magnetic reconnection: In a plasma, the process by which plasma particles riding along two different field lines find themselves sharing the same field line: for instance, solar-wind particles on an interplanetary field line, and magnetospheric particles on a field line attached to Earth, finding themselves united on an “open” field line, which has one end anchored on Earth and the other in distant space. Magnetic reconnection can occur when plasma flows through a neutral point or a neutral line where the intensity of the magnetic field is zero and its direction is not defined. Magnetohydrodynamics (MHD): The study of the interactions between a conducting fluid or plasma and a magnetic field. Plasmas, being ionized, are good electrical conductors. An electric current is induced in a plasma when the plasma tries to cross lines of magnetic force, and the plasma tends to follow the magnetic field lines; alternatively, the magnetic field may be dragged along by the plasma. The plasma is said to be “frozen in” the magnetic field or vice versa.
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Maxwell-Boltzmann distribution: A function describing the distribution of speeds among the molecules of a gas in thermodynamic equilibrium. Metallic hydrogen: A form of hydrogen in which the atoms are highly compressed, as in the interiors of massive gaseous planets such as Jupiter and Saturn. Under such conditions, hydrogen behaves like a liquid metal and hence can conduct electricity and generate a magnetic field. Monte Carlo simulation: A type of calculation involving random sampling for the mathematical simulation of physical systems. Monte Carlo simulations are applied to problems that can be formulated in terms of probability. National Ignition Facility (NIF): A 192-beam, 1.8-MJ solid-state laser under construction by the Department of Energy’s National Nuclear Security Administration at Lawrence Livermore National Laboratory. NIF will create conditions of extreme temperature and pressure in the laboratory that can be used for stockpile stewardship research, high energy density physics research, and inertial confinement fusion ignition. Navier-Stokes equation: Describes the flow of a Newtonian fluid; also known as Gaussian distribution. Neutron star: A star at such high density and pressure that its atoms have been completely crushed so that the nuclei merge and most of the electrons have been squeezed onto the protons, forming neutron-rich material. NIKE: Named after the Greek goddess of victory, NIKE is a krypton-fluoride (KrF) gas laser that produces 4,000 to 5,000 J of ultraviolet light out of the large amplifier in a 4-ns pulse. NIKE is a facility at the Naval Research Laboratory. Nucleosynthesis: The process of creating elements in nuclear reactions, such as in a nuclear fission reactor, the interior of a star, or at the time of the big bang. OMEGA: Situated at the Laboratory for Laser Energetics at the University of Rochester, OMEGA’s 60 laser beams focus up to 40,000 J of energy onto a target that measures less than 1 mm in diameter in approximately 1 billionth of a second. Opacity: The extent to which a medium is opaque to electromagnetic radiation. Opacity is the reciprocal of transmittance. Particle-in-cell (PIC) simulation: A simulation method that follows the self-consistent nonlinear dynamics of a large number of charged particles interacting with applied and self-generated electric and magnetic fields. Pauli exclusion principle: The quantum-mechanical principle, applying to fermions but not to bosons, that no two identical particles in a system, such as electrons in an atom or quarks in a hadron, can possess an identical set of quantum numbers. The origin of the Pauli exclusion principle lies in the spin-statistics theorem of relativistic quantum field theory. Perveance: A measure of the importance of self fields to the dynamics of a
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particle beam. The perveance K is proportional to I/V3/2, where I is the beam current and V is the voltage. Phase transition: A change in a feature that characterizes a system. Examples include the changes from solid to liquid, liquid to gas, and the reverse transitions. Other examples include the transition from being a normally conducting material to being a superconductor. Phase transitions can be classified by their order. If there is a nonzero latent heat (i.e., the quantity of heat absorbed or released when a substance changes its physical phase at constant temperature) at the transition, it is said to be a first-order transition. If the latent heat is zero, then it is said to be a second-order transition. Pion: A member of a hadron subclass called mesons. A pion (or pi-meson) is an elementary particle that exists in three forms: neutral, positively charged, and negatively charged. The charged pions decay into muons and neutrinos. The neutral pion decays into two photons. Plasma: A state of matter consisting of free (unbound) ions and electrons moving freely. Plasma, quark-gluon: Quarks are bound together in nucleons (protons or neutrons), though during the first 10 µs of the big bang the temperature of the universe was so high that unbound quarks moved freely in a state of matter called quark-gluon plasma. It may also be possible to artificially create a quark-gluon plasma by colliding two heavy nuclei at very high energies so that the nucleons dissolve into their quark and gluon parts. Ponderomotive channeling: The process by which a beam of light forms a channel in a plasma due to its radiation pressure, or so-called ponderomotive force. Principle, Hugoniot: The regions of parameter space (density, pressure, and so on) that can be accessed by a shock wave passing through a material. Quantum statistics: A statistical description of a system of particles that obeys the rules of quantum mechanics rather than those of classical mechanics. In quantum statistics, energy states are considered to be quantized. Bose-Einstein statistics apply if any number of particles can occupy a given quantum state; such particles are known as bosons. Fermi-Dirac statistics apply when only one particle may occupy each quantum state; such particles are called fermions. Quasar: A very compact and extraordinarily luminous source of radiation in the nucleus of a distant galaxy. Quasars are believed to be powered by the accretion of gas onto massive black holes (see Black hole, accreting). Radiative shocks: Nonlinearly steepened waves or shocks in which the electromagnetic field pressure or radiation pressure dominates over the usual thermal pressure. Radio lobes: Bright, diffuse areas of radio emission seen on one or both sides of the nucleus of a galaxy. The emission is mainly synchrotron radiation (electro-
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magnetic radiation emitted by charged particles in circular orbits at relativistic speeds in a magnetic field), and the lobe is thought to consist of material ejected from the nucleus of a galaxy and transported into intergalactic space along a jet. Rankine-Hugoniot relations: The relationship between a material’s properties on either side of a shock wave. The relationship is based on conservation of energy and momentum. Rayleigh scattering: The elastic scattering of photons (light) by particles smaller than the wavelength of light, in which the photons do not lose any energy but do change phase. Rayleigh scattering of sunlight by the atmosphere is responsible for daylight and for the sky’s being blue. Relativistic speed: When the speed of a body is such that its mass becomes significantly greater than its rest mass, it is said to be moving at relativistic speed. Relativistic speed is usually expressed as a proportion of the speed of light and is typically greater than 80 percent of the speed of light. A body traveling at these speeds exhibits significant relativistic effects such as an apparent increase in mass, length contraction, and so on. Reynolds number(Re): The ratio of the inertial forces to the viscous forces in a fluid. where ρ is the fluid density, v is the characteristic fluid velocity, L is a characteristic length, and µ is the viscosity. Fluids with low values of Re are stabile to laminar flow. Reynolds number, magnetic(Rm): where v is the characteristic fluid velocity, τ is the magnetic diffusion time, and L is a characteristic length. When Rm is large compared to unity, the magnetic field lines are frozen to the moving fluid. The Reynolds number can be very large for astrophysical situations. In magnetic stars, Rm~1010, whereas for ionized hydrogen the magnetic Reynolds number is small, Rm~10−2. Scattering, Raman: Similar to Brillouin scattering except that the scattering is off electron plasma density waves rather than ion waves. Scattering, stimulated Brillouin: Scattering in a plasma of a light wave off of ion acoustic wave density fluctuations (noise). A positive feedback can then develop such that the scattered wave and the unscattered photons beat to further enhance the ion fluctuation, leading to an instability (stimulated scattering). Such processes are important loss mechanisms in the attempt to get laser light to penetrate far enough to heat a fusion pellet. Schwinger field: The focused electric field strength sufficient to break down vacuum; given by (m2c3)/(eℏ)=1016 V/cm. Stefan’s law: Also known as the Stefan-Boltzman law, Stefan’s law states that the total energy radiated per unit surface area of a blackbody (i.e., a perfect emitter
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or transmitter of energy) in unit time is proportional to the fourth power of its thermodynamic temperature. Synchrotron radiation: The electromagnetic radiation emitted by charged particles in circular orbits at relativistic speeds in a magnetic field. Thermonuclear ignition: The point at which the energy generated in a plasma is sufficient to sustain continued nuclear fusion. Tokamak: Doughnut-shaped magnetic confinement configuration with toroidal symmetry, where the magnetic field in the toroidal direction is much stronger than that in the transverse direction. TRIDENT: A neodymium-glass laser (1,054-nm fundamental wavelength) facility at Los Alamos National Laboratory providing three beams, with the two main drive beams frequency-doubled to a 527-nm wavelength with energies up to 250 J/beam. Twinning transition: A deformation mechanism in solid-state samples that occurs when a region in the crystal changes its collective orientation along a line of reflection symmetry. Unruh radiation: Radiation from an accelerated charge which exceeds that predicted by Maxwell. The source of the radiation is an effective temperature kt=ℏa/c associated with a particle undergoing acceleration a, causing it to emit blackbody radiation (given by Stefan’s law, σ T4). Vacuum polarization: A process in which an electromagnetic field gives rise to virtual electron-positron pairs that effectively alter the distribution of charges and current that generated the original electromagnetic field. Viscous damping: Damping of a wave in fluid due to the viscosity or friction in the fluid that is supporting the wave. VULCAN: The main high-power laser facility operated by the United Kingdom’s Central Laser Facility is a neodymium-glass laser system capable of delivering up to 2.6 kJ of laser energy in nanosecond pulses and over 100 TW power in subpicosecond pulses at 1,054 nm. Frequency conversion to the second harmonic gives 1 kJ at 527 nm. Pulse durations between 700 fs and 5 ns are available. The subpicosecond pulse is produced using the technique of chirped pulse amplification. Wakefield acceleration: A mechanism of particle acceleration whereby particles surf on the (electric field) wake left by a laser or electron pulse in a plasma. The wake is formed as a result of radiation pressure displacing plasma electrons in the path of the laser. Laser wakefield acceleration is particularly promising for generating beams of short-pulse, high-energy electrons for applications in femtosecond electron diffraction, medical imaging, and miniature free-electron x-ray lasers. Warm dense matter: A region of parameter space in which the plasma temperature is close to Fermi temperature and where the plasma temperature is comparable to interparticle coulomb energy.
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Z-pinch: A plasma device with cylindrical symmetry carrying a large current in the axial (z) direction, often created by an electric discharge in a low-pressure gas tube. The self-magnetic field of the plasma in the azimuthal direction causes the plasma to pinch.
Representative terms from entire chapter: