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Subject: Conventional Fusion FAQ Glossary Part 13/26 (M)

This article was archived around: 11 Nov 1999 12:25:47 GMT

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Archive-name: fusion-faq/glossary/m Last-modified: 18-Feb-1995 Posting-frequency: More-or-less-quarterly Disclaimer: While this section is still evolving, it should be useful to many people, and I encourage you to distribute it to anyone who might be interested (and willing to help!!!).
=============================================================== Glossary Part 13: Terms beginning with "M" FREQUENTLY USED TERMS IN CONVENTIONAL FUSION RESEARCH AND PLASMA PHYSICS Edited by Robert F. Heeter, rfheeter@pppl.gov Guide to Categories: * = plasma/fusion/energy vocabulary & = basic physics vocabulary > = device type or machine name # = name of a constant or variable ! = scientists @ = acronym % = labs & political organizations $ = unit of measurement The list of Acknowledgements is in Part 0 (intro). ================================================================== MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM # m, M: variable typically used for mass. $ MA: MegaAmpere or MegAmpere; see Mega, Ampere $ m: meters; SI unit of distance $ M: metric prefix "mega", meaning million @ MARS: Mirror Advanced Reactor Study; see entry $ MeV: Millions of electron volts; see mega, electron-volt. @ MFE: Magnetic Fusion Energy @ MFTF-B: Mirror Fusion Test Facility; see entry @ MHD: Magnetohydrodynamics; see entry @ MHD Instability: see Magnetohydrodynamic instability. @ MHTGR: Modular High-Temperature Gas-Cooled Reactor; see entry. @ MIT: Massachusetts Institute of Technology; see entry @ MITL: Magnetically Insulated Transmission Line; see entry @ MIX 1: see entry under "MIX 1" @ MKS: Meters, Kilometers, Seconds - see SI Units @ MKSA: Meters, Kilometers, Seconds, Amperes - See SI Units. @ MMX: Multiple Mirror eXperiment; see entry @ MS: Maryland Spheromak; see entry @ MTX: Microwave Tokamak eXperiment; see entry $ MW: Megawatt; one million watts; see entry for watts. * Mach-Zender Interferometer: This is a variation of the Michelson interferometer which is used mainly in measuring the spatial variation in the refractive index of a gas (or plasma). A Mach-Zender interferometer uses two semi-transparent mirrors and two fully reflective mirrors located at the corners of a rectangle. The incoming beam is split in two at the first semi-transparent mirror, and the two halves of the beam travel along separate paths around the edge of the rectangle, meeting at the opposite corner. Typically one beam is a control, and the other travels through the system under study. The two beams meet at the second semi-transparent mirror, after which they are mixed together and interfere. % Madison: See University of Wisconsin-Madison * Magnetic Axis: This typically refers to the location of the innermost flux "surface" in a toroidal device, the one which encloses no volume and has therefore degenerated from a flux surface into a single field line. Roughly, the circle through the middle of the dough of the donut. Additionally, in systems with magnetic islands (see entry below), each island has a local magnetic axis, distinct from the overall magnetic axis of the torus. * Magnetic Bottle: Colorful term used to describe a magnetic field structure which confines a plasma "like in a bottle". * Magnetic Confinement: Use of magnetic fields to confine a plasma. (Confinement involves restricting the volume of the plasma and/or restricting particle or energy transport from the center of the plasma to the edge.) * Magnetic Confinement Fusion: Method of fusion which uses magnetic fields / magnetic bottles to confine a hot plasma until fusion occurs. * Magnetic Diffusion: * Magnetic Field: * Magnetic Field Coil: Coiled current-carrying wires used to generate magnetic fields. * Magnetic Flux Surfaces: * Magnetic Force Parameter: A dimensionless number equal to [(magnetic permeability squared) * (magnetic field strength squared) * electrical conductivity * characteristic length of system in question] / [(mass density) * (fluid velocity)]. This measures the strength of magnetic forces relative to the plasma's inertia. * Magnetic Island: A magnetic topology near a "rational surface" (see entry) where the flux surface is broken up into tubes which are not connected with each other poloidally. Islands may develop in non-ideal magnetohydrodynamic fluids, where electrical resistance becomes important and magnetic field lines are no longer "frozen-in" to the fluid. Then magnetic tearing and reconnection may allow field lines to link up and form "islands" with a local magnetic axis (see entry) in a narrow region near a rational surface (see entry). (See also MHD, frozen-in law). The development of islands may be caused by a small perturbation, whether internal or external, whether deliberate or accidental, and is usually associated with enhanced transport (i.e., reduced confinement). The centers of the islands are magnetic O-points, while the boundaries between islands are marked by X-points (see entries). * Magnetic Limiter: See divertor. * Magnetic Mach Number: A dimensionless number equal to the ratio of the velocity of a fluid to the velocity of Alfven waves in that fluid. (See also entry for Alfen waves.) > Magnetic Mirror: See mirror effect, mirror device * Magnetic Moment: (a) A vector associated with a magnet, current loop, or particle; the cross product of this vector with the magnetic field is equal to the torque which the field exerts on the system. (b) The adiabatic invariant associated with the rapid gyromotion of a charged particle in a slowly varying magnetic field. (The value of the magnetic moment in sense (b) is the magnitude of the vector in sense (a).) * Magnetic Number: A dimensionless number equal to the square root of the magnetic force parameter. * Magnetic Pressure: Pressure which a magnetic field is capable of exerting on a plasma; equal to the magnetic energy density; proportional to B^2. (The proportionality constant is 1/(2*mu-o) in SI units, 1/8pi in CGS units). * Magnetic Probe: A conducting coil (sometimes insulated and inserted into the plasma) will have an induced voltage due to changes in the magnetic flux through the coil, and can therefore be used to measure changes in magnetic field strength. Small coils used to measure the local field strength are known as probes. (Other plasma diagnostics using this effect are the Rogowski coil, the voltage loop, and the diamagnetic loop.) Magnetic probes placed outside a toroidal plasma which are used to measure the poloidal magnetic field are also called Mirnov coils. * Magnetic Pumping: Form of plasma heating where the plasma is successively compressed and expanded by means of a fluctuating external magnetic field. (See also adiabatic compression, frozen-in law.) * Magnetic Reconnection: (entry by John Cobb, with some modifications) When a plasma has some resistivity, then the frozen-in flow requirement is relaxed (see frozen-in flow). In that case, the magnetic field can move through the plasma fluid on the resistive (magnetic diffusion) time scale. (Typically slow compared to MHD timescales.) This allows field lines to reconnect with each other to change their topology in response to magnetic and other forces in the plasma. (see also Helicity, which is not conserved when reconnection is significant.) The predominant theory for solar flares is based on the transfer of energy from magnetic fields to plasma particles which can occur in reconnection. Reconnection can also be studied in the laboratory. * Magnetic Stress Tensor: A second-rank tensor, proportional to the dyadic product of the magnetic field (B) with itself. The divergence of the magnetic stress tensor gives that part of the force which a magnetic field exerts on a unit volume of conducting fluid due to the curvature of the magnetic field lines. * Magnetic Switching: The use as switches of saturable inductors for producing high power pulses without electrical arcs. This is a principal technology for extending single-shot accelerators in light-ion-beam-driven inertial confinement fusion to repetitively pulsed devices for possible reactors. Three terawatt, 200 KJ magnetic switches have been developed for fusion drivers at Sandia National Laboratories. (Info from the 1985 OSTI Glossary of Fusion Energy; may be out of date.) * Magnetic Viscosity: A magnetic field in a conducting fluid will damp fluid motions perpendicular to the field lines, similar to ordinary viscosity, even in the absence of sizeable mechanical forces or electric fields. * Magnetic Well: see Minimum-B Configuration. * Magnetically Insulated Transmission Line (MITL): Used to transport power efficiently in vacuum lines at very high power densities. Although the cathode is a space-charge limited electron emitter, the electron flow is confined by self-generated or applied magnetic fields. MITL's are used extensively in light-ion-driven inertial confinement fusion. * Magnetohydrodynamics (MHD): Physical model describing the properties of electrically conducting fluids interacting with magnetic and electric fields. MHD theory is relevant at relatively low frequencies and for distance scales larger than the Larmor radius. Also known as hydromagnetics. * Magnetohydrodynamic Generator: A device that extracts kinetic energy from a jet of plasma and generates electricity. * Magnetohydrodynamic Instability (MHD instability): Class of unstable (growing, not damped) waves and other modes of oscillation which are described by MHD theory. * Magnetohydrodynamic Turbulence: Motion of a plasma in which velocities and pressures fluctuate irregularly. * Magnetohydrodynamic Waves: Material waves in an electrically conducting fluid in the presence of a magnetic field, which are described by magnetohydrodynamics. * Marx Generator: A pulsed-power device invented by Erwin Marx. Capacitors are charged in parallel and then quickly discharged in series to produce high voltage, high current (and thus high power) pulses. Used in light-ion-driven and some laser-driven inertial confinement fusion systems. > Maryland Spheromak: A University of Maryland spheromak facility, used to investigate the production, equilibrium, stability, and confinement properties of spheromaks. (What happened to it?) * Mass Defect: The energy from fusion reactions comes from the difference in mass between the reactants and the products. In an energy-releasing reaction, some mass is converted to energy via Einsteins famous equation E (energy) = m (mass) * c^2 (speed of light squared). The energy released is the difference between the binding energies of the reactants and the products (see entry on binding energy). % Massachusetts Institute of Technology (MIT): Located in Cambridge, MA (just outside Boston). Home of the Plasma Fusion Center and the Alcator series of compact tokmaks. % Max Planck Institute for Plasma Physics (IPP): In Garching (near Munich). The largest plasma physics institute in Germany. Presently home of ASDEX-Upgrade and Wendelstein-7AS. (See entries) ! Maxwell, James Clerk: 19th-century British physicist, responsible for the synthesis of the equations of electromagnetism and the prediction of electromagnetic waves, among other things. & Maxwell-Boltzmann Distribution: Distribution function of particle velocities (or energies) corresponding to a system in thermal equilibrium with a temperature value of T. See also: distribution functions, temperature. & Maxwellian Distribution: see Maxwell-Boltzmann Distribution & Maxwell ('s) Equations: The key equations governing electrical and magnetic phenomena. These are a set of four vector partial differential equations relating electric and magnetic fields to each other and to electric charges and currents. & Mean Free Path (for a given event, e.g., collisons): Average distance a particle travels between occurrences of the given event; e.g., between collisions. For collisions, the mean free path is roughly equal to unity divided by the product of the collision cross section times the particle density. & Mega-: Metric prefix indicating 1,000,000 times a given quantity. e.g., a megawatt is 1,000,000 watts. * Meltdown: In a fission reactor, if there is insufficient coolant or the fission chain reaction proceeds too rapidly, heat can build up in the reactor fuel, causing it to melt. In extreme cases the whole fission core can melt down to (or even through) the reactor floor. Fusion reactors are not vulnerable to this. & Metastable state: several types Electronic Nuclear & Micro-: Metric prefix indicating 1/1,000,000th of a given quantity. e.g., a microampere is 1/1,000,000th of an ampere. * Microinstability: Instabilities due to particle / kinetic- theoretical effects, typically occuring on small scales, as opposed to those derivable from fluid models valid on larger scales. As with other instabilities, these are driven by various types of available free energy. (See also kinetic theory.) * Microwave Interferometer: See interferometer, interferometry. A microwave interferometer uses radio waves in the microwave frequency (or wavelength) range as the electromagnetic signal. Microwave interferometers are used to measure the line-averaged density of a plasma along the path through which the microwave beam is passed, through phase shifts in the propagated beam. * Microwave Tokamak eXperiment (MTX): a reincarnation of Alcator C at LLNL, now shut down. > Migma devices: Non-thermal, non-pulsed devices in which fusion occurs among the ions of a self-colliding particle beam. $ mill: financial unit equal to 0.1 cents or 0.001 dollars; standard unit which electrical utilities use in charging for electricity (e.g., 50 mills/kwh = $0.05/kwh). > Minimum-B Configuration: Confinement configuration where the magnetic field strength is a minimum where the plasma is to be confined, and increases in all directions away from the confinement region. Stability is favorable in such a configuration because the magnetic pressure increases in all directions away from the plasma. * Mirnov Oscillations: Fluctuations in the poloidal magnetic field (of a toroidal magnetic confinement system) which rotate in the electron diamagnetic drift direction at a speed comparable to the electron diagmagnetic drift velocity and with frequencies due to 5-20 kHz. Mirnov oscillations arise from tearing modes. Poloidal magnetic probes used to measure the poloidal field in order to diagnose Mirnov oscillations (and other MHD phenomena) are often called Mirnov coils or Mirnov loops. See relevant entries... > Mirror Advanced Reactor Study (MARS): This was a collaborative effort between government, academia, and industry to design a commercial-scale tandem mirror fusion power plant. Participants included the Department of Energy (LLNL); University of Wisconsin; TRW, Inc.; General Dynamics; EBASCO Services; Science Applications, Inc.; and Grumman Aerospace Corp. System was never actually built. > Mirror device, mirror machine: Generally, linear fusion machines which confine the plasma using the mirror effect. Basically there is a weak field in the center, and strong fields at the ends. Particles are then reflected at the ends by the strong fields, and are confined in the center of the device. (Some particles will have enough velocity along the axis of the device to escape from the mirror, however.) * Mirror effect: A charged particle travelling into an increasing magnetic field will (if the field becomes strong enough) reverse direction and be reflected back. This is a direct result of the adiabatic invariance (see entry) of the magnetic moment (see entry). Plasmas can be confined by devices which utilize this effect; see entry above for mirror device. The effect also occurs in some toroidal plasmas, since the toroidal magnetic field is stronger on the inboard side than on the outboard side; in this case it gives rise to so-called "neoclassical" effects. The strength of the mirror is determined by the mirror ratio. (See relevant entries. Consult an introductory plasma physics text for a more technical explanation.) > Mirror Fusion Test Facility (MFTF): A large mirror device built at LLNL from the late 1970s to the mid-1980s, but mothballed for political reasons (decrease in magnetic fusion funding) just before it was to begin operation. * Mirror Ratio: In a magnetic mirror, the mirror ratio is the ratio between the strongest value of the magnetic field on the mirror's axis, and the value at some other point on the axis. In a mirror confinement device, the "other point" is taken to be the location of weakest field strength between two confining mirrors. The mirror ratio is a key factor in determining confinement properties of the system. > MIX-1: A small, gun-injected mirror machine at the University of Maryland; was used to study the drift-cyclotron loss cone instability (see entries for DCLC, DCLC instability). * Mobility: The ease with which a charge in a medium (e.g. a plasma) moves in response to an electric field. Related to diffusivity and to resistivity. Measured by the average equilibrium drift velocity attained by the charged particle when subjected to acceleration by a unit electric field and the opposing frictional force of collisions with other particles. * Mode Rational Surface: A magnetic surface on which field lines resonate with the helicity of a particular perturbation or instability; see also rational surface. * Moderator: Substance used in a fission reactor to slow down ("moderate") energetic fission neutrons so that they are more easily captured within the reactor and therefore maintain the fission chain-reaction. > Modular High-Temperature Gas-Cooled Reactor: Class of fission reactors under study in the U.S.; designed to run at higher temperatures and use gas cooling to achieve greater efficiency of conversion from thermal to electric energy. & Mole: The amount of given substance such that the mass in grams is equal to its [atomic weight, molecular weight, mass number]. The number of particles in a mole of a substance is Avogadro's Number N = 6.02497 x 10^23 (see entry). For instance, one mole of water weighs 18 grams, since water is H2O, the H's weigh one apiece, and the O weighs 16. Heavy water, or D2O, weighs 20 grams/mole, because each D weighs 2 instead of 1. * Molecular ion injection: Heating concept for magnetic confinement fusion in which energetic (accelerated) molecular ions are injected into the plasma, dissociate, and heat the plasma while building up the population of trapped high-energy ions. Not widely used (see neutral beam injection). & Momentum: Basic physical quantity measuring motion; generally defined as momentum = mass * velocity. The total momentum of all bodies in a system is conserved in all physical processes known so far, I believe. Momentum is related to force in that force = rate of change of momentum with time. See also force. * Motor-Generator: Device used to store energy by accelerating a rotating flywheel to high speeds; energy may be rapidly discharged and converted to shorter-pulse energy. (Used to power TFTR; the electric utility would be a little unhappy if TFTR were to suddenly draw its 30 MW+ of power at random intervals. :) > Multiple Mirror eXperiment (MMX): A 10-meter long simple mirror facility which was located at the University of California, Berkeley. > Muon-Catalyzed Fusion: Alternative approach to fusion where muons are introduced to D-T fluid. The muon is heavy enough that it binds more strongly to the D or T than an electron would, and the result is that the D and T nuclei in the molecule are drawn more tightly together, and fusion results. More detailed discussion is given in section 4B.