| magnetic island | <physics> A magnetic topology near a rational surface 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 in a narrow region near a rational surface. (See also magnetohydrodynamic, 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 centres of the islands are magnetic O-points, while the boundaries between islands are marked by X-points. (09 Oct 1997) |
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| magnetic mach number | <physics> A dimensionless number equal to the ratio of the velocity of a fluid to the velocity of Alfven waves in that fluid. (13 Nov 1997) |
| magnetic moment | <physics> (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).) (13 Nov 1997) |
| magnetic probe | <radiobiology> 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. (09 Oct 1997) |
| magnetic pumping | <radiobiology> 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.) (09 Oct 1997) |
| magnetic reconnection | 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 magnetohydrodynamic 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. (09 Oct 1997) |
| magnetic resonance angiography | Non-invasive method of vascular imaging and determination of internal anatomy without injection of contrast media or radiation exposure. The technique is used especially in cerebral angiography as well as for studies of other vascular structures. (12 Dec 1998) |
| Magnetic Resonance Imaging | A special imaging technique used to image internal stuctures of the body, particularly the soft tissues. An MRI image is often superior to a normal X-ray image. It uses the influence of a large magnet to polarize hydrogen atoms in the tissues and then monitors the summation of the spinning energies within living cells. Images are very clear and are particularly good for soft tissue, brain and spinal cord, joints and abdomen. These scans may be used for detecting some cancers or for following their progress. Acronym: MRI (11 Nov 1997) |
| magnetic resonance imaging, cine | A type of imaging technique used primarily in the field of cardiology. By coordinating the fast gradient-echo mri sequence with retrospective ecg-gating, numerous short time frames evenly spaced in the cardiac cycle are produced. These images are laced together in a cinematic display so that wall motion of the ventricles, valve motion, and blood flow patterns in the heart and great vessels can be visualised. (12 Dec 1998) |
| magnetic resonance scanning | A special imaging technique used to image internal stuctures of the body, particularly the soft tissues. An MRI image is often superior to a normal X-ray image. It uses the influence of a large magnet to polarize hydrogen atoms in the tissues and then monitors the summation of the spinning energies within living cells. Images are very clear and are particularly good for soft tissue, brain and spinal cord, joints and abdomen. These scans may be used for detecting some cancers or for following their progress. Acronym: MRI (11 Nov 1997) |
| magnetic resonance spectroscopy | Detection and measurement of the resonant spectra of molecular species in a tissue or sample. (05 Mar 2000) |
| magnetic stress tensor | <radiobiology> 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. (09 Oct 1997) |
| magnetic switching | <radiobiology> 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.) (09 Oct 1997) |
| magnetic viscosity | <physics> 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. (09 Oct 1997) |
| nuclear magnetic resonance | Spectroscopic method of measuring the magnetic moment of atomic nuclei in a covalent bond. Clinical application is in biochemical, metabolic, and physiologic studies of living tissue. It includes proton and electron spin-echo and spin-relaxation times. (12 Dec 1998) |