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Conventional Fusion FAQ Glossary Part 13/26 (M)


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Archive-name: fusion-faq/glossary/m
Last-modified: 18-Feb-1995
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===============================================================
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.


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