A magnetic mirror
is a magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
configuration where the field strength changes when moving along a field line. The mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
results in a tendency for charged particles to bounce back from the high field region.
Charged particles with a velocity component perpendicular to the field will gyrate around a field line in a generally circular or helical orbit and thus sample some of the field lines that are converging to create the field gradient. The radial component of these field lines, coupled with the azimuthal motion of the particle, will result in a force parallel to the field and directed toward the region of smaller field strength.
A more mathematical treatment of the mirror effect describes it as a result of the adiabatic invariance of the magnetic moment. For a given mirror ratio
(the maximum field strength divided by the minimum field strength), particles with a pitch angle
(angle between the particle velocity and the magnetic field) greater than a critical value will be reflected, those with a smaller pitch angle will escape. In particular,
This result may be surprising because one might intuitively expect that heavier and faster particles, or those with less electric charge, would be harder to reflect, or that a smaller magnetic field would reflect particles less efficiently. However, it must be remembered that the gyroradius
ρ = mv
in those circumstances is also larger, so that the radial component of the magnetic field seen by the particle is also larger. It is true that the minimum volume and magnetic energy is larger for the case of fast particles and weak fields, but the mirror ratio required remains the same.
In the 1960s and 1970s, magnetic mirror confinement was considered a viable technique for producing fusion energy. The concept was eventually largely abandoned because it proved to be impractical to maintain the necessary non-Maxwellian velocity distribution, for several reasons. First, instead of many high energy ions hitting one another, the ion energy spread out into a bell curve. The ions then thermalized, leaving most of the material too cold to fuse. Collisions also scattered the charged particles so much that they could not be contained. Lastly, velocity space instabilities contributed to the escape of the plasma
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
. (For a modern attempt, see the Polywell
A polywell device is a type of fusion reactor that was originated by Robert Bussard under a U.S. Navy research contract. It traps electrons in a magnetic confinement inside its hollow center. The negatively charged electrons then accelerate positively charged ions for the purpose of achieving...
inertial electrostatic confinement
Inertial electrostatic confinement is a concept for retaining a plasma using an electrostatic field. The field accelerates charged particles radially inward, usually in a spherical but sometimes in a cylindrical geometry. Ions can be confined with IEC in order to achieve controlled nuclear fusion...
design by Robert Bussard.)
Magnetic mirrors play an important role in other types of magnetic fusion energy devices such as tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...
s, where the toroidal
The earliest use of these terms cited by the Oxford English Dictionary is by Walter M. Elsasser in the context of the generation of the Earth's magnetic field by currents in the core, with "toroidal" being parallel to lines of latitude and "poloidal" being in the direction of the magnetic field...
magnetic field is stronger on the inboard side than on the outboard side. The resulting effects are known as neoclassical
Magnetic mirrors also occur in nature. Electrons and ions in the magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
, for example, will bounce back and forth between the stronger fields at the poles.
A magnetic bottle
is the superposition of two magnetic mirrors. For example, two parallel coils separated by a small distance, carrying the same current in the same direction will produce a magnetic bottle between them. Particles near either end of the bottle experience a magnetic force towards the center of the region; particles with appropriate speeds spiral repeatedly from one end of the region to the other and back. Magnetic bottles can be used to temporarily trap charged particles. This technique is used to confine very hot plasmas with temperatures of the order of 106
K. In a similar way, the Earth's non-uniform magnetic field traps charged particles coming from the sun in doughnut shaped regions around the earth called the "Van Allen radiation belt
The Van Allen radiation belt is a torus of energetic charged particles around Earth, which is held in place by Earth's magnetic field. It is believed that most of the particles that form the belts come from solar wind, and other particles by cosmic rays. It is named after its discoverer, James...
s", which were discovered in 1958 using data obtained by instruments aboard the Explorer 1 satellite.