A magnetic mirror, also known as a magnetic trap or sometimes as a pyrotron, is a type of magnetic confinement fusion device used in fusion power to trap high-temperature plasma using magnetic fields. The mirror was one of the earliest major approaches to fusion power, along with the stellarator and Z-pinch machines. In a classic magnetic mirror, a configuration of electromagnets is used to create an area with an increasing density of magnetic field lines at either end of a confinement volume. Particles approaching the ends experience an increasing force that eventually causes them to reverse direction and return to the confinement area. This mirror effect will occur only for particles within a limited range of velocities and angles of approach, while those outside the limits will escape, making mirrors inherently "leaky". An analysis of early fusion devices by Edward Teller pointed out that the basic mirror concept is inherently unstable. In 1960, Soviet researchers introduced a new "minimum-B" configuration to address this, which was then modified by UK researchers into the "baseball coil" and by the US to "yin-yang magnet" layout. Each of these introductions led to further increases in performance, damping out various instabilities, but requiring ever-larger magnet systems. The tandem mirror concept, developed in the US and Russia at about the same time, offered a way to make energy-positive machines without requiring enormous magnets and power input. By the late 1970s, many of the design problems were considered solved, and Lawrence Livermore Laboratory began the design of the Mirror Fusion Test Facility (MFTF) based on these concepts. The machine was completed in 1986, but by this time, experiments on the smaller Tandem Mirror Experiment revealed new problems. In a round of budget cuts, MFTF was mothballed, and eventually scrapped. A fusion reactor concept called the Bumpy torus made use of a series of magnetic mirrors joined in a ring. It was investigated at the Oak Ridge National Laboratory until 1986. The mirror approach has since seen less development, in favor of the tokamak, but mirror research continues today in countries like Japan and Russia.
History
Early work
The concept of magnetic-mirror plasma confinement was proposed in the early-1950s independently by Gersh Budker at the Kurchatov Institute, Russia and Richard F. Post at the Lawrence Livermore National Laboratory in the US. With the formation of Project Sherwood in 1951, Post began the development of a small device to test the mirror configuration. This consisted of a linear pyrex tube with magnets around the outside. The magnets were arranged in two sets, one set of small magnets spaced evenly along the length of the tube, and another pair of much larger magnets at either end. In 1952 they were able to demonstrate that plasma within the tube was confined for much longer times when the mirror magnets at the end were turned on. At the time, he referred to this device as the "pyrotron", but this name did not catch on.
Instabilities In a now-famous talk on fusion in 1954, Edward Teller noted that any device with convex magnetic field lines would likely be unstable, a problem today known as the flute instability. The mirror has precisely such a configuration; the magnetic field was highly convex at the ends where the field strength increased. This led to serious concern by Post, but over the next year, his team could find no sign of these problems. In October 1955 he went so far as to state that "it is now becoming clear that in the case of the mirror machine at least these calculations do not apply in detail." In Russia, the first small-scale mirror ("probkotron") was built in 1959 at the Budker Institute of Nuclear Physics in Novosibirsk, Russia. They immediately saw the problem Teller had warned about. This led to something of a mystery, as the US teams under Post continued to lack any evidence of such problems. In 1960, Post and Marshall Rosenbluth published a report "providing evidence for the existence of a stability confined plasma... where the simplest hydromagnetic theory predicts instability." At a meeting on plasma physics in Saltzberg in 1961, the Soviet delegation presented considerable data showing the instability, while the US teams continued to show none. An offhand question by Lev Artsimovich settled the matter; when he asked if the charts being produced from the instruments in the US machines were adjusted for a well-known delay in the output of the detectors being used, it suddenly became clear that the apparent 1 ms stability was, in fact, a 1 ms delay in the measurements. Artsimovich went so far as to claim "we now do not have a single experimental fact indicating long and stable confinement of plasma with hot ions within a simple magnetic mirror geometry."
New geometries
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