Magnetic mirror
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 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 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
The issue of the potential instabilities had been considered in the field for some time and a number of possible solutions had been introduced. These generally worked by changing the shape of the magnetic field so it was concave everywhere, the so-called "minimum-B" configuration.At the same 1961 meeting, Mikhail Ioffe introduced data from a minimum-B experiment. His design used a series of six additional current-carrying bars in the interior of an otherwise typical mirror to bend the plasma into the shape of a twisted bow-tie to produce a minimum-B configuration. They demonstrated that this greatly improved the confinement times to the order of milliseconds. Today this arrangement is known as "Ioffe bars".
A group at the Culham Centre for Fusion Energy noted that Ioffe's arrangement could be improved by combining the original rings and the bars into a single new arrangement similar to the seam on a tennis ball. This concept was picked up in the US where it was renamed after the stitching on a baseball. These "baseball coils" had the great advantage that they left the internal volume of the reactor open, allowing easy access for diagnostic instruments. On the downside, the size of the magnet in comparison to the volume of plasma was inconvenient and required very powerful magnets. Post later introduced a further improvement, the "yin-yang coils", which used two C-shaped magnets to produce the same field configuration, but in a smaller volume.
In the US, major changes to the fusion program were underway. Robert Hirsch and his assistant Stephen O. Dean were excited by the huge performance advance seen in the Soviet tokamaks, which suggested power production was now a real possibility. Hirsch began to change the program from one he derided as a series of uncoordinated science experiments into a planned effort to ultimately reach breakeven. As part of this change, he began to demand that the current systems demonstrate real progress or they would be cancelled. The bumpy torus, levitron and Astron were all abandoned, not without a fight.
Dean met with Livermore's team and made it clear that Astron would likely be cut, and mirrors had to improve or face cutting as well, which would have left the lab with no major fusion projects. In December 1972, Dean met with the mirror team and made a series of demands; their systems would have to demonstrate an nT value of 1012, compared to the current best number on 2XII of 8x109. After considerable concern from the researchers that this would be impossible, Dean backed off to 1011 being demonstrated by the end of 1975.
DCLC
Although 2XII was nowhere near the level needed by Dean's demands, it was nevertheless extremely successful in demonstrating that the yin-yang arrangement was workable and suppressed the major instabilities seen in earlier mirrors. But as experiments continued through 1973, the results were not improving as expected. Plans emerged to brute-force the performance through the addition of neutral-beam injection to quickly raise the temperature to reach Dean's conditions. The result was 2XIIB, the B for "beams".While 2XIIB was being set up, in November 1974, Fowler received a letter from Ioffe containing a series of photographs of oscilloscope traces with no other explanation. Fowler realized they demonstrated that injecting warm plasma during the run improved confinement. This appeared to be due to a long-expected but so-far unseen instability known as "drift-cyclotron loss-cone", or DCLC. Ioffe's photographs demonstrated that DCLC was being seen in Soviet reactors and that warm plasma appeared to stabilize it.
2XIIB reactor started real experiments in 1975, and significant DCLC was immediately seen. Annoyingly, the effect grew stronger as they improved the operating conditions with better vacuum and cleaning of the interior. Fowler recognized the performance was identical to that of Ioffe's photographs, and 2XIIB was modified to inject warm plasma during the center of the run. When the results were seen, they were described as "sunlight was breaking through the clouds and there was the chance that everything would be all right."
Q-enhancement and tandem mirrors
In July 1975, the 2XIIB team presented their results for nT at 7x1010, an order of magnitude better than 2XII and close enough to Dean's requirements. By this time, the Princeton Large Torus had come online and was setting record after record, prompting Hirsch to begin planning for even larger machines for the early 1980s with the explicit goal of hitting breakeven, or Q=1. This became known as the Tokamak Fusion Test Reactor, whose goal was to run on deuterium-tritium fuel and reach Q=1, while future machines would be Q>10.With the latest results on 2XIIB, it appeared that a larger yin-yang design would also improve performance. However, calculations showed it would only reach Q=0.03. Even the most developed versions of the basic concept, with leakage at the absolute lower limit allowed by theory, could only reach Q=1.2. This made these designs largely useless for power production, and Hirsch demanded that this be improved if the program were to continue. This problem became known as "Q-enhancement".
In March 1976, the Livermore team decided to organize a working group on the topic of Q-enhancement at the October 1976 international fusion meeting in Germany. Over the July 4th weekend, Fowler and Post came up with the idea of the tandem mirror, a system consisting of two mirrors at either end of a large chamber that held large amounts fusion fuel at lower magnetic pressure. They returned to LLNL on Monday to find the idea had been developed independently by a staff physicist, Grant Logan. They brought further developed versions of these ideas to Germany to find a Soviet researcher proposing exactly the same solution.
Upon their return from the meeting, Dean met with the team and decided to shut down the Baseball II system and direct its funding to a tandem mirror project. This emerged as the Tandem Mirror Experiment, or TMX. The final design was presented and approved in January 1977. Construction of what was then the largest experiment at Livermore was completed by October 1978. By July 1979, experiments were demonstrating that TMX was operating as expected.