Magnetic mirror
Particles approaching the ends experience an increasing force that eventually causes them to reverse direction and return to the confinement area.
An analysis of early fusion devices by Edward Teller pointed out that the basic mirror concept is inherently unstable.
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.
A fusion reactor concept called the Bumpy torus made use of a series of magnetic mirrors joined in a ring.
[3] The concept of magnetic-mirror plasma confinement was proposed in the early-1950s independently by Gersh Budker[4] at the Kurchatov Institute, Russia and Richard F. Post at the Lawrence Livermore National Laboratory in the US.
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.
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.
[9] 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.
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.
These "baseball coils" had the great advantage that they left the internal volume of the reactor open, allowing easy access for diagnostic instruments.
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.
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.
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.
[11] 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.
Plans emerged to brute-force the performance through the addition of neutral-beam injection to quickly raise the temperature to reach Dean's conditions.
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.
This became known as the Tokamak Fusion Test Reactor (TFTR), whose goal was to run on deuterium-tritium fuel and reach Q=1, while future machines would be Q>10.
[16] 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.
[17] 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.
With the success of the TMX concept, the design was modified to become MFTF-B, using two of the largest yin-yang magnets anyone could figure out how to build in an enormous tandem configuration.
Through late 1978 when the teams began to actually consider the steps in scaling up the TMX, it became clear that it simply would not hit the required goals.
the lab decided they should begin construction, lacking any experimental evidence that the concept worked, in order to get a competitive machine out around the same time as TFTR.
[26] Construction on MFTF, already budgeted, continued and the system was declared officially complete on 21 February 1986, at a final price of $372 million.
While thanking the team for their contributions in building the system, the new director of the DOE, John Clarke, also announced that there would be no funding to run it.
One example is the Gas Dynamic Trap, an experimental fusion machine used at the Budker Institute of Nuclear Physics in Akademgorodok (Academic Town) in Novosibirsk (New Siberia), Russia.
[citation needed] In September 2022, University of Wisconsin–Madison researchers incorporated a spin-off startup company named Realta Fusion to develop and commercialize tandem mirror reactors to supply industrial process heat with smaller power plants.
Electrons and ions in the magnetosphere, for example, will bounce back and forth between the stronger fields at the poles, leading to the Van Allen radiation belts.
One concept could achieve 1 to 10 newtons of thrust with 10,000 to 30000 specific impulse, some 30-100x more fuel efficient than chemical propulsion, dramatically decreasing journey times.
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.
