Migma

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Migma, sometimes migmatron, was a proposed colliding beam fusion reactor designed by Bogdan Maglich in the early 1970s.[1] Migma uses self-intersecting beams of ions from small particle accelerators to force the ions to fuse. Similar systems using larger collections of particles were referred to as "macrons". Migma was an area of some research in the 1970s and early 1980s, but lack of funding precluded further development.

Conventional fusion

Fusion takes place when atoms come into close proximity and the nuclear strong force pulls their nuclei together. Counteracting this process is the fact that the nuclei are all positively charged, and thus repel each other due to the electrostatic force. In order for fusion to occur, the nuclei must have enough energy to overcome this coulomb barrier. The barrier is lowered for atoms with less positive charge, those with the fewest number of protons, and the strong force is increased with additional nucleons, the total number of protons and neutrons. This means that a combination of deuterium and tritium has the lowest coulomb barrier, at about 100 keV (see requirements for fusion).

When the fuel is heated to high energies the electrons disassociate from the nuclei, which are left as ions in a gas-like plasma. Any particles in a gas are distributed across a wide range of energies in a spectrum known as the Maxwell–Boltzmann distribution. At any given temperature the majority of the particles are at lower energies, with a "long tail" containing smaller numbers of particles at much higher energies. So while 100 KeV represents a temperature of over one billion degrees, in order to produce fusion events the fuel does not have to be heated to this temperature as a whole. Even at a much lower temperature, the rate of fusion may be high enough to provide useful power output as long as it is confined for some period of time. Increased density also increases the rate, as the energy from the reactions will heat the surrounding fuel and potentially incite fusion in it as well. The combination of temperature, density and confinement time is known as the Lawson criterion.

Two primary approaches have developed to attack the fusion energy problem. In the inertial confinement approach the fuel is quickly squeezed to extremely high densities, increasing the internal temperature in the process. There is no attempt to maintain these conditions for any period of time, the fuel explodes outward as soon as the force is released. The confinement time is on the order of nanoseconds, so the temperatures and density have to be very high in order to any appreciable amount of the fuel to undergo fusion. This approach has been successful in producing fusion reactions, but to date the devices that can provide the compression, typically lasers, require more energy than the reactions produce.

In the more widely studied magnetic confinement approach, the plasma, which is electrically charged, is confined with magnetic fields. The fuel is slowly heated until some of the fuel in the tail of the temperature distribution starts undergoing fusion. At the temperatures and densities that are possible using magnets the fusion process is fairly slow, so this approach requires long confinement times on the order of tens of seconds, or even minutes. Confining a gas at millions of degrees for this sort of time scale has proven difficult, although modern experimental machines are approaching the conditions needed for net power production.

Migma fusion

The colliding beam approach avoided the problem of heating the mass of fuel to these temperatures by accelerating the ions directly in a particle accelerator. Accelerators capable of 100 keV are fairly simple to build, although in order to make up for various losses the energy provided is generally higher. The original colliding beam concept used two small accelerators arranged so the beams would intersect, but this reaction has very low cross-sections and most of the particles exited the experimental chamber without colliding. The obvious solution was to capture ions that "missed" and circulate them back into the reaction area in a storage ring, but those ions that came close to a reaction tended to scatter out at high angles that made them exit the rings. Simple mathematics showed this approach would not work, the loss rate from these reactions would always be much higher than the energy gained from fusion reactions.[2]

Maglich's concept modified the arrangement based on a new particle storage concept he had co-invented, known as the "precetron". In a typical storage ring concept, the particles are fired into the ring "end on" with a specific energy so they follow the path of the ring. In contrast, in the precetron the storage area is a magnetic mirror. In most magnetic mirror arrangements the average particle energy is relatively low and the ions and electrons have relatively small orbits around the magnetic lines of force, much smaller in radius than the diameter of the mirror as a whole. In the precetron, the ions have much higher energies, and thus much larger orbits, taking up a significant portion of the mirror's diameter, about 13 to 12. In this arrangement, the ions will tend to move towards the center of the mirror volume, instead of reflecting back and forth between the ends as in the classical mirror setup.[3]

Additionally, due to the arrangement of the fields, with the field being stronger at the outside of the volume, the ion orbits will precess around the inner area. This causes the circular path to move its center of rotation so that it rotates around the volume as well. For instance, if the particle is initially fired into the storage area so that it is orbiting around the bottom half of the mirror area, it will slowly move so the orbit is on one side, then the top, the other side, and then the bottom again. If one traces out the path of a single ion over time, it forms a pattern similar to that of a Spirograph, creating a series of circles that fill the volume.[4]

The key to using this concept in the migma system was to fire the ions into the chamber with the right energy so that their paths passed through the geometric center of the mirror. After a short time, this orbit would precess away from the initial entrance point. When another ion is fired in, it takes up the original orbit. Over time, the chamber would fill with ions orbiting within what was effectively an infinite number of storage rings all intersecting in the center. And because they met in the center, ions on the other side of the chamber were moving in opposite directions when the met, so a single accelerator produced an effect similar to two accelerators and two storage rings in the conventional layout.[4]

A great advantage of this approach is that forward scattering of the ions in "missed" reactions would simply move them to a different orbit, but their natural movement in the mirror field would bring them back to the center over time. It was only those ions that scattered to a large off-axis angle that would escape. As a result, it was expected that any given ion would take about 108 orbits through the reaction area before scattering out of the system.[5] The term "migma", from the Greek word for "mixture", was chosen to distinguish this mass of orbiting ions from the plasma in conventional machines.[2]

Several Migma experimental devices were built in the 1970s; the original in 1972, Migma II in 1975, Migma III in 1978, and eventually culminating with the Migma IV in 1982. These devices were relatively small, only a few meters long along the accelerator beamline with a disk-shaped target chamber about 2 metres (6 ft 7 in) in diameter and 1 metre (3 ft 3 in) thick. Migma testbed devices used accelerators of about 1 MeV,[6] to 2 MeV.[2] This device achieved the record fusion triple product (density × energy-confinement-time × mean energy) of 4 × 1014  keV sec cm−3 in 1982, a record that was not approached by a conventional tokamak until JET achieved 3 × 1014 keV sec cm−3 in 1987.

Maglich attempted to secure funding for a follow-on version for some time, unsuccessfully. According to an article in The Scientist, Maglich has been involved in an acrimonious debate with the various funding agencies since the 1980s.[7]

Migma drawbacks

One more recent concern with the Migma design is that the particles lose energy through collisions with other particles in the reaction area, and through other interactions that only become an issue at very high energies, notably bremsstrahlung. These processes remove energy from the fast particles being injected, lowering their temperature and feeding it into the surrounding fuel mass. It appears there is no obvious way to fix this problem.[8]

References

  1. ^ Maglich, Bogdan (1973). "The Migma principle of controlled fusion". Nuclear Instruments and Methods III: 213–235. 
  2. ^ a b c Maglich 1973, p. 213.
  3. ^ Rostoker, N.; Wessel, F.; Maglich, B.; Fisher, A. (June 1992). Magnetic Fusion with High Energy Self-Colliding Ion Beams (Technical report). University of Texas. p. 3. 
  4. ^ a b Maglich 1973, p. 214.
  5. ^ Maglich 1973, p. 215.
  6. ^ Migma IV High Energy Fusion Apperatus
  7. ^ Crease, Robert (27 November 1989). "Visionary Physicist's Crusade Serves As Lesson In Futility". The Scientist. 
  8. ^ Rider, Todd (June 1995). "Fundamental Limitations on Plasma Fusion Systems Not in Thermodynamic Equilibrium". Thesis (Ph.D.) -- MIT Department of Electrical Engineering and Computer Science. 

External links

  • Visionary Physicist's Crusade Serves As Lesson In Futility in The Scientist
  • Patent 4788024: Apparatus and method for obtaining a self-colliding beam of charged particles operating above the space charge limit
  • "Figure Eights for Fusion: The Migma's Mix," by Dietrick E. Thomsen, Science News, 1973
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