Migma, sometimes migmatron, was a proposed colliding beam fusion reactor designed by Bogdan Maglich in the early 1970s. 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.
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.
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. Later Migma testbed devices used accelerators of about 1 MeV, fairly small compared to the large research colliders like Tevatron, which are a million times more powerful.
The original colliding beam concept used two small accelerators arranged so the beams would intersect, but this reaction proved to have fairly low cross-sections and most of the particles exited the experimental chamber without colliding. Maglich's concept modified the arrangement to include a powerful magnetic confinement system in the target area; ions injected into the chamber would orbit around the center for some time, thereby greatly increasing the chance that any given particle would undergo a collision given a long enough confinement time. It was not obvious that this approach could work, as positively charged ions would all orbit the magnetic field in the same direction. However, Maglich showed that it was nevertheless possible to produce self-intersecting orbital paths in such a system, and he was able to point to experimental results from the intersecting beams at CERN to back up the proposal with real-world numbers.
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. 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, so far unsuccessfully. According to an article in The Scientist, Maglich has been involved in an apparently acrimonious debate with the various funding agencies since the 1980s.
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.
- The Migma principle of controlled fusion, Bogdan C. Maglich, Nuclear Instruments and Methods III (1973), p 213-235
- Migma IV High Energy Fusion Apperatus
- Rider, Todd H., Fundamental Limitations on Plasma Fusion Systems Not in Thermodynamic Equilibrium, Thesis (Ph.D.) -- MIT Department of Electrical Engineering and Computer Science, June 1995