A magnetic quadriloop mining ship in orbit around Saturn, 190 a.t.
The interaction of high energy cosmic rays with the atmosphere, ring system, or regolith of a planet or its moons will result in a shower of high energy particles. Among these particles are anti-neutrons. Like neutrons, free anti-neutrons are unstable with a half-life of fifteen minutes, and they will decay into an anti-proton, a positron, and an electron neutrino. If this decay occurs within the radiation belts of a planet, the anti-proton will be trapped by the planet's magnetic field, resulting in the anti-particle spiraling around the field lines while bouncing back and forth between the magnetic poles.
Anti-protons are the most difficult component of antimatter to produce below the second singularity. This naturally occurring source of anti-protons thus forms a valuable renewable resource than can be captured and extracted. The quantities of antimatter stored at any one time around a planet are too low for significant direct energy production, propulsion, or industrial use. For example, a typical baseline-habitable planet will store only a few tens of megajoules worth of anti-protons, and fill the radiation belts with anti-protons at a rate of a few tens of milliwatts. However, small quantities of antimatter can be used to initiate fission in sub-critical masses of actinide elements (typically U-238) if injected as a particle beam. The ability to leverage the antimatter as a fission trigger makes mining the radiation belts worthwhile.
Antimatter Mining Techniques
A typical antimatter mining station consists of a plasma antenna in equatorial orbit passing through the radiation belts of a planet with high natural antimatter production. A plasma antenna consists of a series of magnetic loops which produce a rotating magnetic field. This field induces a current in the plasma trapped in the radiation belts, and this induced current produces a much more significant magnetic field than could be obtained by the direct current in a physical loop of similar size. (Reference http://www.niac.usra.edu/files/studies/abstracts/860Slough.pdf )
As anti-protons impinge on the magnetic field, they will be channeled along the field lines. If they can be slowed down, they will be captured in the field, endlessly circling around the plasma current produced by the antenna. There are several techniques for doing this, the most effective of which is to resonantly de-excite the anti-protons with a radio frequency field.
Once captured, the plasma antenna itself may act as the antimatter containment device. However, it is more typical for the anti-protons to be neutralized with positrons and collected in a physical trap. This allows the antimatter to be conveniently transported without moving the mining station, and also allows simplified extraction for the fission-inducing particle beam.
In order to trap antimatter, a planet must possess a magnetic field. Larger magnetic fields can trap higher energy anti-protons and allow more time for the anti-neutrons to decay before capture. However, stronger magnetic fields can also reduce the cosmic rays impinging on the planet's atmosphere.
Larger planets present a greater surface area for cosmic ray interactions, and thus will generate more antimatter than smaller planets. The presence of moons also increases the interactions with cosmic rays, but moons can remove antimatter by colliding with the trapped anti-protons and annihilating them. Dense extended ring systems provide a significant increase in the rate of anti-proton production both because of the significant surface area of the rings and because anti-neutrons produced in the rings do not have to scatter back out of the rings to be captured - those that punch through also exit the rings and enter the planet's magnetosphere.
An ideal planet for establishing an antimatter mining operation, therefore, is a large gas giant with a significant ring system, yielding production rates of several hundred micrograms of antimatter per year. Jovian planets without significant ring systems can produce several micrograms per year, while ice giants will produce on the order of a microgram per year. Large terrestrial planets with magnetic fields have production rates on the order of a few nanograms per year.
It is much cheaper to mine antimatter than to manufacture it. However, as demand for antimatter increases, mining is not able to keep up and antimatter production factories become economical. Eventually, antimatter manufacturing becomes the dominant source, and mining becomes a minor, although profitable, contribution to the supply.
Antimatter induced fission must compete with fusion as a source of energy and space propulsion. Deuterium/helium-3 fusion is more technically difficult to achieve, but has nearly unlimited supplies of fuel from ice giant and gas giant planets. However, all of the helium-3 fuel is at the bottom of the very large potential well of these massive planets. Fusion for energy production does not require gas giant helium, but can get away with fusing deuterium found in hydrogen containing ices or liquids, such as water or ammonia, especially if a source of lithium is available for breeding tritium. The high neutron flux of deuterium or deuterium-lithium fueled reactors produces environmental issues due to the radioactivity of neutron activated reactor components as well as the possibility of tritium leaks. However, since these forms of fusion produce most of their energy in the form of neutrons, their performance for space propulsion is significantly reduced and the fusion of deuterium with helium-3 is preferred.
As noted, antimatter is also mined primarily around gas giants, but since only small quantites are needed the price to take the antimatter out of the gravity well is small. Antimatter induced fission also requires fissionables, which are only available from terrestrial planets with (present or past) active geologic and hydrologic cycles to concentrate the actinide ores. Since terrestrial planets have a much smaller potential well than gas or ice giants, antimatter/fission rocket fuel is less expensive for most applications except for those in close orbit around gas giants. The fission of these actinides leads to the production of highly radioactive fission fragments as well as heavier actinides from neutron capture. This imposes an environmental cost on habitable worlds where the waste cannot simply be disposed of in space. However, antimatter/fission drives operating in an external pulse drive with a magnetic nozzle can achieve much higher thrusts than any modosophont fusion drive, although direct exhaust deuterium/helium-3 fusion drives tend to have a slight edge in total delta-V available for a mission.
First singularity technology can induce CNO fusion. This largely eliminates the market for antimatter when S1 transapients make these reactors available to modosophonts. Conversion reactors largely supplant fusion and antimatter/fission reactors whenever monopoles become available.
Legal and Social Issues
In frontier areas and loosely regulated societies, anyone can set up an antimatter mining antenna and begin extracting antimatter. However, since antimatter is produced at a finite rate, once too many people start trapping what is available the supplies per miner will get stretched thin. This often leads to conflicts, and antimatter wars are not uncommon in new colonies that do not have conversion technology. Typically, this will lead to establishment of some set of laws regulating antimatter extraction, usually in the form of granting stakeholders antimatter rights.
Antimatter mining was an important source of energy and propulsion in the pre-Technocalypse era. Records indicate that the first antimatter/fission spacecraft in the Sol system spent time orbiting Earth to collect antimatter, before boosting off to Saturn to fill up their collectors. Soon, Saturn became the major source of antimatter in the Sol system, with Jupiter a distant second. For several decades, antimatter was the limiting factor on interplanetary transport, until deuterium/helium-3 fusion became practical.
The first major conflict for antimatter resources occured at Jupiter [in 218 A.T.]. Antimatter interests wanted to disassemble the moon Amalthea into a ring to boost antimatter production. This would have allowed the production at Jupiter to exceed that of Saturn. However, it would have made much of Jupiter's close orbital space useless for other purposes. Jovian colonists had already established tether power stations to extract energy from Jupiter's inner moons as they cut through the strong magnetic field - these would have been destroyed in the process of creating a Jovian ring.
Initially the miners, with their antimatter/fission drives, had the advantage of maneuverability. Meanwhile, the colonists had the edge in energy production, allowing them to assemble several powerful lasers to protect their interests. The colonists could not directly threaten the miners' antimatter collection antennae because the range of their lasers was too short. This led to a siege, where the kinetic strikes of the miners were shot down by the colonist's lasers, but any traffic to or from the colonist stations could be intercepted and destroyed. This situation lasted until interests at Earth sided with the colonists and cut off the supply of uranium to the miners.
At this time, Lunar production of thorium and uranium from Mare Imbrium was only sufficient to supply the domestic market. The Martian ore beds at Elysium had either not been discovered or were not yet in production (records are not clear), and Venus was still inaccessible, so that Earth was still the main source of fissionables. This action tilted the balance in favor of the colonists, who were able to send out task forces against the miners' stations and antennae and force a surrender [in 220 A.T].
By the time of the Technocalypse antimatter mining was no longer significant, with most antimatter being produced by photovoltaic-powered accelerators in close solar orbit. By this time as well, deuterium/helium-3 fusion and proton/boron fusion threatened to supplant antimatter/fission propulsion and had already largely replaced antimatter/fission power production.
After the Technocalypse many of the refugees set up antimatter mining facilities at their new home systems. This often provided necessary resources to get started, before advanced antimatter production factories and fusion drives could be produced.
To this day many midtech and marginal colonies have amat mining industries, especially those in the Outer Volumes, even when other forms of production are available.