Note: several technologies that could work out for providing commercial nuclear fusion would not lead to cheap and abundant nuclear fusion. They would have power that is about the same price as current 3rd generation nuclear fission. The regular ITER project is such a system. For low cost and more availability, there needs to be factory mass produced nuclear fusion generators. There are designs for factory mass produced deep burn (burn most of the fuel) nuclear fission which could be cheaper than many forms of nuclear fusion. Cheap nuclear power needs to be as common as small planes. Production volumes need to be a few thousand per year or more.
How it could happen and how cheap could the energy be?
1. Lawrenceville Plasma Physics (LPP) succeeds as they have described A Focus Fusion reactor would produce electricity very differently. The energy from fusion reactions is released mainly in the form of a high-energy, pulsed beam of helium nuclei. Since the nuclei are electrically charged, this beam is already an electric current. All that is needed is to capture this electric energy into an electric circuit. This can be done by allowing the pulsed beam to generate electric currents in a series of coils as it passes through them. This is much the same way that a transformer works, stepping electric power down from the high voltage of a transmission line to the low voltage used in homes and factories. It is also like a particle accelerator run in reverse. Such an electrical transformation can be highly efficient, probably around 70%. What is most important is that it is exceedingly cheap and compact. The steam turbines and electrical generators are eliminated. A 5 MW Focus Fusion reactor may cost around $300,000 and produce electricity for 1/10th of a cent per kWh. This is fifty times less than current electric costs. Fuel costs will be negligible because a 5 MW plant will require only five pounds of fuel per year. [About 40 million kWh per year from a 5 MWe plant and 5 MWe is equal to 6705 horsepower]
Past Dense Plasma Fusion work which shows why LPP looks reasonable
The story board of what the LPP focus fusion process would be doing
2. Inertial electrostatic confinement (bussard/IEC) fusion is targeting commercialization at 2-5 cents per kWh.
However, many people can make the simple fusor technology which is being scaled up. Material and components costs go up, but future manufacturing capability (nanofactories) and superconductor technology could make the full commercial scale IEC fusion reactors cheap. A 100 MWe reactor for $6 million would be comparable to the Focus Fusion reactor scenario. The hobbyist nature of the simple fusor suggests that even though the high power systems would involve a lot more safety issues and costs, reasonably skilled and dedicated teams of engineers would be able to replicate any IEC fusor success.
3. DARPA had a funded project for Chip-Scale High Energy Atomic Beams.
Develop 0.5 MeV [mega electron-volt] proton beams and collide onto microscale B-11 target with a fusion Q (energy ratio) > 20, possibly leading to self-sustained fusion.
There is progress towards a 1 meter long 10 GeV particle accelerator using plasma wakefield technology
If the distance and power were linear, then a 1 millimeter long system would generate 10 MeV particles. You would then need to work out miniturizing the laser system and the targets. Laser technology is advancing quickly and better targets could be made from advancing nanotechnology.
Cheap and Abundant Access to Space
IEC fusion at the 2 cents per kWh level would be providing $27/kg access single stage access to orbit.
This kind of single stage to orbit ship would still cost $2-5 billion. High availability of cheap graphene, carbon nanotube or diamondoid or nanofactory capability would greatly reduce the costs and simplify the production of the spaceship because of superior materials and manufacturing.
Easy access to space with a lot of high powered ships and equipment means easy space mining.
One NASA report estimates that the mineral wealth of the asteroids in the asteroid belt might exceed $100 billion for each of the six billion people on Earth.
Fully developing the capabilities of nuclear fusion and nanofactories and accessing these resources in the solar system is the end of scarcity scenario.
Also, use the mundane singularity technology like cement jet printing buildings.
Nuclear Bombs and Weapons would be Easy
If you have a nuclear fusion generator then you can generate a lot of neutrons. With a lot of neutrons you can transmute uranium isotopes.
Non-electric uses for nuclear fusion.
If you have fusion powered transportation around the solar system, then you can make all kinds of kinetic energy weapons. ie bombarding things with accelerated asteroids.
So What Would Be Safe ?
Live in the cheap mobile fusion spaceships.
Have ones big enough for a few tens of thousands of people or move around in fleets.
Use metamaterials (invisibility) or at least alter the albedo (space camoflage) to make them harder to spot. (The solar system is a big place, we are still spotting objects bigger than Pluto at about the distance of Pluto.)
Initially the hard to spot spaceships would be like nuclear missile submarines now, your deterrent force, but eventually a large fraction of the population would be mobile in the solar system for commerce and for safety. There would also be less strategic purpose in going after those people who were still on Earth.
In the long range scenario with nanofactories and cheap fusion, then you could not just manufacture big ships with rotating sections for gravity and carrying plenty of supplies but you would have manufacturing capability and resources to make decoy/redundant ships/colonies. The fully capable redundant ships would also be places to move to if for some reason some of the primary ships were damaged.