I believe before 2018 there will be many viable nuclear fusion approaches that will be competing in commercial markets. More than one could achieve technical and commercial success by leveraging different niches. Just like there is more than one kind of viable form of nuclear fission. There are metal reactors in Russian submarines, Pressure Water Reactors (PWR), Boiler Water Reactors (BWR), Heavy water reactors (candu), Pebble bed reactors, Liquid flouride thorium, and various breeder reactors.
The technology takeoff point for nuclear fusion should hit by 2015 and there is even a chance for significant proof before the end of 2011. Also, once a few prove viable then the other approaches could also be more fundable and be adapted to be made viable. One of the biggest argument against all of them is Bremsstrahlung radiation, which is too much energy leaking away via microwaves. Once a few show how to overcome the problem then all of them can get tweaked if they would not already work
I like Focus fusion (lawrenceville plasma physics) they could achieve good results late this year or next year. Simple devices $200,000 for the materials to make it.
EMC2 (bussard/IEC/polywell fusion). Awaiting news but make or break could be known by the end of 2011 (actual schedule is beginning of 2011 but delays are not unreasonable.).
General fusion (in Canada, magnetized target fusion variant)
Tri-alpha Energy (secretive - they were saying some news could be released this year. 2015-2020 target for full scale success first pilot plant. Field Reversed Colliding beam fusion)
Helion Energy (need next level of funding but have built one third scale)
Japan Muon fusion
Recent work suggests that impact fusion is more viable than previously believed.
It looks promising but I guess it is at least 5 years away from a full scale test and more likely ten+ years. Unless one of the current linear colliders could be adapted.
The researchers are from China, so this could be attempted by China. I also see this work being helped with the improvement and lowering of costs for superconducting magnets. Higher Tesla field strengths. Iron superconductors might reach 200 tesla. There is a project to build 32 tesla superconducting magnet with YCBO variant wire. 2015 should see the Superpower Inc ramping up of 2G wire production payoff with far lower costs. Far more powerful and affordable superconducting magnets makes most of the nuclear fusion designs smaller and better to implement.
There is the possibility of a breakthrough with Low energy nuclear reactions (what was called cold fusion). I like the nickel 64 - stripping reactions theory.
The potential of a viable implementation of nickel-64 stripping reactions.
All of the nuclear fusion approaches have the potential to enable hybrid fusion-fission to close the fission fuel cycle (virtually no nuclear waste) even if they fall short of pure commercial fusion.
Plus they can enable nuclear space propulsion with far lower requirements than commercial energy. If the system can run for 3 minutes it could power a launch into orbit. Commericial energy needs it to work 60+% of the time 24-7. Also, space launch application does not matter if it costs 100 times more than regular electricity, it would still be way cheaper and better than chemical launch.
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