I know there is a way go go yet before this technology is proven,however as the EMC2 team appear to be making real progress towards a demonstration design, and as this is the Advanced Concept thread, is it time to start thinking about how this technology might extend capabilities.
If the energy production does scale by the 7th power then would Orion class vehicles be feasible?
Orion class vehicles use explosive nuclear pulses from small fission or fusion shaped charge warheads so the explosive energy pushes against a giant pusher plate with giant shock absorbers. So Polywell would not be useful in this class of vehicle because Polywell produces constant or near constant fusion reactions (there has been reported a pulsing effect in polywell fusion activity, but this is extremely minor compared to nuclear explosives).
However, a polywell reactor in space would, if scaled up to produce 2 GW of power, be able to power a variety of propulsion schemes. 2 GW is enough to power the full scale VASIMR engines that would allow for 39 day trip times to Mars from Earth.
Then you have the reactor itself as part of the engine...
Firstly, hydrogen, or water, could be used as a reactor coolant and then ejected as a propellant much like fission engines like NERVA, PLUTO, etc. however as most of the energy produced by pB11 fusion, the preferred fuel choice for low radiation fusion, is in the form of electrons, this allows for direct conversion of fusion reaction energy into electric power, with no need for turbine generators, brayton cycle converters, or humongous radiators. Whereas fission reactors have higher thermal power output than electric, due to the inefficiency of converting thermal energy to electricity, polywell reactors would produce significantly more power as electricity, with a small fraction as heat.
You could therefore use a concentrator system to use your coolant to concentrate the heat in less mass via a heat pump system, then subject that mass of propellant to electron beams, which would convert it into a very energetic plasma at such high temperatures that you would need magnetic nozzles like VASIMR has, and would result in very very high Isp, in the hundreds of thousands.
Theoretically polywell fusion permits efficiencies as high as several million Isp if you simply took the fusion plasma products, dumped the electricity back into them, and expelled them.
Carrying your own on-board fuel, you could potentially reach up to 20% of light speed with this technology, enabling trips to Alpha Centauri in 22 years trip time.
If you were to use this technology as the core of a bussard ramjet spacecraft, you would carry an onboard store of Boron 11 fuel, and use the hydrogen collected from interstellar space as your source of protons. Since brehmsrahlung thermal issues require your reactor run hydrogen rich, you could collect a mass of hydrogen equal in mass to the on board boron fuel, which would double your effective Isp. This combined fuel ramjet stage would potentially enable you to light up the ramjets fusion from a dead stop and reach velocities high enough where the amount of hydrogen collected, and speed its collected at, would permit pure H-H fusion to be achieved, and the bussard ramjet could potentially reach near to light speed depending on trip length and acceleration rate.
At this point you run into issues of radiation from the relative velocity of your ship vs the gasses in interstellar space hitting your ship and/or the magnetic field around it thats used to scoop up that gas. There are some calculations showing that at some high fraction of light speed, no amount of mass or magnetic shielding would protect a human crew from this radiation that would be feasible for enabling a ship capable of any significant acceleration.
Now, beyond fusion propulsion, there is also the potential to use polywell reactors to power more exotic field drives or mach effect drives. Mach-Effect thrusters at low power levels are believed to generate simple thrust from an input of electricity. Laboratory evidence supports this part of the theory so far, though this is not yet rock solid and experiments are ongoing. Furthermore, this sort of technology is also supposed to allow for generating warp fields like those described by Alcubierre and shown in Star Trek. Finally, they potentially (and this is far future speculation given a lot of advancement of the technology and no show stoppers that have not yet been thought of) are the technology needed to generate stargate type wormhole travel. These three levels of propulsion are at different levels of power and efficiency with this sort of device, and a lot of this is at this point just theory, as I previously said. However, if current low power experiments confirm that part of the theory, then it is likely that the rest of the theory is also valid and only depend on future advancements to achieve (much akin to the distinction between ancient roman steam wheel toys and modern turbine jet engines).
