(07-14-2015, 07:07 AM)radtech497 Wrote: With the Daedalus, Longshot, and other ICF-based drives, the "propellant" is heated and compressed by lasers/masers/etc. located within the ship's structure. The fusion reaction takes place within a "combustion chamber" that is generally also located within the ship's structure.
The drive beams may be inside the structure, but their intersection point is not always so. In many of these cases, the fuel pellet is shot through the forward end of the pusher field, and ignited on the aft side of it. The principle benefit of this sort of propulsive system is that the vast majority of the unwanted radiation from the reaction does not hit the ship. Any internally detonating system would have none of this benefit; moreover the power levels required to confine the blast would be most unwelcome.
Longshot is unusual in that it does have an internal reaction chamber... Daedalus, VISTA, MiniMag Orion and ICAN-II all very clearly have all reaction take place in the nozzle, not in a confined chamber. Magneto-inertial fusion is less clear (as there's a chamber behind the nozzle into which the fuel pellet is injected) but fusion only takes place as the pellet leaves the chamber and enters the nozzle. The HOPE Z-pinch fusion system is even less clear, but I do believe that the most energetic part of the reaction happens in the nozzle as trying to confine the fusion fireball is inconvenient. It is notable that with the exception of Daedalus, all of these other designs postdate Longshot, and I'd be inclined to discount it as obsolete (but that's just me).
Magnetic confinement fusion does indeed have an enclosed and internal chamber, as I described above. I agree that it is "internal"; electrostatic confinement systems would fall into the same category.