Yes, external magnetic fields do influence the DPF if they are strong, but Focus Fusion will have its own field generator, and is otherwise magnetically isolated. In an ordinary situation of power generation, this will not produce thrust, since you can have a barrier to stop the alphas after they pass through the coil.
A VASIMR VF-200 is made from two 100 kW engines, strapped together with opposite magnetic polarity. So 250-500 W/kg from a solar array will weigh in at 400-800 kg for each VF-200. Yes, if you want to strap a dozen or so of engines together for a faster flight, a 5MW focus fusion reactor will beat the solar array.
The hotter you can run the reactor, the smaller your radiator.
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