The DFD is a variant of the Princeton Field-Reversed Configuration (PFRC), a fusion-reactor concept invented in the early 2000s by Samuel Cohen of the Princeton Plasma Physics Laboratory (PPPL). The DFD is basically a PFRC reactor with an open end, through which exhaust flows to generate thrust, Thomas explained. 

The DFD's interior will feature a magnetically contained hot plasma of helium-3 and deuterium, a special "heavy" type of hydrogen that has one neutron in its nucleus (as opposed to "normal" hydrogen, which has no neutrons). Atoms of these elements will fuse within this plasma, generating lots of energy — and very little dangerous radiation, Thomas said.

The fusing plasma heats up cool propellant flowing outside the confinement region. This propellant is directed out a nozzle at the back of the engine, producing thrust. 

All that heat translates to a lot of power — likely between 1 and 10 megawatts, Thomas said. The DFD will tap into that power, using a "Brayton cycle" engine to convert much of the heat into electricity.  

That means a DFD mission would be able to do a great deal of science work after reaching its destination. For example, a fusion-equipped Pluto orbiter could beam down power to a lander on the dwarf planet's surface and also send high-definition video back to Earth, Thomas said.

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