A design proposal by Rodney A. Clark and Robert B. Sheldon theoretically increases efficiency and decreases complexity of a fission fragment rocket at the same time over the bundle proposal. In their design, nanoparticles of fissionable fuel are kept in a vacuum chamber subject to an axial magnetic field (acting as a magnetic mirror) and an external electric field. 

A fission fragments ejected for propulsion
B reactor
C fission fragments decelerated for power generation
d moderator (BeO or LiH)
e containment field generator
f RF induction coil
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