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"Using a more sophisticated silicon structure, still for subrelativistic electrons (96.3 keV), the highest gradient achieved is 370 MV/m over a 5.4 μm interaction length in the dual pillar silicon structure with two-sided illumination"

— Source: https://opg.optica.org/oe/fulltext.cfm?uri=oe-30-1-505&id=46...

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I heard about photon rockets, I heard about ion drives, but an electron rocket motor is new. Can't be bothered to do the math here but Isp doesn't sound too bad eyeballing it...? 1 eV is 5.94 × 10^6 m/s but multiplying this by 96.3k looks like it needs a relativistic correction.

We've got no shortage of electron beams to use in an engine. The problem is that the engine runs out of electrons very quickly and will become positively charged. Meaning all your propellant will come flying back. You would have to capture 'free' electrons from space to make use of it.

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