Yep, there was a physicist who did the math on the Epstein Drive (unfortunate name) in The Expanse and found that it was at the edge of possibility if you use very efficient lasers, ICF, and D+He3 fuel. There is mention of helium-3 and fuel pellets in the books, so it sounds like that was the idea.
The biggest un-realism in The Expanse is the lack of huge heat-sinks, at least in the show. (They aren't mentioned in the books but I assumed they'd be there.) Without heat-sinks even if the drives were >95% efficient the ship would melt. Also in the show the thrust plumes from the engines are portrayed as looking like grill flames. In reality they'd look more like beams of light fading off into space.
the argument is that "highly efficient laser" might be an oxymoron and if you were serious about commercial fusion you might trade lasers for much more efficient particle accelerators that run at a viable shot rate. Trouble is that you need heavy ions (lead) at 8GeV and it takes multiple barrels that are a km long or so... A huge machine that might be competitive with lasers for a commercial power plant but that can't be built in a subscale prototype. It might not be compatible with the magnetic nozzle though as 8GeV is not relativistic for lead ions.
That’s what I think. The efficiency of lasers is awful and they take hours to cool off after a shot whereas a commercial fusion power plant needs a shot rate between one every few seconds to several per second.
A heavy ion power plant is possible in terms of the physics but needs to be the scale of a fission power plant to work at all and is projected to cost maybe 2x what an AP1000 costs assuming everything goes well and we know things usually work worse than you expect. So nobody is interested in funding a full-scale prototype, a reasonable development plan is you build several linac barrels and a test fusion facility and expect to rebuild that and add more barrels. It probably costs about what Musk spent on Twitter in the end.
See https://cds.cern.ch/record/2743157/files/Seeman2020_Chapter_... you can get better Q for superconducting linacs and also operate them in CW mode without fear of burning them up. Off the cuff I'd speculate that superconducting magnets that can support 2x the magnetic field might support 2x the accelerating gradient but we are talking AC operation here, not DC.
Maybe they have some sort of thermally excited laser material and use hybrid electro/thermal lasers to "heat pump" waste heat back into the laser system.
Now tell me about all the waste heat such a scheme would generate. Yes, you can generate work from a temperature difference. You cannot convert that thermal energy to work with 100% efficiency.
The biggest un-realism in The Expanse is the lack of huge heat-sinks, at least in the show. (They aren't mentioned in the books but I assumed they'd be there.) Without heat-sinks even if the drives were >95% efficient the ship would melt. Also in the show the thrust plumes from the engines are portrayed as looking like grill flames. In reality they'd look more like beams of light fading off into space.