No thermodynamics in Traveller - soft sci fi
How do you get man portable laser weapons with barrels instead of mirrors {ever see the laser weapons in the D&D adventure Expedition to the Barrier Peaks?)
How do you get fusion to work?
How do you shrink it down to vehicle size?
Now you're talking about Traveller art, which wasn't in the original black books. Nothing in the description says the lasers have barrels, just a length and a weight. Still more hard than soft, given the need for an external power pack.
Again, a Tokomak is not much larger than the 4 Td of an A power plant. The original black books didn't have smaller fusion power plants, except implied that small craft have smaller power plants.
I agree that the Trav paradigm of proton fusion is soft sf. Proton fusion is weak force controlled and barely works in stars. The energy density in the core of Sol is less than 300 W/m³. Every time I hear some sci-pop dude/chick say the sun is a "thermonuclear bomb" I laugh. It's more like a brick kiln.
However, the original LBBs didn't mention proton fusion specifically (that I can find). The idea of "refined fuel" would make sense if that meant enriched D content, but it doesn't. The power plant runs just fine on unrefined hydrogen, and refined doesn't increase the power output or reduce fuel usage rate.
Elsewhere I showed that D-D fusion could account for all the power per ton of fuel, based on EP = 250 MW and fuel usage. It just doesn't work to try to fuse the D in it's natural concentration. It would send the Lawson PT criteria up by an order of magnitude. Same for D-T with external source of T. The low concentration of natural D wouldn't work.
And, you're right about thermodynamics. I've said elsewhere that the real handwavium isn't fusion or jump drive or grav tech. It's heat. Even at 99% efficiency a 1 EP power plant would be dumping 2.5 MW of waste heat into the ship. The engineering section would be slag in a day.
If you're insisting on current theories nothing could ever be hard sf and involve anything that requires such power levels.