Lore behind fusion/am drives

Drive discussions got me thinking: is there any established lore about high-power fusion/am drives operation?

They are called "torches" meaning there is something expelled in a flamy fashion, but they are also "closed cycle" meaning reactor mass aren't used as is a case for burner line drives.

I can only think of AM/Fusion drives as a mega-VASIMR kind of an engines. Having a gigawatt/terawatt reactor and a superconducting everything means you can throw all this energy into hydrogen plasma EM-accelerator and expell it with stupendous speed for thrust.