Let's assume problems of running sustained nuclear fusion are overcome (be it by making the mechanism highly energy-positive, or just supplying all the energy deficit externally, like beamed power.) Let's also assume we managed to direct all products (and none of the substrates) of the fusion in one direction (through the nozzle). Plus matters of cooling, safety etc, all the engineering trivia.
Essentially, the drive reacts Deuterium and Tritium, converting them into Helium through nuclear fusion, and the newly created extremely hot helium is ejected through the nozzle, as reaction mass.
What would be the exhaust speed of such a drive - speed at which the atoms of helium would be ejected; the specific impulse of such a drive?
I tried to ballpark typical temperatures of fusion plasma into average speeds of particles, and got nowhere, as the gas equations don't really work with plasma. Could you give me a ballpark value?