The engine functions in various ways at various speeds. At low speed interplanetary runs, the drive works like a conventional ion rocket or mass driver. Stored reaction mass is feed into the electromagnetic or electro static accelerator core. Unlike normal thermal rockets, an ion thruster works more efficiently with heavy atom ions. So I'll assume we are storing iron for reaction mass.
Specific impulse varies depending on the exhaust velocity of the expended mass.
Acceleration To InterStellar Cruise. For acceleration out of the Sol system the ship will deploy its ramscoop and switch to the externally fueled RAIR configuration I came up with. As mentioned above, a normal RAIR doesn't try to fuse anything it scoops up. But you still need a lot of fuel to power the accelerator. The fuel is launched at measured and timed speeds, so that the ship will encounter a fairly steady stream of fuel during its run down the fuel loaded acceleration track.