You still have to run the cryogenic system for the superconductors and the pittance of power to keep the confinement coil power supplies. Q=inf means ignition:

You still have to run the cryogenic system for the superconductors and the pittance of power to keep the confinement coil power supplies. Q=inf means ignition: an entirely self-heated plasma. The collisions of the helium ash on the un-burnt fuel are enough to bring the fuel above the flash point. In this way it is like a campfire.