It's not just that the fusion only occurs in the core, but the fusion rate is also very low; it's only because the sun is so incredibly massive that this slow r

It's not just that the fusion only occurs in the core, but the fusion rate is also very low; it's only because the sun is so incredibly massive that this slow reaction rate can self-sustain. With a much smaller fusion reactor, the reaction rate needs to be much higher to keep itself going and to create enough power to run the electromagnets that keep it confined.