We can't make it so large that its own gravity will contain the reaction mass, so it has to be kept inside a very strong magnetic field created by huge magnets. You can't put solar panels inside the reaction chamber, they would get destroyed.

Yep! And fun fact, online encryption relies on basically exactly this technology (radioactive decay, not fusion, but hey it's close enough if you squint). Radiophotovoltaic batteries provide uninterrupted current, which is used to ensure that encryption keys (stored in highly volatile memory for security) are not lost due to a brief power flicker.